Baseline Susceptibility for Sugarcane Aphid, Melanaphis Sacchari (Zehntner) (Homoptera: Aphididae), Adults to Select Insecticides, 2016

The WSCA is capable of explosive population growth and can cause significant damage to sorghum in a short amount of time. Currently, there are only two labeled insecticides (Sivanto and Transform) for effective control of WSCA as others are either marginally effective or unavailable because of pre-harvest application restrictions. For these reasons, multiple applications of the same chemicals are being applied in the same growing season. To characterize WSCA response to key insecticides and develop accurate diagnostic doses for use on field populations, we (1) used diet incorporated or leaf dip bioassays (dependent on chemistry) to describe the responses of adults of a susceptible laboratory colony to key insecticides including acetamiprid (Intruder WSP), dicrotophos (Bidrin 8), flonicamid (Carbine 50WG), imidacloprid (Admire Pro), flupyradifurone (Sivanto), sulfoxaflor (Transform WG), and thiamethoxam (Centric 40); (2) development of three diagnostic doses, LD₅₀, LD₇₅, and LD₉₀; (3) verify diagnostic doses with ~600 WSCA from a single susceptible population using diet incorporated/residual bioassays to determine whether observed survivorship differed from the expected level of 50%, 25%, and 10%, respectively. Commercial formulations of Transform WG, Centric 40 WG, Carbine 50WG, Admire Pro, Sivanto, and Bidrin 8 were diluted in de-ionized water for use in diet-incorporated or leaf-dip bioassays dependent on chemistry. Baseline dose response curves were estimated and used for estimation of diagnostic doses for LD₅₀, LD₇₅, and LD₉₀ values. A series of dilutions were prepared from a 1000 ppm stock solution. Based on preliminary data, 6 to 9 concentrations (0.01–10 ppm, sulfoxaflor; 0.01–15 ppm, thiamethoxam; 0.01–10 ppm, flonicamid; 0.01–10 ppm, flupyradifurone; 0.01–10 ppm, acetamiprid; 0.01–20 ppm, imidacloprid; and 0–10 ppm), dicrotophos and deonized water as an untreated check were used in each bioassay. Five adult WSCAs were collected from the susceptible greenhouse-reared colony by a hand-held aspirator. WSCAs were placed on sorghum plants from three leaf to boot stage and allowed to breed uncontrolled in a greenhouse setting. Pieces of sorghum leaf (2.5 × 2.5 cm) were dipped into deionized water or a dilution of the representative chemical for 5 s and allowed to air dry before placing them individually into a vented 60 mm Petri dish. For diet-incorporated bioassays, the dilutions of insecticide were combined at 15% with deionized water containing 1.24 ml green food coloring per 1 liter of water as a food attractant. The representative chemical (150 µl) of each concentration was placed into the cap of a 1.5 ml micro-centrifuge tube, and a small piece (2.5 cm × 2.5 cm) of parafilm sealed the solution in the cap. Five aphids were placed into the 1.5 ml micro-centrifuge tube, and the cap sealed the aphids with diet-incorporated treatment. Dishes or vials were then placed into an environmental chamber at 26.8 ± 1°C, 60% RH, and a photoperiod of 16:8 (L:D) h, and mortality was assessed after 48 h by prodding the aphids with a blunt probe and observing their movement. An aphid was considered to be alive if it was able to move its antennae, legs, or head when prodded. Aphids that exhibited only rapid twitching of the legs, abdomen, or antennae were considered to be moribund and combined with dead insects (no movement) for analysis. Each bioassay had a minimum of four replicates. Dose–response (mortality) relationships were estimated with PoloJR (LeOra Software, LLC, Parma, MO). Data were analyzed assuming the probit model. Plots of standardized residuals were examined for outliers. Only concentrations between the lowest concentration that caused 100% mortality and the highest concentration that caused 0% mortality were used in analyses. Slopes, LD₅₀’s, LD₇₅’s, and LD₉₅’s were estimated for each bioassay (Table 1). Diagnostic doses were administered to ~100 aphids per dose on a single date, and we compared observed survivorship with expected survivorship. Mortality was corrected with formula of Abbott (1925). Means of survivorship were tested against the expected level with a one-sample t-test and a two-tailed probability distribution (PROC TTEST, SAS Institute, P < 0.05). Results reported here provide baseline data for future monitoring of resistance development.

To determine the baseline susceptibility of WSCA to these insecticides, we estimated the statistical parameters of concentration–response relationships observed in binary bioassays. We selected insecticides commonly used as foliar treatments for other sucking insects including aphids. There were no significant differences between the organophosphate, Bidrin, and Sivanto. The latter is in a new class of insecticides, butenolides, that are similar to neonicotinoids in that it is a systemic insecticide that targets piercing, sucking insects. Intruder, another neonicotinoid, was slightly less toxic (0.52 ppm) than Bidrin and Sivanto (Table 1). Carbine showed similar toxicity to Intruder, is likewise systemic, but is in a different class of insecticides (pyridincarboxamide); Transform WG and Centric were threefold and twofold less toxic, respectively, as the before-mentioned groups of insecticides. Admire Pro was 15-, 13-, 9-, 8-, 4- and 2.8-fold less toxic than Bidrin, Sivanto, Intruder, Carbine, Transform, and Centric, respectively. Out of the most toxic chemicals for WSCA (Bidrin, Sivanto, Intruder, and Carbine), only Silvanto is currently labeled for use in sorghum. Baseline susceptibility data are important to allow future monitoring of development of resistance resulting from selection pressure from insecticide use in the field. Continued monitoring of WSCA response to these reduced-risk insecticides will be essential to detect and manage resistance. This research was supported in part by industry gifts of pesticide and research funding.

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