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, these insecticides are frequently over-used, and this can lead to environmental contamination 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 are comparing the effects of standard and reduced rates of nitrogen and phosphorous as well as an adjusted application timing of nitrogen on thrips populations and bulb yield, as well as understanding the plant growth metrics associated with thrips colonization.
In 2018, we completed our field trial and examined the effect of 5 different rates of nitrogen (with different timings) and 4 rates of phosphorus on a muck soil type in northwestern New York. In a randomized complete block design, we explored the effects of these different rates of fertilizer on onion thrips densities (early in the season and total season average), plant growth, and onion bulb yield. Fertilizer treatments were applied early in the spring when onions were planted, mid-April 2018, and again in early June 2018 for the nitrogen treatments.
Thrips populations in 2018 contrasted greatly from the populations we recorded in 2017. In 2017, onion thrips populations remained below one thrips larvae per plant for most of the growing season, but in 2018 we observed mean populations as high as 818 thrips per plant. Similar to 2017, we found no significant effect of fertility amendments on total larval onion thrips density in the phosphorus and nitrogen trials in 2018.
In 2018, Onion plant growth and yield were very similar across all treatments regardless of rate of phosphorus or nitrogen fertilizer. While plants fertilized with nitrogen had greater weights and longer leaves in June, there was no season-long effect of nitrogen fertilizer. Yields were statistically similar across all nitrogen rates, and only insecticide program significant impacted yield. Similarly, onion yield was not increased with phosphorus fertilizer, but was increased with insecticide use. These findings are similar to those from 2017 and suggest that onion growers can reduce their rates of phosphorus and nitrogen in muck soils.
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) Develop a nitrogen fertility plan that reduces onion thrips densities in onion.
- a) 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.
- b) Evaluate reduced rates of nitrogen fertilizer and different application timings of nitrogen fertilizer on onion thrips reproduction and feeding levels in onion in a controlled no-choice experiment in the greenhouse.
2) Develop a phosphorus fertility plan that reduces onion thrips densities in onion.
- a) Evaluate reduced rates of phosphorus fertilizer on onion thrips densities, damage, and bulb yield in onion in a field study.
- b) Evaluate reduced rates of phosphorus fertilizer on onion thrips reproduction and feeding levels in onion in a controlled no-choice experiment in the greenhouse.
3) Disseminate research results to Northeastern onion growers.
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 Pantoea 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 a number of biological attributes that makes it highly likely for developing insecticide resistance: short-generation time, high reproductive rates, parthenogenesis and polyphagous feeding. Indeed, 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 these treatments on onion thrips reproduction and feeding in a controlled no-choice experiment in the greenhouse. 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.
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 randomized complete block design, five rates of nitrogen 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 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. At the first appearance of onion thrips adults in the field, larval onion thrips densities were recorded weekly until onions lodged (8 weeks of sampling). Twenty plants per plot were visually examined and the number of larval onion thrips recorded. Additionally, Larval emergence data was collected at thrips colonization to the field (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 other 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.
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 was collected in the same manner as discussed in the nitrogen ‘onion thrips densities’ in the nitrogen trial above.
Plant growth and yield. Data was collected in the same manner as discussed in the nitrogen ‘Plant growth and yield’ in the nitrogen trial above.
Soil sampling. Data was 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.
Experiments from all objectives have been initiated. Results from the fertility field trials (objectives 1a and 2a) are shown below. Objectives 1b and 2b are still underway and will be presented in the final report.
Onion thrips densities. Onion thrips populations were very high in 2018, and seasonal means were well above the economic injury level of 2.2 thrips per leaf (Figure 1 and Figure 3). However, we found no significant differences in onion thrips densities between the nitrogen treatments (Figure 1). Early in the season, we looked at the emergence of thrips from a subsample of onion plants from 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 (F1,94=12.39, P= 0.0004297) (Figure 2). However, nitrogen rate did not significantly impact seasonal mean onion thrips densities (P>0.05) (Figure 1) and insecticide program was the only significant effect on thrips densities (Figure 3).
Plant growth and yield. Plants were statistically similar between all nitrogen treatments in July and August, and only significantly differed in June (Table 1). Onion plants that received nitrogen in June weighed 33% more compared to the unfertilized control. 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). On average, yields were 63% less than those recorded in 2017 which is likely due to the dry growing conditions and high thrips pressure. Unlike 2017, onion yields were statistically similar between all nitrogen treatments and averaged 9.4 kg/plot in 2018 (P>0.05) (Figure 4). 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. 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.
Onion thrips densities. Similar to the nitrogen trial, onion thrips populations in the phosphorus trial were very high in 2018. Onion thrips densities were well above the economic injury level of 2.2 thrips per leaf and weekly thrips counts exceeded densities of 500 thrips per plant (Figure 5 and 7). However, seasonal thrips densities were not significantly impacted by phosphorus treatments (P>0.05) but by insecticide program (F1,3367=26.35, P<0.00001) (Figure 7). Interestingly, 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. Plant growth was statistically similar at all points during the growing season (P>0.05). 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). Yields in 2018 were approximately 66% lower compared to 2017. Insecticide significantly impacted yield, and treated plots had 50% greater yields as compared to untreated controls (Figure 7). 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 7a).
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).
- 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.
Education & Outreach Activities and Participation Summary
Expo and society presentations:
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
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
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)
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 needs additional data before project outcomes can be recorded.