Two insecticide trials conducted near Fruita in 2000 demonstrated that onion thrips are at least partially resistant to two formulations of Warrior insecticide. Lannate LV was partially effective in controlling onion thrips, but control was only 53% and 60% at five and ten days after treatment. The combination of Lannate and Warrior was no more effective than Lannate alone. The lack of effective insecticides for onion thrips control is cause for concern for onion producers in the future.

Onion thrips, Thrips tabaci, are a major concern to onion producers. They feed in the neck area of onions. Their piercing and rasping type feeding behavior causes scarring of leaves and leads to reduced bulb size and corresponding lower yield. Onion thrips have a history of resistance to insecticides after prolonged use of a single mode of action. Several formulations of pyrethroid insecticides have been used extensively on onion thrips in western Colorado since the early 1990's. These insecticides have been the only effective alternatives for onion thrips control in the past few years. Warrior (lambda-cyhalothrin) was the most commonly used insecticide on western Colorado onion thrips in the past few years. The manufacturer changed the formulation in 1999 from Warrior EC to Warrior T, and some growers felt there was a decrease in performance of the reformulated product. One objective of this research was to gather data on the performance of the old and new Warrior formulations. The other objective was to evaluate commonly used insecticides for onion thrips control during the 2000 growing season. Data was also collected for one insecticide for future emergency/special local needs registration.

Three trials were conducted in the fields of grower/cooperators during the 2000 growing season. All treatments were applied with a hand held CO₂ pressured sprayer calibrated to apply 12 gal/A of finished spray material. Plot width was 7.5 ft (three 30" beds) in all experiments. A spray adjuvant (Activator 90) was added to all treatments except the Furadan 4F at a rate of 2 pt/100 gal. All plots were sampled by choosing five random onions from the center bed of the plot and counting the number of thrips present in the neck area of the plant. All statistical analysis was conducted with MSTAT-C, a statistical program developed by Michigan State University.

• Trial # 1. This experiment was a simple comparison of Warrior EC and Warrior T insecticides and an untreated control. Treatments were applied to onions on 13 June 2000. Plots were arranged as a randomized complete block with four replications and three treatments. Thrips abundance was evaluated 7 and 14 days after treatment.
• Trial # 2. Treatments included Warrior EC, Warrior T, Lannate LV, and the combination of Warrior EC and Lannate LV, all at label rates. The experiment was arranged as a randomized complete block with four replications and five treatments. Treatments were applied on 30 June 2000. Thrips abundance was evaluated 5 and 10 days after treatment.
• Trial # 3. This simple trial was an evaluation of Furadan 4F plus crop oil for onion thrips control. Treatments were applied on 21 July 2000, and evaluated 3,5,7 and 10 days after treatment. There were four replications of the treatments. The data was analyzed using a paired t-test.

Table 1. Onion thrips counts from three insecticide trials during 2000. Cells marked with an asterisks (*) were not included in that particular trial.

Means within a column followed by the same letter are not significantly different.
¹ DAT= Days After Treatment
² LSD = 12.2; P=0.10
³ LSD=19.1; P=0.05

Trial # 1. There were no differences between the two Warrior formulations and the untreated control on either sample date. The treatments were ineffective in controlling onion thrips.

Trial # 2. There were no significant differences in thrips numbers between Lannate LV and Warrior EC treatments or the combination of the two at 5 DAT. There were no statistical differences in thrips counts between the two Warrior formulations on that sample date, but there were fewer thrips in the Warrior EC treatment than the untreated control, while there were no differences in thrips numbers in the Warrior T and untreated control. Lannate LV was more effective in controlling onion thrips than either Warrior formulation 10 days after treatment. Control with Lannate LV was calculated to be 53% and 60% at 5 and 10 days respectively. Addition of Warrior EC to Lannate LV did not improve performance. There were fewer thrips in the Warrior EC than the Warrior T treatments on both sample dates , but the differences were not statistically significant. While the Warrior EC treatment showed limited activity against onion thrips at 5 DAT, neither formulation gave control that could be considered acceptable in a commercial onion production setting.

Trial # 3. Furadan 4F plus crop oil was effective in reducing onion thrips abundance on all sample dates. These trials have shown that onion thrips have gained resistance to Warrior insecticide, and that Lannate is at best only partially effective in controlling onion thrips. Alternatives to pyrethroid insecticides are needed for successful control of onion thrips in the future.