Cultural Management of Onion Thrips and Iris yellow Spot Virus

Final Report for SW08-076

Project Type: Research and Education
Funds awarded in 2008: $133,441.00
Projected End Date: 12/31/2011
Region: Western
State: Utah
Principal Investigator:
Dr. Jennifer Reeve
Utah State University
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Project Information


Our farm survey showed no correlation between input intensity and yield or onion size. Thrips incidence was greater in fields fertilized with higher nitrogen (N). Moreover, considerable N was present in the soil at harvest. The impact of higher N on thrips and N leaching was confirmed in replicated trials. Yields and onion size were similar between higher and lower N rates when onions were planted in a timely manner. Thrips populations were lower on onions planted after corn than after wheat, and we saw no effect of a soil biostimulant. Lacy phacelia and buckwheat show potential as trap crops, although synchronization of trap crop blooming period with peak thrips densities may be critical.

Project Objectives:

In order to meet the multiple goals of increased environmental and economic sustainability, enhanced quality of life and worker safety, and increased farmland diversification in onion cropping systems, we studied the following six objectives:

1) work with local growers to determine onion thrips and IYSV pressure in primary Utah onion growing areas and correlate thrips and IYSV pressure to common farm practices;

2) determine the effects of crop and pest management strategies on thrips survival and population size;

3) evaluate nitrogen (N) inputs, N leaching potential, alternative fertilizers, trap crops and crop rotation on thrips, IYSV, onion yields and storage quality;

4) conduct grower workshops and field days on control options of onion thrips and IYSV;

5) conduct economic cost-benefit analysis of proposed changes to management of onion thrips and IYSV; and

6) disseminate results through extension bulletins, the Internet, trade journals and scientific literature.


Onion (Allium cepa L.) is produced on 60,000 – 70,000 hectares annually in the United States. Over half of this acreage is located in the western states, with Utah growing between 647 – 1,133 hectares annually, at a farm value of $4-10 million (NASS, 2006). Because of the high value of the crop, onions are intensively managed for weeds, insects and diseases, largely through regular applications of pesticide.

Infestations of onion thrips, Thrips tabaci Lindeman, are the most common cause for insecticide use in dry bulb onion production in the western U.S. (Mayer et al. 1987). Onion thrips feed on onion foliage and interfere with the transport of nutrients to the bulb (Kendall and Capinera 1987). Economic yield losses occur with substantial feeding during bulbing stages of growth (Fournier et al. 1995). Insecticides used in thrips management (carbamates, organophosphates, pyrethroids) have high health and environmental risks. Despite widespread and frequent (three+ applications per season) use of insecticides, thrips continue to cause increasing damage to onion because of the development of insecticide resistance (Allen et al. 2005).

In addition to feeding injury to onion leaves, onion thrips also vector Iris yellow spot virus (IYSV). IYSV causes “straw bleaching” of leaves and has become an increasingly devastating and widespread disease of onion. The disease is now reported throughout the western states, and yield losses can be as high as 60% in heavily infected fields (Gent et al. 2007). The disease spreads extremely rapidly. Initially detected in only 6% of fields in Colorado, 73% of surveyed fields were found to be infected only two years later (Gent et al. 2007). Revenue losses of up to 15% are projected as a result of IYSV (Gent et al. 2007). However, this figure does not include the risk to the environment and human health associated with intensive use of insecticides.

The current body of scientific knowledge is devoid of adequate management strategies for IYSV (Gent et al., 2004). In the absence of sound management strategies, growers have implemented even more intensive thrips insecticide programs. In conversations with local onion growers, some reported that shortening the interval between insecticide spray applications for onion thrips had little effect on thrips control, IYSV incidence or severity. However, one grower (Morgan Reeder, Corinne, Utah) reported that he has not had to spray for onion thrips during the last three growing seasons, potentially due to recent changes in crop management.

Alternative management strategies have been evaluated in Oregon and Colorado with some success (Jensen et al., 2003; Gent et al. 2007). Biological-based insecticides, (spinosad and azadirachtin), provide poor thrips suppression when used alone, but they show promise when combined with straw mulch. This suggests that mulches, combined with insecticides that have low impact on natural enemies of thrips, may be more successful and sustainable in onion production systems than conventional insecticide spray programs alone. Managing N fertilization has successfully reduced thrips populations on numerous crops (e.g. Brodbeck et al., 2001; Mollema and Cole, 1996) and may also be useful in tospovirus management (Stavisky et al., 2002). Recently, Malik et al., (2009) showed 70% higher thrips in fertilized than unfertilized onions. Other soil parameters may also impact both IYSV severity and thrips pressure. In Colorado onion fields, IYSV symptoms increased with an increase in soil pH, organic matter, sodium and other micronutrients (Schwartz et al., 2010).

Rotation effects on thrips populations have not been adequately studied (Gent et al. 2007). Alternative hosts provide a green bridge by which thrips can successfully overwinter, so effects of planting a less attractive crop prior to onion needs investigation. Use of trap crops for reducing pest damage has been tried with some success in various vegetable crops, but there is little published work of this kind in onion. Trap crops can be planted around the perimeter or in strips throughout the field. A good trap crop would be more attractive to thrips than the main crop or would reduce their chance of locating the main crop through disorientation. One approach used successfully in Europe is to under-sow a low growing crop such as clover; however, yields can be affected due to plant competition (Belder et al. 2000). Trdan et al. (2006) showed buckwheat and lacy phacelia competed too strongly with onion to serve as effective under-sown crops. They were highly attractive to thrips, however, so there is potential for these species to be strip cropped. While strip-cropping a highly attractive trap crop at specified intervals through a field could prove to be economically justifiable, use of income generating trap crops such as carrot or buckwheat with onion would improve the economic viability of the system.

Our goal for this project was to better characterize the severity of onion thrips damage and IYSV incidence in transplanted and seeded onions throughout the major growing areas of Utah. Thirty-two commercial onion fields were surveyed for thrips, IYSV and soil properties, and cultural practices (irrigation, nutrition, weed control, insecticide management strategies) were determined for each field through grower interviews. A replicated field trial was conducted to test the effects of reduced N inputs and a biostumulant, MoreLife, on onion growth and yield, soil microbial activity, nitrate leaching, thrips populations and incidence of IYSV. The efficacy of three trap crops (lacy phacelia, buckwheat and carrot) in reducing thrips and IYSV were also evaluated.


Click linked name(s) to expand
  • Diane Alston
  • Dan Drost
  • Kent Evans
  • Ruby Ward


Materials and methods:

Objective 1

An on-farm survey was conducted during the 2008 and 2009 growing seasons. Fifteen commercial onion fields were surveyed in 2008 and, another 16 in 2009 were surveyed, all in the three primary onion-producing counties in northern Utah. The following parameters were collected:

Thrips Population Densities

Bi-weekly changes in thrips populations were determined from five samples of two plants each randomly collected in each field (10 plants per field per sample date). Plants for samples were cut below the neck, placed in jars with soapy water, and the soapy water solution poured through a 220-mesh sieve to collect the motile life stages of thrips for counting under a dissecting microscope at 10-20 magnification. The mean numbers of adults and larvae per plant were determined. Fields were sampled from early June to early August, for a total of five sample dates.

IYSV Incidence

Visual incidence (% plants infected) and severity of IYSV (scale of 0 to 4, where 0 = no symptoms, 1 = 1 to 2 small lesions per leaf, 2 = 2 medium-sized lesions per leaf, 3 = lesions coalescing on more than 25% of the leaf and 4 = more than 50% leaf death, particularly in the neck region, as a result of lesions) were measured every two weeks from mid-May to harvest from the same fields in northern Utah. Severity in a field was measured along transects by visually evaluating 25 plants at 20 foot intervals. Leaves from 30 IYSV symptomatic plants were selected each sampling period. These leaves were used for serological confirmation of visual identification of IYSV infection using enzyme-linked immunosorbent assay (ELISA) (Agdia, Elkhart, IN)

Plant Nitrogen Status

Plant N status was assessed monthly from May to September. Thirty randomly collected plants from each field were collected and used to assess plant growth response and tissue N status. Plant growth response (leaf number, fresh & dry weights) and leaf area (using leaf imaging software) were collected at each sample period. Onion tissue was dried at 70C for dry weight determination, ground and total N measured by combustion (LECO TruSpec CN, LECO Inc., St. Joseph, MI).

Soil Nitrogen Status

Soil nitrate and ammonium were measured during the growing season. Baseline N levels were collected after emergence (May) and at monthly intervals through September. Twenty randomly selected soil samples from the crop root zone (0-30 cm) in the center of the bed were combined from each field, dried and analyzed.

Onion Yield and Storage Quality

At harvest, four 3 m sections of bed were bagged, graded and weighed before being placed in storage. Storage potential (weight loss, sprouting and rots) was assessed after a 90-day storage period. After storage, a random sample of bulbs was evaluated for storage breakdown, single centers, the presence of thrips in bulbs and IYSV carry-over. While only cursory evaluation was conducted on this data, with the assistance of our new pathologist, future evaluations of the changes in storage losses will occur and will be part of our on-going evaluation of the effects of nitrogen rate and prior cropping history on onion productivity.

Statistical Analysis

On-farm survey data were analyzed with random forests (Cutler et al.2007) to classify relationships among crop management practices, soil, insect, disease and yield parameters.

Objective 2

The effects of crop and pest management practices on thrips egg survival and adult dispersal were evaluated in all fields over the two growing seasons. Egg densities were measured in the third youngest leaf of each of the 10 sample plants per field (see Obj. 1) by staining leaves with an acid fuchsin-lactic acid technique (REF). Leaves were pressed between glass plates and the number of eggs counted under a dissecting microscope at 30 magnification. To assess the influence of thrips adult migration on field populations over the season, eight aerial traps were placed in each field. To compare immigration and dispersal on field edges versus interiors, four traps were placed in each border and interior location. Traps were constructed of a 15 cm-long section of 10 cm diameter PVC pipe covered with flat white paint and removable sticky fly paper, mounted on a wooden post. Traps were placed in onion rows with the tops of traps protruding 10-15 cm above plant canopy height. Effects of field edges on thrips densities were compared for adult densities on aerial traps using a three-way factorial design (field, date and trap location within field) and analysis of variance (Proc Mixed, SAS version 9.2, SAS Institute, Cary, NC).

Objective 3

Replicated Field Trial 1 – Crop Rotation and Nitrogen Management

The field trial was located on 0.69 ha of land at the USU experiment station in Kaysville. Twelve plots of wheat and 12 plots of corn were established on half of this area in a completely randomized design in the spring of 2008 in order to provide Year 1 of the rotation component of the study. Individual plot sizes were 7.6 x 15 m, consisting of six four-row beds 15 m in length, maintained by standard cultural practices. At a 10 cm seed spacing, this resulted in approximately 4,800 onion plants per plot. In 2009, onions were seeded in March in the following six treatments: 1) standard grower N application after wheat, 2) standard grower N application after corn, 3) 50% reduced N application after wheat, 4) 50% reduced N application after corn, 5) 50% reduced N application plus biostimulant (MoreLife) after wheat, and 6) 50% reduced N application plus biostimulant after corn. Wheat and corn plots (2nd year rotation) were established in 2009 on the second half of the field followed by the second year of the same onion treatments in 2010.

Soil and plant N, thrips pressure and IYSV incidence were monitored in all plots as described in Objective 1. An additional measure of effects of cultural practices on onion thrips survival was used to determine hatching rates of thrips eggs over time. The third youngest leaf was collected from five plants per plot, washed with water to remove motile stages and placed into a 27C hatching chamber. After one week, the leaves and inside of the chamber were washed with water to remove hatched larval thrips and collected on a 220-mesh sieve for counting as previously described.

To evaluate N leaching potential under the six different treatments, two soil water lysimeters were placed at two and four foot depth in each plot, and the leachate collected every month from May to July and analyzed for nitrate-N. Soil cores to 90 cm were also collected in March and October to assess N leaching. Microbial activity was assessed by measuring dehydrogenase enzyme activity in the top 10 cm of the soil, monthly from May to September according to Tabatabai (1994). Soil respiration, readily mineralizable carbon and microbial biomass were determined in May and July of both years according to Anderson and Domsch (1978).

Replicated Field trial 2 – Trap Crop Assessment

Three different trap crops were assessed for their effectiveness in luring thrips away from onions. Plantings of carrot (Daucus carota subsp. Sativus), buckwheat (Fagopyrum sagittatum (Gilib.)) and lacy phacelia (Phacelia tanacetifolia) measuring 3 x 12 feet with four replicates were established in a random complete block design within a commercial onion field in each 2009 and 2010. Sampling for thrips and IYSV was carried out along four transects from each plot at the following locations: within the trap crop, and at 0, 0.8, 6.4 and 12.8 m distance. Four onion-only areas greater than 30.5 m from any edge or trap crop were also established. Potential herbicide injury to the trap crops was prevented by covering the plots with plastic sheeting before spraying.

Statistical Analyses

Onion thrips densities were compared among fertilizer, crop rotation and trap crop treatments with analysis of variance (Proc Mixed, SAS version 9.2, SAS Institute, Cary, NC). Data were square root transformed prior to analysis to meet assumptions of normality, and when different, means separated with Tukey’s test (?=0.1).

Objective 4

Field days and workshops: See details under Results/Discussion below.

Objective 5

Economic Analysis

Crop yield were used to calculate the cost of onion production based on present grower inputs. Production costs were then calculated for a low nitrogen application – low pesticide spray management scenario partial budgets generated. We also evaluated the costs associated with a high nitrogen application – increased pesticide spray use scenario. Growers using a reduced N, reduced spray program would save approximately $300/acre, while growers utilizing the increased N, more sprays program would decrease their returns by about $200/acre. The most cost effective management options in terms of economic return, environmental benefit and human health was determined. Potential costs and benefits of widespread adoption of the new systems relative to current practice were calculated.

Objective 6

Dissemination of results: See details under Results/Discussion below.

Research results and discussion:

Objective 1

Thirty-one commercial onion fields were surveyed over the course of two growing seasons for soil properties, onion growth, thrips abundance and IYSV incidence. In addition, growers were interviewed to gather details on their inputs, crop rotations and other management practices. The results of this survey are still being analyzed with the help of a professional statistician. Preliminary correlation analysis revealed no relationship between input rate and yield or onion size. IYSV was positively correlated with thrips incidence, and low- or no-spray fields had similar or lower thrips than high-input fields. A positive correlation between thrips and soil pH was observed in 2008 only. High-input fields contained considerable residual N at the end of the growing season, with high potential for loss to the environment over the winter. These findings were confirmed in the replicated trial (see objective 3).

Objective 2

In the 31 on-farm survey fields, only field and date effects were significant with adult thrips increasing in numbers as the season progressed. There were no edge effects and no two-way interactions. The experimental design may not have been ideal for teasing out edge effects. In ongoing sampling efforts funded through an onion IPM PIPE project (USDA SCRI), new data is showing some edge effects, especially earlier in the season.

Objective 3

Replicated Field Trial 1

Crop Rotation and Nitrogen Management

Onion growth, yield and storage

The relative response of onion growth to fertilizer and crop rotation was similar between years; however, overall onion yield in all treatments was much lower in 2010 due to a wet and cold spring and a resulting later planting date. In 2009, the normal planting year, neither total yields nor bulb size were affected by crop rotation or fertilizer treatment. However, in 2010, both crop rotation and fertilizer treatments did significantly affect yield. Yields were higher in the standard fertilized treatments, and there were more cull onions in the reduced and MoreLife treatments than the standard fertilizer treatment. All other size class categories were similar among treatments. Onions grown after corn had higher yields than after wheat in 2010. There was no significant effect of crop rotation or fertilizer treatment on storage loss in either year.

Fertilizer treatment, but not crop rotation, strongly influenced the amount of total tissue N as well as the timing of onion maturation in both years. In 2009, onions fertilized at the standard rate had significantly more total tissue N than either the MoreLife or reduced fertilizer treatments in the months of May, June and August. In 2010, onions fertilized with the standard rate had greater tissue N than both the reduced and the reduced + MoreLife treatments in May and July; but in August, the standard rate treatment contained more tissue N than the reduced rate treatment only. In 2009, visual ratings on September 1 showed the standard rate treatments had significantly more onion lodging than other fertilizer treatments. By September 20, lodging in all the treatments exceeded 90%. In 2010, a greater percentage of onions were lodged in the standard rate treatment on September 1 and 20.

Thrips and IYSV

High rates of N fertilizer significantly increased densities of adult and immature onion thrips (OT) two-fold in both years. Thrips populations were generally higher in 2010 than 2009, and populations peaked later in the season in 2010 than in 2009. The treatment response of adult OT was significant in June only with the differences in immature OT evident approximately one month later. There appeared to be an intermediate response of thrips to the MoreLife treatment, suggesting that MoreLife may have attracted thrips in some way. In 2009, both leaf area and onion dry weight in June were positively correlated with the number of adult OT on June 1. Similarly, onion growth in July 2010 was positively correlated with the number of adult OT on July 1.

Crop rotation influenced adult OT densities in both years over a wider time period than did fertilizer treatments. In 2009, onions planted after wheat had more adult OT than after corn in mid-June, early July and early August. In 2010, there was significantly greater adult OT after wheat than after corn in mid-August. Corn and wheat rotations did not significantly affect immature thrips except in 2009 in early August, where onions after wheat had greater immature counts than after corn.

Neither fertilizer rate nor crop rotation significantly affected the number of thrips eggs per plant or the number of immature thrips that hatched from eggs within leaves. Similar to OT, the number of adult western flower thrips was somewhat greater after wheat than after corn. Total insect biodiversity on onion plants showed effects of crop rotation only. Plots planted to wheat prior to onions had greater biodiversity in early and mid-August in 2009 and at mid-June in 2010. All other sample periods showed no significant differences in diversity.

Neither crop rotation nor fertilizer treatment influenced the incidence of onion plants that tested positive for IYSV in 2010. Overall incidence rates were low. In August 2010, onions positive for IYSV ranged from 0 to 6% of tested samples, whereas by September 2010 the incidence rate was 0 to 20%. Onion samples collected in 2009 for DAS-ELISA testing were not included in analysis due to data quality problems. Visual ratings for IYSV showed that the virus was not symptomatic until late in the season; however, virus was detected using ELISA testing as early as April.

Soil Properties

Chemical and biological soil properties were affected by crop rotation and fertilizer treatment. Crop rotation influenced soil NO3-, phosphorus (P) and potassium (K) levels, mostly in 2009. Plots planted to wheat rather than corn prior to onions had higher levels of soil NO3- in May, June, July and August, 2009, as well as in May, 2010. In 2009, both P and K were significantly greater after wheat than corn. There were no other significant rotational effects on soil chemical properties.

Levels of extractable soil NO3- and NO4+ in both growing seasons were strongly influenced by fertilizer treatment. In June of both 2009 and 2010, soil NO3- was greater in the standard than the MoreLife treatment. In July and August 2009, soil NO3- was higher in the standard than in both the reduced and MoreLife treatments. Similarly, soil NO3- was much greater in the standard than MoreLife and reduced fertilizer treatments in May and July, 2010. Differences in soil NO4+ levels were noted only in the months of June and July of each year, following fertilizer applications of ammonium sulfate ((NH4)2SO4) in early June. In June of both 2009 and 2010, extractable NO4+ was greater in the standard than the reduced or reduced + MoreLife treatments. In July 2009, NO4+ was greater in the standard than reduced +MoreLife treatments but not the reduced treatment. Soil NO4+ in July, 2010 was significantly greater in the standard than both the reduced and reduced + MoreLife treatments. Fertilizer treatment did not significantly impact soil P or K levels in either year.

The movement of excess NO3- through the soil profile was captured at three different times during the course of the experiment. Cumulative NO3- leaching collected in lysimeters from May through July, 2009 was not significantly affected by crop rotation or fertilizer rate. Following snow melt in the spring of 2010, extractable NO3- levels at 90 cm sample depth were significantly greater in the standard than in the reduced and reduced + MoreLife treatments, indicating that excess N had moved below the rooting depth during the winter. Treatments following wheat also contained greater NO3- at 90 cm than treatments following corn. In fall 2010, extractable soil NO3- was significantly greater in the standard treatment than both the reduced and reduced + MoreLife treatment in the top 60 cm, with significantly greater concentrations after corn than wheat.

Not only were differences in soil chemical properties observed, but soil microbial parameters were also influenced by both fertilizer treatment and crop rotation in both years. Dehydrogenase activity was significantly greater after wheat than corn in both years; however this difference disappeared by the end of the season. In contrast, there was greater dehydrogenase activity in both the reduced and reduced + MoreLife treatments at the end of the growing season. There were no effects of MoreLife on dehydrogenase activity observed at any time.

Other measures of microbial activity were also impacted by both crop rotation and fertilizer treatment. Following wheat, soils had greater mineralizable C than after corn in May 2009 and July 2010. The only fertilizer effects on mineralizable C were in July, 2010 where the standard treatment had greater Min C than the reduced treatment. Microbial biomass was also greater after wheat in May and July of both years. Fertilizer treatment effects on microbial biomass were only observed in May, 2010 when the standard rate was higher in microbial biomass than both the reduced and reduced + MoreLife treatments. Again, there were no effects of MoreLife on mineralizable C or microbial biomass at any time.

Replicated Field trial 2

Trap Crop Assessment

The trap crop trial was located on two separate grower fields in 2009 and 2010. Fields were both located in Box Elder County, Utah, within approximately 15 km of each other. Soil types were sandy loam and silt loam in 2009 and 2010, respectively. Cumulative extractable soil nitrate levels were slightly lower than local averages in 2009 and much lower than averages in 2010. Onion growth data shown for July of each year was very different between years. Overall, the growing season in 2010 was much colder with more moisture early in the season. Onion growth was delayed when compared with the 2009 growth data. Neither grower sprayed for thrips in either year.

In 2009, relative seasonal thrips populations were monitored at four control points within the field relative to 0, 0.8, 6.4 and 12.8 m from one of the three trap crops. Thrips numbers were low, with the highest adult populations observed on the July 14th sample date. With a few exceptions, most of the whole plant samples for adults, immatures and eggs were lower in the trap crop rows than at any sample distances from the traps. IYSV symptoms were also low throughout the season.

Buckwheat was attractive to adult OT on July 30 when the buckwheat was at full height and in bloom. Thrips populations on buckwheat were greater than on onions at any of the transect distances. However, on June 29 before the buckwheat crop was well-established, there were fewer adult OT within the buckwheat than on onions at all distances from the trap crop. The number of immature thrips was lower within the buckwheat than at any other distance.

Egg counts in the buckwheat plots suggested an increasing attraction as the season progressed. The number of eggs was notably lower in buckwheat rows on June 29 than in rows at 6.4 and 12.8 m from the buckwheat plants, and significantly lower on July 14 than on onions at all distances. However, by July 30, which had the highest egg counts of the season, egg densities in buckwheat were greatest over onions at all distances. There were also greater numbers of eggs at a distance of 0.8 m than at 6.4 m. These results suggest that buckwheat in bloom can be a highly attractive trap crop to thrips adults, but that it is only a moderately suitable host for thrips reproduction and larval establishment.

Unlike buckwheat, there was not an effect of carrot on adult thrips populations. Adult OT populations in carrot rows showed a consistent pattern with the lowest counts as compared to onion rows on all the sample dates. Densities of immature thrips were negatively impacted by carrot. On July 14 and 30, there were fewer immature thrips on carrot than on onion at all distances along the transects. Egg numbers showed an early season increase, but then declined to lower counts on carrot than on onions at any distance.

Lacy phacelia showed the greatest influence on early season thrips, with increased adult OT, immature and egg counts within the trap crop rows. On June 6 and 29, adult OT populations were greater within the trap crop than on onion at all distances. However, by July 14, the attraction was no longer observed as phacelia rows had less adult OT than onion at all distances. Immature thrips counts followed a similar trend, where numbers on phacelia were higher early in the season and then declined. There were fewer immature thrips on phacelia than onions after July 14. The only significant differences in egg counts were observed on June 29, where there were more eggs on phacelia than onion. Lacy phacelia was a highly attractive trap crop to onion thrips when it was in bloom and full vegetative growth, and it was also a good reproductive host; however, the length of its attractiveness was limited to about 30 days during June. To extend the attraction period to thrips, in 2010 we seeded successive plantings of the trap crops.

While there were overall higher thrips numbers in 2010 than in 2009, trap crop growth was reduced and had less influence within the field. This may have been due to the cold, wet summer but also potentially to stunting caused by residual herbicides in the soil. Buckwheat plots were somewhat attractive at close range; however, the effect of both carrots and phacelia were limited. There were significantly greater numbers of adult OT in buckwheat than the control plots on both July 15 and August 4; however, similar to 2009, there were fewer immature thrips on buckwheat than onion, again supporting its low suitability as a reproductive host for thrips.

Carrot and phacelia were much less attractive to adult and immature thrips than buckwheat in 2010. There were fewer adult OT in carrots than the control plots on June 24 and August 18. The only attraction observed was on August 18 when adult OT were greater at a distance of 0.8 m from carrots than in control onions; however, carrot supported fewer immature thrips than onions on June 24, August 4 and August 18, again supporting 2009 results that carrot is not attractive to thrips adults and is a poor reproductive host. Thrips were impacted least by phacelia in 2010. Adult OT populations were only greater on phacelia than on onions on June 24. Immature thrips counts were not significantly different in any sample period.

There was no impact of trap crop on IYSV incidence. Overall, levels of virus ranged from 0-35% positive, which is a fairly low incidence rate.

Objective 4

Field days and workshops

Over the course of the project, we conducted three winter growers’ meetings and three summer field days. In addition, we held two evening observation and discussion sessions per season on individual farms.

The 2008 summer onion field day was conducted on August 12, at which we introduced the project and presented the idea of a whole farm approach to onion thrips and IYSV management. IYSV field identification and thrips sampling techniques were demonstrated. Approximately 55 onion growers and agricultural agency and extension personnel attended, including out of state guests. The event was very well received.

A winter meeting was held on February 17, 2009, which was attended by 42 growers, agency and extension personnel on a very snowy day. Presentations were given on IYSV, the onion thrips survey and nitrogen levels, and a whole farm approach to IYSV management. Post-meeting evaluations ranked the research being conducted on this grant as the most interesting and valuable information presented. A research meeting was held with grower cooperator Morgan Reeder in April 2009 to discuss the 2009 field season and trap crop trial.

One hundred copies of a pre-project survey were mailed out to growers and industry personnel in late January 2009; however, only 14 were returned despite a reminder note being sent out. Due to low participation in our pre-project survey, we repeated the same survey at the 2009 winter meeting where another 23 completed surveys were collected.

A second summer onion field day was conducted on August 11, 2009 at which we presented updates on the early findings generated by the project. IYSV field identification and thrips sampling techniques were also demonstrated. Approximately 60 onion growers and agency extension personnel attended, including out of state guests. The event was very well received.

In addition to the field day, two evening walks were held. The first evening walk on June 25 was conducted on Morgan Reeder’s farm and focused on the trap crop study. Twelve growers participated. The second evening walk was held on September 1 and was focused on thrips and IYSV identification, as well as a discussion of cultural management of onion thrips and Iris yellow spot virus. While these evening walks had relative low attendance they were very well received, and we feel they were particularly valuable due to the high degree of interaction that occurred between project leaders and growers and between individual growers. Two evening walks were also held in 2010.

A second winter meeting was held on February 16, 2010 which was attended by 45 growers and agency and extension personnel. Three presentations were given highlighting early results from this project including the on-farm survey, early findings on the effects of soil N and trap crops on onion thrips, and early results on the effects of high and reduced N on onion yields, quality and soil processes. Topics identified by the onion growers as being of primary interest were those studies that focused on the relationship between farm nitrogen use and onion thrips.

A third winter meeting was held on February 15, 2011 at which we presented the final results from the farm survey and replicated field trials. Growers were shocked by the levels of N recorded in their fields, and this generated intense discussion on how or why this was happening. They did not feel this accurately reflected their application rates. The consensus emerged that accumulated N from repeated applications over the years might be moving up from lower layers in the soil profile during the course of the season. Soil N levels were so high in some cases it is likely plant toxicity was occurring, possibly explaining the lack of correlation between N inputs, yield and onion size. This discussion continued with Dr. Drost over the course of the summer.

Objective 5

Economic Analysis: See below under section #7.

Objective 6

Dissemination of results: See below under section #8.

Research conclusions:

The post-meeting survey completed by attendees of the 2009 winter regional onion meeting listed the reports of the research coming out of this project as the most interesting and useful information presented. Onion growers are struggling in Utah. Our 2008 farm survey showed that not a single field surveyed was free of IYSV. Growers indicated that even though they were harvesting good yields, the onions were rotting in storage, a problem associated with IYSV infection. Several growers indicated on meeting evaluation forms they would now be paying closer attention to nitrogen applications and soil and weed management. One of the two new weedy hosts of the virus (Green Foxtail) is a grass, and represents the first time a grass has been confirmed as a host for IYSV. This is a very important discovery that has come directly out of this research.

Forty-five onion growers, industry leaders and service providers from around the western U.S. attended the 2010 Utah Onion School on February 16 in Brigham City, Utah. The grower/participant evaluation collected at the end of the sessions noted that most growers are paying more attention to crop and soil nitrogen levels since these have been shown to influence thrips populations. Our findings demonstrate that farms and replicated plots with lower N additions tend to have fewer problems with onion thrips, and this could reduce the development of Iris yellow spot virus. Growers noted that reducing N applications will save on thrips sprays, thus there is both cost savings and improvements in sustainability. Some growers reported that the idea of reducing N applications to control thrips goes against the common theory and practice that healthy vigorous onion growth (more N applied) and regular thrips sprays are the best ways to reduce IYSV. However, the USU research results and several farmers practicing reduced N applications strongly show this not to be completely true. At present, about 25% of the onion acreage in Utah (375 acres) is grown using more appropriate N applications with almost no reduction in productivity, improved storage quality and significantly fewer pesticide applications. Growers using the system report that they apply 0-1 pesticide applications per year for onion thrips. This has a significant bearing on onion productivity and greatly reduces the likelihood of nitrogen movement off-farm and the total pesticides used on the farm.

Additional impacts: see grower adoption.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

In 2008-09, posters introducing the project were presented at 1) the Western SARE regional meeting in Cheyenne, WY; 2) at the National Allium Meeting in Savannah, GA in 2008; in Austin, TX in December 2009 and Reno, NV in December 2010; and 3) at the Pacific Branch Entomological Society of America Conference in San Diego, CA. Two additional posters were presented in 2010, one to the American Society of Horticultural Sciences meeting in Palm Desert CA in August and a second to the American Society of Agronomy in Long Beach CA in November.

Dr. Alston gave four invited presentations external to Utah that disseminated results from this project: 1)‘Onion thrips: contributions of egg survival and adult dispersal to populations on plants’ at the Rocky Mountain Agribusiness Association 56th Annual Convention and Trade Show, January 8, 2008, Denver Colorado; 2) ‘Biology and control of onion thrips’, February 2, 2010, Ontario, OR; 3) ‘Susceptibility of onion thrips life stages to insecticides, crop nitrogen, and rotation’, Pacific Northwest Vegetable Association (PNVA), November 17, 2010, Kennewick, WA; and 4) ‘Thrips control’, PNVA, November 18, 2010, Kennewick, WA.

Dr. Dan Drost was the keynote speaker in the onion section of Empire State Fruit and Vegetable Expo with his talk titled ‘Growing Onions in a Reduced Nitrogen System: Effects on Productivity and Thrips’, January 26-28, 2011, Syracuse, NY.

Dr. Kent Evans was invited to give a presentation on this work at the ID Plant Pathology meeting titled ‘IYSV incidence and severity in Utah onions’, November 6-7, 2008, Jerome, ID and at the Treasure Valley Onion meetings titled ‘IYSV incidence and severity in Utah onions’, February 3, 2009, Ontario, Oregon.

Fact sheets

Evans, C and E. Frank. Iris Yellow Spot Virus in Onions. (PLP-010-PR) September 2008
Alston, D and D. Drost. Onion Thrips. (ENT-117-08PR) March 2008
Drost, D and R. Ward. Onion Budget 2011. December 2011. (appendix)

Two introductory fact sheets ‘Iris Yellow Spot Virus in Onions’ (PLP-010-PR) and ‘Onion Thrips’ (ENT-117-08PR) were published in March and September 2008, and two short papers have been published in Plant Disease titled ‘Natural Infection of Iris yellow spot virus in Twoscale Saltbush (Atriplex micrantha) Growing in Utah’ April 2009 Vol 93:460 and ‘Green Foxtail (Setaria viridis), A Naturally Infected Grass Host of Iris yellow spot virus in Utah’ June 2009, Vol 93:6, announcing the discovery of two new weedy hosts for IYSV. One of these hosts was a grass and represents the first time a grass has been confirmed as a host for IYSV.

A summary of our research findings as presented in the 2009 winter onion meeting was published in the trade magazine Onion World. Two abstracts on the project findings were submitted and accepted; one to the American Society of Horticultural Sciences meeting in Palm Desert CA in August and a second to the American Society of Agronomy in Long Beach CA in November.

Final results are in the process of being published through two additional extension factsheets, two research publications and on USU Extension WebPages. Images of thrips and IYSV have been posted to the Bugwood image gallery and a grower vegetable management advisory service has been established. A web based tool to allow growers to determine economic costs vs. benefits of different management decisions is being developed and is near completion.

Project Outcomes

Project outcomes:

Detailed economic analysis has been completed for an updated onion production system. This is posted on the Department of Applied Economics web page ( and will be published by the Utah Department of Agriculture and Food as part of their annual collection of enterprise budgets in 2012. A pre-publication draft has been included in the appendix. For a normal onion production (base nutrient and pesticide use) system given 2011 production values and prices gathered from USDA-AMS database, onion growers may expect net returns to land, labor and management of $1,925/acre. Included in the enterprise budget is a breakeven table that provides projected returns at selling prices that are 10 or 25% above or below the base. It also estimates net returns if the yield was 10 or 25% above or below the 2011 levels of productivity.

As part of this Western SARE-funded project, we assessed low- and high-input production systems (Table 1). In the low-input system, growers use a reduced amount of nitrogen and many have completely eliminated pesticide sprays for onion thrips. In the high-input system, growers apply additional nitrogen (? more) in an attempt to increase onion bulb yields, and most stated that they also increased the number of onion pesticide sprays needed to control onion thrips. Using this information we created budget information to reflect these changes. Buckland (2011) reported that there was no change in production (yield) when onions were grown in a low nitrogen input system. Using our 2011 productivity values, assuming no yield reduction, a 50% decrease in nitrogen application and no pesticide sprays, we calculated the percent change in operating costs and returns. If growers adopted a low-input system, they may expect a 16% increase in returns compared to the base onion production system. If growers used the high-input system to increase yields, they may expect a 10% decrease in returns compared to the base onion production system. For growers using the high-input system, nitrogen costs would increase by 32% and pesticide costs by 64%. In contrast, growers using the low-input system would reduce their nitrogen use by 50% and would incur no pesticide costs. Since onions are known to have low nitrogen use efficiency, the application of extra nitrogen could lead to greater nitrogen losses through leaching or runoff on farm, which can be damaging to the environment. In addition, if growers apply more pesticides, this increases the amount of pesticide used, increases the overall cost of the materials, increases the likelihood of onion thrips resistance to the chemicals applied and ultimately decreases the return on investment.

Farmer Adoption

A post-project survey is currently being administered to help gauge farmer adoption of reduced N management strategies for helping to manage onion thrips. Though all of the results from this are not available at this time, more information will be forthcoming as it is available.

Onion growers from Utah were the primary beneficiaries of this project. There are approximately 1,500 acres of onion grown in Utah by 20 growers (average 75 acres per grower). As of 2011, about 375 acres (25%) of onions are being grown using the low-input production system. Newell Norman states “we appreciate what USU has done for onion growers over the years. This research has helped us save a lot of money by reducing our nitrogen and pesticide costs.” Ed Brewer of Utah Onion Inc. notes “that onion thrips are getting harder to control, and it takes more sprays which are getting more expensive. We have to figure out how to reduce the number of sprays applied without damaging the crop.” Morgan Reeder, President of the Utah Onion Association notes “USU onion research helps verify what I was seeing on my farm. Significant reductions in nitrogen do result in fewer onion thrips sprays. This is good for my bottom line.” Most growers in Utah were exposed to the onion research (on-farm and at the research station) during this study. Each year we held a half-day Winter Onion Meetings, and each summer we had a field tour. On average from 2008-2011, 50-60 growers, industry representatives and government employees with farm related responsibilities attended one or both of these events each year. In addition, in 2008 and 2009, evening field visits were attended by 8-10 growers. During these formal and informal events, growers learned about nutrient management, onion thrips and their control, and Iris Yellow Spot Virus, and had the opportunity to interact with and share their experiences with others attending. In addition to our outreach efforts in Utah, the PIs running this study shared their findings with growers in Idaho, Washington, Utah, Nevada, New York and Colorado. As a result of our findings, researchers in Washington and New York have begun investigating the effects of reduced nitrogen applications on onion crop performance and onion thrips. Therefore, growers across the U.S. are being educated on the impacts that nitrogen can have on pesticide use and onion thrips and IYSV management.


Areas needing additional study

Further study is needed to explore the ecological and farm management drivers behind thrips population dynamics and IYSV outbreaks. This will require surveys of a larger number of farmers’ fields and their management practices, as well a careful survey of surrounding crops and weeds. A more precise understanding of the factors involved in thrips and IYSV outbreaks would enable the development of a decision making tool to assist growers with management practice decisions.

Preliminary findings from this project show that prior crop history (corn or wheat) before onions has a significant impact on onion thrips populations. More research on the role of crop rotation is needed. Grower-cooperator Morgan Reeder has employed an eight-year crop rotation which includes alfalfa (five years), corn (one year), wheat (one year), then onions. While funding for this type of rotation would be extremely difficult to secure, we are employing a shorter version with three years of alfalfa, before growing corn or wheat. We see a need to continue exploring rotational effects and also further evaluate the role nitrogen nutrition has on plant performance, thrips predation and Iris Yellow Spot Virus incidence.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.