INSECTICIDE TRIALS FOR ONION THRIPS (THRIPS TABACI) CONTROL Lynn Jensen Malheur County Extension Service Oregon State University Ontario, OR

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1 INSECTICIDE TRIALS FOR ONION THRIPS (THRIPS TABACI) CONTROL 2004 Lynn Jensen Malheur County Extension Service Oregon State University Ontario, OR Introduction During the past 4 years alternative insecticides have demonstrated superior control of onion thrips when compared to conventional insecticides. Alternative insecticides in this trial are azadirachtin (Aza-Direct and Ecozin ) an extract from the neem tree (Azadirachia indica, A. Juss.), and spinosad (Success ), a bacterial fermentation product. Conventional insecticides are the currently registered products in the synthetic pyrethroid (Warrior, Mustang ), organophosphate (parathion, malathion, Guthion, Diazinon), and carbamate (Lannate, Vydate ) classes. Different rates and combinations of these insecticides were tested for efficacy against onion thrips. Materials and Methods A 36.7-ft-wide by 500-ft long block was planted to onion (cv. Vaquero, Nunhems, Parma, ID) on March 23, The onions were planted as 2 double rows on a 44-inch bed. The double rows were spaced 2 inches apart. The seeding rate was 137,000 seeds/acre. Lorsban 15G was applied in a 6-inch band over each double row at planting at a rate of 3.7 oz/1,000 ft of row for onion maggot control. Water was applied by furrow irrigation. The plots were 7.3 ft wide (2 beds) by 25 ft long and were replicated 4 times. There were 14 treatments as outlined in Table 1. Acephate is an older insecticide that is now manufactured by several companies. It is not currently registered for use on onions. Insecticide applications were made with a CO 2 -pressurized plot sprayer with 4 nozzles spaced 19 inches apart. All treatments were made with water as a carrier at 38.9 gal/acre. Thrips counts were made weekly through the growing season by counting the total number of thrips on 20 plants. The onion bulbs were harvested by hand on September 10 and graded on October 11. The plot area harvested was 20 ft of the center 2 double rows. 71

2 Treatment differences were compared using ANOVA and least significant differences at the 5 percent probability level, LSD (0.05). Means were also compared using Duncan s multiple range test. Results and Discussion Thrips populations in June were fairly high (Fig.1). The acephate treatments provided the best thrips control. Table 2 contains yield and grade information. All of the yield classes had significant differences except for medium-size onions. Acephate treatments had the highest yield of supercolossal plus colossal bulbs at both the 8.0-oz and 16.0-oz rate, and the 6.0 oz rate had the highest yield of colossal bulbs. The Aza Direct plus Success (10.0 oz) had the overall highest yield followed by treatment 13, which was a combination of Penncap M plus and Warrior plus. Acephate at the 6.0-oz rate also produced high yields. Aza-Direct by itself produced the lowest yields, followed by the late June and mid-july applications of Warrior plus and Warrior plus Lannate (treatment 2). Compost tea by itself was not better than the untreated check. Aza-Direct plus the 6.0-oz rate of Success was not better than the untreated check, whether applied as a weekly spray mix or alternated weekly. The iris yellow spot virus infected the plot area late in the season. The treatments were evaluated for resistance to disease expression and the data are shown in Table 3. Generally, the treatments with the highest yields had less incidence of the disease although the correlation was not very strong. Conclusions Azadirect (20 oz) plus Success applied at the 10.0-oz rate (treatment 14) and acephate at the 8.0-oz rate (treatment 12) were the best treatments. Neither Success or acephate is currently registered for use on onions although a section 18 emergency registration for Success was granted in 2004 and is anticipated again in There were significant differences between treatments in all onion size classes except mediums. 72

3 Table 1. Insecticides evaluated for thrips control, Malheur Experiment Station, Oregon State University, Ontario, OR, Treatment no. Insecticide applied Formulated product Treatment date Rate/acre 6/7 6/28 7/16 7/29 1 Aza-Direct 20.0 oz X X X X 2.0 pt X 2 Lannate 3.0 pt X 3 Untreated check 4 Aza-Direct 20.0 oz X X Success 6.0 oz X X 5 Compost Tea 4.0 gal X X X X 6 Warrior X Lannate 3.0 pt X 7 Aza-Direct + Success 20.0 oz 6.0 oz X X X X 8 Acephate 16.0 oz X X X X 9 Success 6.0 oz X X 10 Success 6.0 oz X X X X 11 Warrior X 2.0 pt X Lannate 3.0 pt X 12 Acephate 8.0 oz X X X X 13 Penncap M pt 2.0 pt X X X 2.0 pt X 14 Aza-Direct + Success 20.0 oz 10.0 oz X X X X 73

4 Table 2. Effects of different thrips treatments on onion yield and grade, Malheur Experiment Station, Oregon State University, Ontario, OR, Treatment No. Medium Jumbo Colossal Supercolossal Colossal + S-Col. Jumbo + Col. + S-Col. Total Yield cwt/acre LSD (0.05) ns

5 Table 3. Evaluation of iris yellow spot virus disease severity with different insecticide treatments for the thrips vector, Malheur Experiment Station, Oregon State University, Ontario, OR, Treatment Iris yellow spot virus severity Number 0 = dead; 5 = no injury LSD (0.05)

6 Average thrips / plant a a a a-b b-c d-e c-e d-e d-e b-d b-e c-e e d-e Treatment no. Figure 1. Treatment effects on thrips populations during June, Malheur Experiment Station, Oregon State University, Ontario, OR,