Toxicity of Rimon and Dimilin to Spotted Wing Drosophila by Three Routes of Exposure, 2016*

SWD is an important pest of sweet cherry due to the deposition of eggs in the skin of ripening fruit. The resulting damage caused by larval feeding render the fruit unmarketable. This test was conducted in July/August 2016 at the Washington State University Tree Fruit Research and Extension Center in Wenatchee, Washington. The flies used in this test came from a laboratory-reared colony of SWD established in 2015 with adult flies captured in an organic cherry orchard. In all treatments, five 7-day-old female SWD per arena were used. A replicate consisted of an arena made from a 1-liter plastic container with previously untreated “Sweetheart” cherry fruit and leaves. Five cherries were suspended from the lids by their stems, and three leaves were stapled to the inside surfaces of the arenas. A 30-ml plastic container was secured with hot-melt glue to the bottom of the arena to secure a small portion of Drosophila medium. We tested three routes of exposure (topical, residual, and per os) for two insecticides, Rimon and Dimilin and an untreated check. The experimental design was a CRD with seven treatments and five replicate arenas per treatment. In the topical treatments, the flies were sprayed with a laboratory sprayer (Potter Spray Tower, Burkard Scientific, Rickmansworth, UK) with 2 ml of insecticide mixture at 6.5 psi before being transferred to an untreated arena. In the residue treatments, cherries were treated two ways before untreated flies were added. Cherries were sprayed with a calibrated handheld mister (MISTO, LifetimeBrands, Inc., Garden City, NY), and rotated to cover all surfaces of the fruit. Leaves were sprayed with the laboratory sprayer before being stapled to the inside of the arena. For per os exposure, the insecticides were mixed with corn syrup (Karo Dark Syrup, ACH Food Companies, Inc., Memphis, TN) and applied in 25-µl droplets to the leaves (20 droplets per arena) after they were secured in the arenas. The check consisted of untreated leaves, fruit, and flies. Flies were allowed to oviposit for 16 h before fruits were removed and the number of eggs per fruit counted. Lids with fruit attached were moved to a new arena, and new lids were placed on the arenas containing flies. A 30-ml plastic container with Drosophila medium was placed into the glued-in container in the arena containing flies. Adult female SWD mortality was evaluated at 48 h after exposure. Fruit were held for 14 days at 21 °C and evaluated for successful adult emergence. All data were subjected to ANOVA for treatment effects with means separated by LSMEANS/PDIFF (P = 0.05).

All treatments caused low levels (≤40%) of mortality; most means were significantly higher than the check except residual and per os Dimilin applications (Table 1). None of the treatment means had significantly different ovipositions per fruit compared to the check. Dimilin residues significantly reduced the number of emerged adults compared to the check, with zero eggs successfully producing adults despite high levels of oviposition. However, many other treatment means (Rimon Topical, Rimon Residual, Dimilin per os, and Rimon per os) were not significantly different from Dimilin Residual.

Author notes