Western flower thrips, Frankliniella occidentalis, are important insect pests of strawberries in California. They use their mouthparts to pierce plant cells and suck out their contents, resulting in decreased yield and quality. Control of this pest has been heavily dependent upon chemical insecticides.

Spinosad has been the most effective insecticide class in management of the thrips on strawberries. In 2009, we surveyed their susceptibility from 6 distinct sites to Radiant insecticide in early season of strawberry production. Based on the results of these surveys, a susceptible site (the Holly Ranch, Watsonville) and a tolerant site (the Beach Road Ranch, Watsonville) were selected for further studies. Here we report the impact of a management strategy on susceptibility of the thrips from the susceptible site.

Materials and methods

Strawberries at this susceptible site were treated with Success rotated with Dibrom. About half an acre of first year strawberries at the Holly Ranch was treated. A large area of wild hosts including trees and weeds were nearby the treatment field. Success was applied at the top label rate to the strawberries on May 29, June 8, Aug. 8 and 15, respectively, while Dibrom at its top label rate was applied on June 16 and 29. Western flower thrips with strawberry flowers were collected on July 23, Sept. 4 and October 21. Collected samples were immediately shipped to the lab for bioassay experiments.

Strawberry seedlings were planted in pots filled with soil mixture in a greenhouse/shadehouse as a source for leaflets on which to conduct the bioassays. Plants used in the experiments were at the three to six trifoliate stages when leaflets were removed for the bioassays, and were never treated. Radiant (spinetoram) was diluted in distilled water and at least 6 concentrations were used to produce a range of 5 percent to 90 percent mortality.

The most-recently fully-expanded strawberry leaflets were dipped for 10 s into a solution containing specific amount of Radiant. Control leaflets were dipped into distilled water only. After the leaf surface was dried, 25-35 adult thrips were transferred from the field collected flowers with an aspirator to the upper surface of a treated leaflet encased in a Munger cell apparatus with a layer of wet paper facing the lower surface of the leaflet (Figure 1).

After the initial exposure, adult mortality was determined at 24, 48 and 72 h, respectively. Thrips that were unable to walk at least a distance equivalent to their body length were considered dead.

The resulting data were corrected for control mortality and analyzed by the probit analysis.

LC50 and LC90 for spinetoram were determined for each 24 h interval (24, 48 and 72 h) after the treatment. Differences in LC50s and LC90s were considered not significant if their respective 95 percent confidence limits overlapped.

Results

After applications of Success on May 29 and June 8 and applications of Dibrom on June 16 and 29, LC50 and LC90 values of spinetoram to adult thrips (sampled on July 23) at 24, 48 and 72 h after the initial exposure were shown in Table 1. Two more Success treatments were applied on August 8 and 15. Compared to their respective LC50 and LC90 at 24, 48 and 72 h after the initial exposure from the July 23 sampling date, the LC50 and LC90 from the Sept.4 sampling date (twenty days after the final application) statistically remained the same (Table 1). The LC50 values for 24, 48 and 72 h post treatment from the October 21 sampling date (10 weeks after the final treatment) were 7.6, 17.0 and 141.9 times lower, respectively, while the LC90 values for the three post treatment intervals were still the same (Table 1).

Since the Holly ranch was treated with spinosad (Success) instead of spinetoram (Radiant), we tested susceptibility of adult western flower thrips to spinetoram compared to spinosad using the discriminating LC50 (26 μg ai/ml) and LC90 (115.2 μg ai/ml) concentrations of spinetoram (Table 2).

At the LC50 concentration, spinetoram caused 20 percent greater mortality of the thrips at 24 h, 24 percent greater at 48 h, and 8 percent more at 72 h, whereas at the LC90 concentration, spinetoram killed 9 percent tp 16 percent more of the thrips from 24 to 72 h after the exposure.

Conclusion

At the susceptible site of western flower thrips, two applications of spinosad did not affect the susceptibility. Ten weeks after 4 sequential applications of spinosad, the susceptibility increased dramatically. There was a large area of wild hosts existed nearby the treatment field and these wild hosts had never been treated for western flower thrips. Our results were likely attributed to the lack of treatments in the wild hosts.