Advancing onion thrips management and production in onion in the Northeast by improving soil fertility programs

Final report for GNE16-126

Project Type: Graduate Student
Funds awarded in 2016: $14,877.00
Projected End Date: 03/31/2019
Grant Recipient: Cornell University
Region: Northeast
State: New York
Graduate Student:
Faculty Advisor:
Dr. Brian Nault
Cornell University
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Project Information

Summary:

Onion thrips (Thrips tabaci Lindeman) is a significant pest of onion in the Northeast. Growers often rely on weekly insecticide applications to control onion thrips. However, insecticides are frequently over-used which can lead to negative off-target effects and insecticide resistance. Therefore, there is a need to supply onion growers with other options that will reduce insecticide applications, while still providing effective onion thrips control. Nitrogen and phosphorus have both been demonstrated to impact thrips densities. In a two-year field trial, we compared the effects of standard and reduced rates of nitrogen and phosphorous on thrips populations and bulb yield. Additionally, in field and greenhouse studies we examined the association of plant growth metrics on early season thrips populations.

In 2017 and 2018, we examined the effect of 5 different rates of nitrogen (with different applications timings) and 4 rates of phosphorus on a muck soil type in northwestern New York. In a split plot design, we evaluated the effect of these different rates of fertilizer and insecticide use on onion thrips densities (early in the season and total season average), plant growth, and onion bulb yield. Results from these trials were communicated to growers through field days, grower meetings, and extension resources in 2017 and 2018.

Early in the season, significantly more thrips were observed on plants fertilized with nitrogen or phosphorus. However, the effect of fertility was only significant during the pre-bulbing stage and we found no significant effect of fertility amendments on total larval onion thrips density in the phosphorus and nitrogen trials in either year. Insecticide use was the only significant, consistent effect on onion thrips densities. Field surveys and greenhouse studies revealed that early season populations may significantly depend on plant size. Specifically, we observed significantly more thrips on plants with greater numbers of leaves and greater levels of leaf chlorophyll (SPAD meter ratings).

Overall, nitrogen and phosphorus fertilizer had a limited impact on plant growth. In 2017, plants fertilized with nitrogen were statistically similar, and produced onion plants with longer leaves and greater weight compared to plants that did not receive any nitrogen fertilizer. In 2018, nitrogen fertilizer did not consistently impact any plant growth metric measured. In 2017 and 2018, phosphorus fertilizer did not consistently impact any plant growth metric in either year. In both years, yields did not increase in plots treated with increasing rates of nitrogen or phosphorus, as lowest rates tested, 60 lbs. N/A and 50 lbs. P/A, had statistically similar yields to the highest fertilizer rates. Notably, soil nitrate and phosphorus values were highest in the plots fertilized with the greatest rates of nitrogen or phosphorus, however the majority of this fertilizer remained in the soil. Our research indicates that currently used fertilizer rates in muck onion production may be too high and merit further evaluation.

Project Objectives:

The long-term goal of this project is to decrease insecticide and fertilizer inputs without compromising bulb yield, and supply growers with complementary agricultural practices that will decrease onion thrips infestations in onion. To achieve this long-term goal, I propose the following objectives:

1) Evaluate reduced rates of nitrogen fertilizer and different application timings of nitrogen fertilizer on onion thrips densities, damage, and bulb yield in onion in a field study.

2) Evaluate reduced rates of phosphorus fertilizer on onion thrips densities, damage, and bulb yield in onion in a field study.

3) Evaluate effects of plant growth on onion thrips ovipositional preferences in a controlled choice and no-choice experiment in the greenhouse.

4) Disseminate research results to Northeastern onion growers.

Introduction:

In the Northeast, onion thrips (Thrips tabaci Lindeman) is the most important pest of onion to control throughout the growing season. Feeding on leaves by onion thrips indirectly reduces onion bulb yield.  If uncontrolled, onion thrips damage can reduce yield by 30-50% in New York. Onion thrips feeding can also result in the transmission of a variety of plant pathogens that cause diseases including purple blotch (Alternaria porri), bacterial center rot (Pantoea ananatis and P. agglomerans), and Iris yellow spot (Iris yellow spot virus). These diseases can lead to complete yield losses (Gill et al 2015).

Onion growers rely on insecticides to manage onion thrips. However, onion thrips has several biological attributes that makes it highly likely for developing insecticide resistance: short-generation time, high reproductive rates, parthenogenesis and polyphagous feeding. Onion thrips have developed resistance to several insecticide classes. Excessive insecticide applications can also result in environmental contamination, and other undesirable non-target effects. Therefore, growers would benefit in the long term by integrating multiple techniques to ease over-reliance on insecticides as the only tool for onion thrips management.

Soil fertility has shown to impact pest populations in many agronomic crops. Specifically, nitrogen and phosphorus amendments have been shown to impact thrips population size. Studies conducted on mineral soil show onion thrips densities in onion increase with increasing rates of nitrogen. Malik et al. (2009) found that onions treated with high rates of nitrogen had 70% more onion thrips than those fields treated with a reduced rate. While these studies illustrate the impact of nitrogen rate on onion thrips, they did not consider the impact of nitrogen application timing, which may have different impacts on plant growth and thrips feeding. Phosphorus fertilizer amendments also warrant consideration in onion thrips management. Chen et al. (2004) reported a 40% increase in the number of western flower thrips (Frankliniella occidentalis) on Impatiens flowers (Impatiens wallerana) when fertigated with a 1.28 mM ate/pot of phosphorus compared with those fertilized with the 0.32 mM rate/pot.  No studies have examined the impact of phosphorous applications to onion on onion thrips populations. Harmonizing lower rates of nitrogen and phosphorus fertilizer could provide growers with a valuable cultural management tactic for onion thrips.

Even a slight reduction in thrips density could have a profound impact on the overall population in onion fields.  For example, a conservative estimate of 10% reduction in onion thrips fecundity (number of viable eggs laid) could reduce the overall population by 33% (i.e., 10% fewer eggs laid for each of 4 generations produced in an onion crop). Consequently, onions will require fewer insecticide applications, thus slowing the onset of insecticide resistance and prolonging the efficacy of current insecticides. Additionally, reduced rates of nitrogen and phosphorus will lower annual fertilizer costs and decrease potential surface water and groundwater pollution. This approach will likely be rapidly adopted on commercial onion farms, as it does not require growers to use new techniques. The cumulative effect of this method would increase agricultural sustainability through reduction of environmental pollution, slowing insecticide resistance onset, and increased grower savings.

The purpose of this research is to identify a fertility program that will reduce onion thrips densities in onion by reducing nitrogen and phosphorus fertilizer and optimizing nitrogen application timing without compromising bulb yield. Specifically, I propose to 1) evaluate the response of onion thrips densities and onion bulb yield to varying rates of nitrogen and phosphorous as well as an adjusted application timing of nitrogen in a field experiment and 2) examine the effect of plant growth on onion thrips oviposition through field surveys and controlled choice and no-choice experiments. This approach will identify a fertility program that supports the lowest thrips density, but that does not compromise bulb yield. Finally, I will 3) present all pertinent findings to onion growers at regional grower meetings throughout the Northeast. Optimizing nutrient management to reduce onion thrips in onion will provide growers with an easily integrated practice that will lower insecticide and fertilizer costs, while slowing the onset of insecticide resistance.

Cooperators

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Research

Materials and methods:

Objective 1: Evaluate reduced rates of nitrogen fertilizer and different application timings of nitrogen fertilizer on onion thrips densities, damage, and bulb yield in onion in a field study.

Site selection and management. Trial was conducted on a commercial onion farm in Elba, NY on ‘muck’ soil. Onions field sites were selected based on low initial values of soil nitrate. In a split plot design, ten treatments combining five rates of nitrogen and two insecticide treatments were replicated five times. These five nitrogen rates were applied throughout the growing season; 1) unfertilized control (0lbs N/acre), 2) 60 lbs. N/A at planting, 3) 60 lbs. N/A at planting and 15 lbs. N/A when onions had 4-5 leaves, 4) 60 lbs. N/A at planting and 45 lbs. N/A when onions had 4-5 leaves, and 5) 60 lbs. N/A at planting and 75 lbs. N/A when onions had 4-5 leaves. Nitrogen was applied in the form of urea (46-0-0). All plots receiving nitrogen fertilizer were supplemented with 60 lbs. N/A at planting (April), and then an additional amount of nitrogen when onion had 4-5 leaves (June). Experimental plots were also supplemented at planting with the appropriate rates of potassium (potassium chloride; 0-0-60; N-P-K) and phosphorus (triple superphosphate; 0-45-0; N-P-K) per current soil tests and corresponding fertility guidelines. Cv. ‘Bradley’ was planted with a vacuum seed planter (646,000 onion seeds per hectare) on 15 Apr 2017 and 21 Apr 2018. To ensure crop establishment, seeds were treated with FarMore FI500 (Mefenoxam (0.15 g ai/kg), Fludioxonil (0.025 g ai/kg), Azoxystrobin (0.025 g ai/kg), Spinosad (0.20 mg ai/seed), Thiamethoxam (0.2 mg ai/seed)) and Pro-Gro (Carboxin (7.50 g ai/kg) and Thiram (12.50 g ai/kg)). This seed treatment package does not impact onion thrips.

Insecticide applications were made in accordance with current recommendations and guidelines (Leach et al. 2018). Every plot was bisected, such that one half of the plot received insecticide and the other half remained an untreated control. In 2017, one insecticide application was made to control onion thrips and five insecticide applications were made in 2018 to manage onion thrips populations. Decisions to apply insecticides were made on a weekly basis. The sequence of insecticides and rates used during each experiment were as follows: spirotetramat at 0.08 kg (AI) ha-1 (Movento; Bayer CropScience, Research Triangle Park, NC), cyantraniliprole at 0.1 kg (AI) ha-1 (Exirel; DuPont, Wilmington, DE), and spinetoram at 0.07 kg (AI) ha-1 (Radiant SC; Dow AgroSciences, Inc., Indianapolis, IN). Each insecticide was applied no more than twice consecutively if the thrips density exceeded the action threshold; if the action threshold was not exceeded for a week, no insecticide was applied. Insecticides were applied with a CO2-pressurized backpack sprayer with four, twin flat-fan nozzles (TJ-60-8003VS; TeeJet Technologies Harrisburg, PA). All insecticides were co-applied with an adjuvant at 0.5% v:v (Induce; Helena, Collierville, TN) to increase efficacy (Nault et al., 2013).

Onion thrips densities. At the first appearance of onion thrips adults in the field, larval onion thrips densities were recorded weekly until onions lodged which amounted to 10 weeks in 2017 (19 June 2017 to 28 Aug 2017) and 8 weeks in 2018 (19 June 2018 to 15 Aug 2018). Twenty plants per plot were visually examined and the number of larval onion thrips recorded. Onion thrips larvae were recorded weekly from 19 June 2017 to 28 Aug 2017 and 19 June 2018 to 15 Aug 2018.

Additionally, larval emergence data was collected at thrips colonization to the field (19 Jun 2017; 10 June 2018), to discern any early season preferences of adults to lay eggs in any of the nitrogen treatments. Three onion plants per plot were removed from field plots, and transported back to the lab in Geneva, NY. In the lab, all onions were washed with ethanol to remove all insects. Once cleaned, onions were placed singly into plastic containers with thrips -proof netting. After 10 days, all onions were inspected for the number of emerged larvae.

Plant growth and yield. Onion plants were assessed at three crop phenological time points; pre-bulbing (June), bulbing (July), and post-bulbing (August). Three plants from each plot were removed at each time point (June, July, and August). The number of leaves, length of longest leaf, and wet weight of each plant was measured. The second application of nitrogen was applied in late June; which precluded the inclusion of nitrogen treatments with split application of nitrogen in the June dataset.

Onions were harvested and graded at the end of the growing season. All onions from the inner rows from each plot were harvested and cured for a week in a screenhouse.  Bulbs were classified according to bulb diameter and assigned a size class of either ‘boiler’ (2.5 cm-4.8 cm), ‘standard’ (4.9 cm-7.6 cm), or ‘jumbo’ (≥7.7 cm). Bulbs that were either ‘standard’ or ‘jumbo’ were considered marketable, and ‘boiler’ bulbs unmarketable.  Marketable yields for treatments were then extrapolated to estimate mean kilograms per plot based on onion stand counts.

Soil sampling. Soil samples were submitted for soil analysis at three crop phenological time points; pre-bulbing (June), bulbing (July), and post-bulbing (August). Five soil samples were taken from every plot, at a depth of 6 and 24 inches, using a soil corer. Samples from each plot were homogenized and then submitted for soil analysis 24 hours after the soil was removed from the field.  Soil samples were submitted to an agricultural soil analysis lab, where they were evaluated for levels of soil nitrate present in the soil. The second application of nitrogen was applied in late June; which precluded the inclusion of nitrogen treatments with split application of nitrogen in the June dataset.

Statistical analysis. Data within each year were analyzed independently since environmental conditions and thrips pressure were different between years in both fertility trials. Data were analyzed using a generalized linear mixed model (R version 3.5.2) ((R package; ‘lme4’ (Bates et al. 2015)). Generalized linear mixed models were fit with fixed effects of fertilization rate and insecticide use, and all interactions, and included random effects of plot nested within row as well as subplot nested within plot nested within row.

The mean number of larvae per leaf (total for the season) were analyzed assuming a negative binomial distribution. Plant weight, plant leaf length, number of leaves per plant, soil nitrate, marketable yield, and adjusted marketable yield were analyzed assuming a normal distribution. Treatments in each analysis were compared using least squared means (P<0.05) (‘emmeans’, Lenth et al. 2018).

Objective 2: Evaluate reduced rates of phosphorus fertilizer on onion thrips densities, damage, and bulb yield in onion in a field study.

Site selection and management. Trial was conducted on a commercial onion farm in Elba, NY on ‘muck’ soil. Onions field sites were selected based on initial low levels of soil phosphorus. In a randomized complete block design, four rates of phosphorus were replicated five times. All phosphorus rates were applied at planting; 1) unfertilized control (0lbs P/acre), 2) 50 lbs. P/A at planting, 3) 100 lbs. P/A, and 4) 150 lbs. P/A. Phosphorus was applied in the form of Triple superphosphate (0-45-0). Experimental plots were also supplemented at planting with the appropriate rates of potassium (potassium chloride; 0-0-60; N-P-K) and nitrogen (Urea; 46-0-0; N-P-K) per current soil tests and corresponding fertility guidelines. Cv. ‘Bradley’ was planted into the field. Onion thrips were managed throughout the growing season, and insecticide applications were applied as needed or when the action threshold of 1 thrips/leaf was exceeded. Every plot was bisected, such that one half of the plot received insecticide and the other half remained an untreated control. In 2018, five insecticide applications were made to manage onion thrips populations.

Onion thrips densities. Data were collected in the same manner as discussed in the nitrogen ‘onion thrips densities’ in the nitrogen trial above.

Plant growth and yield. Data were collected in the same manner as discussed in the nitrogen ‘Plant growth and yield’ in the nitrogen trial above.

Soil sampling. Data were collected in the same manner as discussed in the nitrogen ‘Soil sampling’ in the nitrogen trial above, however soil analysis was conducted to measure levels of soil phosphorus.

Statistical analysis. Data were analyzed in the same manner as discussed in the nitrogen ‘Statistical analysis’ in the nitrogen trial above.

Objective 3: Evaluate effects of plant growth on onion thrips ovipositional preferences in a controlled choice and no-choice experiment in the greenhouse.

Field survey. Onion plants grown under two fertility regimes were observed in a commercial onion field in attempt to correlate specific plant growth metrics with thrips colonization. Fertilized plots were supplemented with nitrogen and phosphorus (135 lbs. N/A, 150 lbs. P/A, 50 lbs. K/A), whereas unfertilized plots did not receive any fertilizer (0 lbs. NPK/A). Onion cultivars were ‘Avalon’ and ‘Bradley’. Plants did not receive any pesticides. In total, 100 plants were observed, 50 plants from unfertilized plots, and 50 plants from fertilized plots. On each plant, the number and placement of onion thrips larvae and adults were recorded on each leaf (as described by Mo et al 2008 and Chittirii et al 2015). Plant growth metrics were recorded from each plant including the number of leaves, length of each leaf, chlorophyll meter (SPAD) rating of each leaf, and onion neck width. To reduce the chance of disrupting the thrips, adults and larvae were counted before collecting the plant growth metric data.

Greenhouse assays. Further research was conducted in the greenhouse to determine the importance of onion chlorophyll and leaf number on thrips oviposition. Onion plants (cvs. ‘Avalon’ and ‘Bradley) were seeded and grown in a greenhouse free from onion thrips. All onions were seeded into 72-cell round propagation trays (TO Plastics INC. item #59-5010) with superfine germination mix (Pro-mix item #20-200400) and then transplanted at the 2-leaf stage into pots (7.6 cm diameter × 31 cm tall) containing Cornell potting mix (peat, perlite and vermiculite in a 4:1:1 ratio). Onion plants were grown using two fertility regimes, unfertilized (no NPK) and fertilized (135 lbs. N/A, 150 lbs. P/A, 50 lbs. K/A). As a result, plants grown under the unfertilized regime had fewer leaves compared to the fertilized plants. We preferentially selected 30 plants with 3 leaves (selected from the unfertilized treatment) and 30 plants with 5 leaves (from the fertilized treatments). We also manipulated the chlorophyll levels of fertilized plants. We selected 60 plants and half were placed in a dark room for 72 hours to reduce chlorophyll levels and the remaining half were kept in the greenhouse with a constant light source for 72 hours. Differences in chlorophyll levels were confirmed using a chlorophyll meter (SPAD 502 Plus Chlorophyll Meter, Spectrum technologies). Plants were then exposed to laboratory-reared thrips in no-choice and choice assays.

No-choice assay. Thrips were confined to thrips dorms (1462W’ bug dorm, Bio Quip, Rancho Dominguez, CA). Five similarly aged adult thrips were placed on an onion plant for 48 hours. After 7 days, onion plants were cut at the neck and placed into a 80% ethanol solution. Plants were agitated in the ethanol solution to dislodge all larvae, and then dislodged thrips were counted. This was replicated on 30 plants per treatment (4 treatments= 120 plants).

Choice assay. Ten similarly agreed thrips were released onto white filter paper in the center of a bug dorm (1462W’ bug dorm, Bio Quip, Rancho Dominguez, CA). Bug dorms contained two plants of the contrasting treatments (i.e. high chlorophyll, low chlorophyll or 3 leaves, 5 leaves). The position of the plants was randomized between bug dorms. After 7 days, onion plants were cut at the neck and then placed into 80% ethanol solution. Plants were agitated in the ethanol solution to dislodge all larvae, and then dislodged thrips were counted. This was replicated 15 times (4 treatments= 60 plants).

Statistical analysis. Data for field survey and choice and no-choice assays were analyzed using a generalized linear mixed model (R package; ‘lme4’) (Bates et al 2015). Models were fit with fixed effects on onion growth metrics (number of leaves, chlorophyll levels, neck width, and leaf length). Plant within trial and treatment was treated as a random effect. When possible, treatments were compared using least squared means (P<0.05) (‘emmeans’, Lenth et al 2018).

Research results and discussion:

Objective 1: Evaluate reduced rates of nitrogen fertilizer and different application timings of nitrogen fertilizer on onion thrips densities, damage, and bulb yield in onion in a field study.

Onion thrips densities. Onion thrips populations were not significantly impacted by nitrogen in 2017 or 2018 (Figure 1). In 2017, populations were low, and remained below 1 thrips per leaf until early August. Populations were higher in 2018 and reached maximum densities of 80 thrips per leaf in August. Early in the season, we recorded the emergence of thrips from onion plants in each nitrogen treatment. There was approximately 2.7 times more onion thrips larvae emerging from plants receiving 60 lbs. N/A at planting compared to those onions that did not receive any fertilizer in both years (2017: F1,143=9.16, P= 0.002478, 2018: F1,94=12.39, P= 0.0004297) (Figure 2). However, this relationship did not persist throughout the growing season and nitrogen rate did not significantly impact seasonal mean onion thrips densities (P>0.05) (Figure 1). Onion thrips densities were also significantly impacted by insecticide use in 2017 and 2018 (2017: F1,44=32.2, P<0.0005, 2018: F1,44=299.4, P<0.0005), and fewer thrips were recorded in treatments with insecticide as compared to untreated controls (data not shown).

Plant growth and yield. Plants treated with nitrogen were statistically similar during all developmental stages in 2017 and 2018 (Table 1). In 2017, onion plants that received nitrogen weighed 2.5, 2.1, and 1.8 times more in June, July, and August, respectively as compared to the unfertilized control. Similarly, onion leaves in fertilized plots were 12-16 cm greater than the control. Number of leaves per plant was not statistically different between nitrogen treatments (Table 1). In 2018, plant growth metrics differed during the prebulbing stage, but not the bulbing or postbulbing stages. In the prebulbing state, fertilized onion plants weighed 33% more compared to the unfertilized control plants. Similarly, onion leaves in fertilized plots were 5-6 cm longer than the control. Number of leaves per plant were not statistically different between nitrogen treatments at any of the time points (Table 1).

In both years, yields did not significantly differ between plants treated with nitrogen. In 2017, onion yields were statistically similar between all nitrogen rates, but plots treated with nitrogen had 66% greater yields compared to the unfertilized control (Figure 3) (F4,38=10.91, P= 0.0257). Additionally, there was a significant interaction between insecticide use and nitrogen fertilizer (F4,35=9.87, P= 0.0489). In 2018, onion yields were statistically similar between all nitrogen treatments and averaged 9.4 kg/plot in 2018 (P>0.05) (Figure 4a). Insecticide significantly impacted yield, and plots treated with insecticide had 50% greater yields compared to the untreated control (F1,76=264.1, P<0.0001) (Figure 4b).

Soil sampling. Soil nitrate in the soil was positively associated with the amounts of urea applied (Table 1), and rates of soil nitrate were higher than those recorded in 2017 and 2018. Plots that received the highest rate of nitrogen, cumulative amount of 135 lbs. N/A, had the highest soil nitrate levels at every sampling period. Similarly, plots that did not receive nitrogen fertilizer had lowest levels of soil nitrate throughout the growing season.

Objective 2: Evaluate reduced rates of phosphorus fertilizer on onion thrips densities, damage, and bulb yield in onion in a field study.

Onion thrips densities. Onion thrips populations were not significantly, consistently impacted by phosphorus fertilizer in 2017 or 2018 (Figure 5). In 2017, Onion thrips in the phosphorus treatments differed little from week to week and remained below 1 thrips per leaf until August (data not shown). Larval emergence early in the season was not significantly impacted by any of the rates of phosphorus (Figure 6). Additionally, total seasonal number of thrips per plant were statistically similar, and differed by less than 4 thrips per plant (P>0.05) (Figure 5).  Similarly, in 2018, seasonal thrips densities were not significantly impacted by phosphorus treatments (P>0.05) but by insecticide program (F1,34=26.35, P<0.00001) (data not shown). Interestingly, in 2018, larval emergence early in the season was significantly impacted by phosphorus rates and significantly more onion thrips larvae were recorded in onion treated with 150 lbs. P/A as compared to 0 lbs. P/A (Figure 6).

Plant growth and yield. In 2017 and 2018, plant growth was statistically similar at all points (P>0.05) (Table 2). Phosphorus rate at planting did not significantly impact the number of leaves, length of the longest leaf, or the weight of onion plants in June, July, or August (Table 2).

Phosphorus fertilizer amendments had a limited impact on yields in 2017 and no effect in 2018 (Figure 7). In 2017, phosphorus fertilizer but not insecticide use had a significant impact on yield. Onions supplemented with 150 lbs. P/A had greater yields than those onions that did not receive any fertilizer (F4,38=10.91, P= 0.0257) (Figure 7). On average, yields in plots treated with phosphorus were 6-10% greater than the unfertilized control. In 2018, insecticide significantly impacted yield, and treated plots had 50% greater yields as compared to untreated controls (Figure 8). Onions supplemented with 150 lbs. P/A tended to greater yields than those onions that did not receive any fertilizer, however this was only marginally significant (Figure 7).

Soil sampling. Phosphorus levels in the soil were positively associated with the amounts of triple superphosphate applied, although not all rates were not statistically different from one another (Table 2).

Objective 3: Evaluate effects of plant growth metrics on onion thrips ovipositional preferences in a controlled choice and no-choice experiment in the greenhouse.

In the field survey, early season thrips populations were positively correlated with number of leaves and the amount of chlorophyll present in the leaves (Figure 9). However, these two plant growth metrics were correlated so further analysis is needed to determine which metric is important in the field. In no-choice and choice assays in the greenhouse, we recorded more thrips larvae on plants with greater numbers of leaves (5+ leaves) as compared to plants with only 3 leaves (Figure 10 and Figure 11). Chlorophyll content did not appear to significantly impact the number of larvae emerging from plants (P>0.05). Additionally, significantly more thrips were recorded on ‘Bradley’ as compared to ‘Avalon’ in no-choice assays (Figure 12). Further evaluation is needed to confirm these relationships.

Research conclusions:
  • Results from 2017 and 2018 field trials suggest that soil fertility is not an effective cultural control tactic for managing onion thrips densities in muck onion production. In 2018, larval onion thrips populations were not consistently impacted by nitrogen or phosphorus fertilizer amendments which is consistent with findings from 2017. Thrips pressure in 2018 was substantially higher than 2017, but even under this higher pressure we failed to record any significant differences in total thrips larvae per plant in either trial.
  • Furthermore, our field trial results suggest that onion growers can reduce rates of nitrogen and phosphorus fertilizer, as we have not found yields increase with higher rates of phosphorus or nitrogen. In 2017, reduction in plant growth and yield were only recorded in unfertilized control treatments. Further, rates of nitrogen as low as 60 lbs. N/A produced high marketable yields which were statistically similar to treatments fertilized with 135 lbs. N/A. Onion plant growth and yield were similar across fertility treatments regardless of nitrogen or phosphorus rate applied in 2018.  Yields in the phosphorus trial followed a similar pattern, however we did find that plots treated with 150 lbs. P/A tended to have higher yields than those left unfertilized, however this amounted to a relatively low, 10% increase in yield.
Participation Summary
1 Farmer participating in research

Education & Outreach Activities and Participation Summary

3 Consultations
1 Curricula, factsheets or educational tools
2 On-farm demonstrations
1 Published press articles, newsletters
6 Webinars / talks / presentations
2 Workshop field days

Participation Summary

200 Farmers
30 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

Grower and scientific presentations:

  • Leach, A., S. Reiners, M. Fuchs, C. Hoepting and B. A. Nault. 2019. Optimizing IPM: development and adoption of programs for onion thrips in onion. Oral presentation. P.J. Chapman Fellowship award presentation. Cornell Entomology department seminar series, Geneva, NY. April 30. (20+ people in attendance)
  • Leach, A., S. Reiners, M. Fuchs, C. Hoepting and B. A. Nault. 2018. IPM and the human element: developing programs to control onion thrips in onion. Oral presentation. Purdue entomology department seminar series, West Lafayette, IN. December 6. (20+ people in attendance)
  • Leach, A., S. Reiners and B. A. Nault. 2018. Impacts of nitrogen fertilizer in muck onion production. Poster presentation. Great Lakes Fruit and Vegetable Expo, Grand Rapids, MI. December 4-6. Great Lakes Expo.poster
  • Ebels, L., Leach, A., and B.A. Nault. 2018. Associations between onion growth characteristics and Thrips tabaci populations. Summer Scholar poster session. Geneva, NY July 28.
  • Leach, A., S. Reiners, F. Hay, M. Fuchs, R. Harding, and B. A. Nault. 2018. Evaluating interactions between onion thrips and associated plant pathogens for improved management in onion. Oral presentation. 67th Annual Muck Vegetable Growers Conference, Bradford, ON. March 28-29. (30 people in attendance)
  • Leach, A., S. Reiners, and B. A. Nault. 2018. Evaluating effects of nitrogen fertilizer and insecticide use in managing onion thrips (Thrips tabaci) in onion. Oral Presentation. Entomological Society of America- Eastern Branch, Annapolis, MD. March 17-19. (30 people in attendance)
  • Leach, A., S. Reiners, M. Fuchs and B. A. Nault. 2018. Unraveling the interactions among variety, fertility, yield, onion thrips and diseases, and implications for improved management practices. Oral Presentation. Empire State Producers Expo, Syracuse, NY. January 16-18. (50 people in attendance) Presentation notes for 2017 EXPO.1_11_17
  • Leach, A., S. Reiners, and B. A. Nault. 2016. Optimizing soil fertility for managing onion thrips in onion. Oral Presentation. Northeastern IPM center conference, Webinar. November 9. (20+ people in attendance)

Field days/ Twilight meetings:

  • Leach, A. and B.A. Nault. 2018 “Evaluating cultivar and nitrogen rates to reduce onion thrips densities and bacterial bulb rot” Oswego Onion Growers Twilight Meeting. (30 people in attendance) Oswego twilight meeting.0820181
  • Leach, A. and B.A. Nault. 2018 “Updates in nitrogen and cultivar trial to reduce onion thrips densities” Extended Elba muck donut hour. (15 people in attendance)

Extension publications:

  • Leach, A., S. Reiners, M. Fuchs and B. A. Nault. 2018. Unravelling the interactions among variety, fertility, yield, onion thrips and diseases, and implications for improved management practices. Empire State Producers Expo proceedings. Syracuse, NY

Project Outcomes

2 Grants applied for that built upon this project
2 Grants received that built upon this project
$10,000.00 Dollar amount of grants received that built upon this project
1 New working collaboration
Project outcomes:
  1. Growers may consider reducing nitrogen and phosphorus fertilizer in muck onion production. All commercial onion growers in the New York state apply fertilizer to their onion crop in order to ensure profitable yields, however growers should critically evaluate their soil fertility programs. Our research has shown that relatively low rates of nitrogen and phosphorus are needed to produce high marketable yields. However, current recommended rates of nitrogen and phosphorus range in muck onion production, but some sources recommend rates exceed 125 lbs. N/acre and 200 lbs. P/acre. Reducing rates of fertilizer could have significant impacts on the environmental sustainability, as less nitrogen will leach or volatilize from the system. In addition to reducing off-target effects of fertilizer, growers may also save money by reducing rates of fertilizer.
  2. We identified plant growth characteristics that may be important factors in early season thrips colonization including the number of leaves and chlorophyll levels (SPAD meter). While epicuticular wax has been identified as a component to thrips preference for certain onion cultivars (Damon et al 2014), plant size and health are also important considerations when choosing to plant a specific onion cultivar.
  3. Pest management practices are not “one size fits all”, and the success of pest management practices may be dependent on the production system in which they are implemented. A number of studies conducted on mineral soil types have demonstrated that fertility may significantly contribute to thrips infestations (e.g. Malik et al 2009; Buckland et al 2013), but we failed to confirm this relationship in muck soils. Therefore, its important that growers rely on regional research to determine the pest management practices that will work best on their farm.
Knowledge Gained:

My knowledge of sustainable agriculture and related integrated pest management (IPM) practices has increased as a result of this project. I have studied numerous components of onion production including but not limited to, onion thrips, onion yield, iris yellow spot virus, Stemphylium leaf blight, bacterial bulb rot, host plant resistance, insecticide products, and types of nitrogen and fertilizer. In short, my research has enabled me to take a systems level approach to pest management evaluation in which IPM is acknowledged as a program installed into an agricultural production system. I feel that my education gained as a result of completing this research has prepared me for a career in integrated pest management and sustainable agriculture.

Importantly, I have learned that agricultural sustainability does not take one form and should be viewed as a continuum of progress. I interacted with growers who challenged my conceptions of onion production, and what was considered “sustainable”. Through these interactions, I have learned to take an open-minded approach to agriculture. I have been able to reach a greater diversity of growers and increase my impact as a scientist. As I move forward, I want to continue to remain open-minded and strive to include all agricultural producers so that we can create sustainable, effective, and realistic solutions for problems in agricultural production.

As a result of this grant, I have presented a total of 9 presentations, 4 scientific presentations, 3 grower presentations, and 2 field days. I have submitted one manuscript for publication, with another in preparation. Additionally, we have one trade journal article submitted and others planned. My participation in this grant has also expanded my opportunities with other disciplines and projects. For example, I have collaborated with extension educators and plant pathologists to further understand the role of soil fertility in the development of certain plant diseases.

Assessment of Project Approach and Areas of Further Study:

Thrips densities were not consistently affected by nitrogen or phosphorus fertilizer amendments; however, plant growth was also unresponsive to fertilizer amendments in most trials (3/4 trials). Previous literature has shown that thrips densities increase with increasing rates of fertilizer, and researchers have posited that thrips may be responding to increased onion plant vigor (Malik et al. 2009; Buckland et al. 2013). The effect of plant growth metrics on onion thrips populations is currently understudied but may have significant implications on onion thrips management. In our studies we found significant effects of leaf number and chlorophyll level in the early colonization of onion thrips. However, further research should identify those plant characteristics associated with thrips colonization including: Does increased size of a specific plant part (e.g. length of leaves, width of onion pseduostem, number of leaves) predict thrips colonization and ovipositional preference? Does ovipositional preference predict larval survival on these plant parts? Does soil fertility influence epicuticular wax development in onion, and is it possible that increased fertility increases thrips attraction to certain onion cultivars?

Soil fertility is likely to impact other major insect pests and pathogens of onions. Therefore, further research should also examine the impact of soil fertility on onion pests like leaf miners, onion maggot, and the new invasive allium leaf miner (Phytomyza gymnostoma). Further research into the trophic implications of increased soil fertility may also be interesting, as onion fields host a suite of natural enemies (Fok et al 2014).

Overall, we found that growers should consider reducing rates of nitrogen and phosphorus fertilizer on muck soil types. Recent survey results in New York reveal that the majority of growers are applying between 100-150 lbs N/A, therefore further intervention and innovation is needed to decrease fertilizer use in muck onion production.

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.