Dry bulb onions are a vital crop in the Treasure Valley of Eastern Oregon and southwestern Idaho. About 10,000 hectares are planted each year, contributing more than $120 million to the region’s economy. But onion farming is expensive — recent production costs have ranged from $12,000 to $14,500 per hectare — and growers face pressure to manage costs while protecting crop quality.
Two of the most significant input costs are insecticides and nitrogen fertilizer. One key pest is the onion thrips — a tiny, slender insect that feeds on onion leaves and can spread iris yellow spot virus, which reduces bulb size and affects storage quality.
To protect their crops, growers apply insecticides several times each season. Meanwhile, nitrogen fertilizer must be carefully managed. Too little reduces yield, while too much raises costs and increases the risk of bulb rot. Fertilizer prices can also swing dramatically from year to year.
To help growers make more informed decisions, researchers from Oregon State University partnered with Greenway Research and Consulting in Parma, Idaho, to conduct field trials from 2019 through 2021 at the Malheur Experiment Station in Ontario.
The study, published in the Journal of Economic Entomology, shows that onion growers can increase profits by using soil and tissue tests to guide nitrogen fertilizer use, rather than applying set amounts. It also found that integrated pest management (IPM) strategies — applying insecticides only when pest levels require it — can reduce chemical use without sacrificing yield or profit.
The team tested three insecticide strategies — no insecticide, weekly calendar-based sprays and IPM, which applies insecticides only when thrips numbers exceed a damaging threshold. They also compared two nitrogen rates — a standard level based on soil and tissue tests and a reduced rate at 50% of that amount.
The most precise result was with nitrogen: across all three years, the standard program consistently produced higher profits. Even when fertilizer prices were high, every dollar spent returned between $2.62 and $4.01 in crop value. The reduced nitrogen program did not improve pest control and consistently resulted in lower profits.
Insecticide results varied by year. In 2019, the most profitable outcome came from applying no insecticide at all — likely due to a cooler, wetter spring and minimal thrips pressure. In 2020, the IPM program performed best. It matched the yield of the calendar-based approach, required fewer applications and earned more profit. In 2021, the calendar-based program produced the highest yield and profit, partly because it led to more jumbo onions — a size that earned premium prices that year.
While not always the top performer, the IPM approach offered solid results in two out of three years. It reduced the number of sprays and provided a sustainable approach to managing pests and delaying insecticide resistance.
This study was supported in part by the Oregon Agricultural Experiment Station in OSU’s College of Agricultural Sciences, and by Specialty Crop Research Initiative grant no. 2018-51181-28435 from the U.S. Department of Agriculture National Institute of Food and Agriculture.
For Oregon’s onion growers, this research this research provides practical, science-based tools for enhancing production efficiency and profitability. By helping growers understand when and how to invest in fertilizer and pest control, the study supports long-term sustainability for one of the state’s most valuable vegetable crops.