Susceptibility of Eggs of Tarnished Plant Bug (Hemiptera: Miridae) to Novaluron and Diflubenzuron, 2015

The tarnished plant bug (TPB) is a very serious pest in the Mid-South in cotton. The damage from their feeding mostly causes pinhead squares to abort, causing significant yield loss. In this study, we compared the baseline toxicity of novaluron and diflubenzuron with eggs from a laboratory-reared, susceptible TPB colony. The laboratory colony was acquired from Mississippi State University (MSU) in spring of 2014. The insecticide susceptible colony has been maintained on an artificial diet of wheat germ and lima bean without insecticide exposure for about 10 yr at MSU. The egg dip bioassay used commercial formulations of novaluron (Diamond 0.83EC) and diflubenzuron (Dimilin). Eggs were collected on parafilm packets filled with Gelcarin exposed to adults for 48 h on a screened lid of an oviposition plastic container. The parafilm was cut into small sections with 7–9 eggs apiece. There were four replicates (novaluron, 12/9/14, 1/6/15, 2/4/15, and 7/6/15; diflubenzuron, 12/9/14, 1/6/15, 2/4/15, and 7/6/15) and each replicate of the bioassay used 14–30 eggs per concentration. The pieces of parafilm with eggs were dipped by hand into deionized water or a dilution of the representative chemical (novaluron or diflubenzuron) for 5 s and allowed to air dry on a paper towel before placing them individually into a vented 60-mm Petri dish with a miniature food packet containing the wheat germ and lima bean diet (2 g of diet, 3.5 × 4 cm). Dishes containing eggs were then placed into an environmental chamber at 26.8 ± 1°C, 60% RH, and a photoperiod of 16:8 (L:D) h. Egg hatch was recorded after 10 days. Eggs that showed no nymph development were recorded as dead, and eggs that hatched were considered to have survived (regardless of subsequent nymph survival). The treatments were prepared in such a way that the representative chemical was combined with de-ionized water to equal a 1,000 ppm stock solution. A series of dilutions were prepared from the stock solution. A range of concentrations (novaluron, 0.01, 0.03, 0.1, 0.3, 1.0, 2.0, 3.0, and 10.0 ppm; diflubenzuron, 0.01, 0.03, 0.1, 0.3, 1.0, 2.0, 3.0, and 10.0 ppm) were prepared for each insecticide, and deonized water was used in the untreated check. Dose–response (mortality) relationships were estimated with probit analysis (PoloPlus). Lethal dose ratios and 95% confidence limits (CL) or intervals for those ratios were calculated to identify differences in responses to novaluron or diflubenzuron. Lethal doses were considered to be significantly different from each other if the 95% confidence interval for the lethal concentration ratio did not include 1.0 (Robertson, et al. 2007).

TPB was equally susceptible to novaluron and diflubenzuron (Table 1). The LD₅₀ and LD₉₀ ratios were 1.0 (95% CL 0.41–2.51) and 1.45 (95% CL 0.29–7.09), respectively. Although the bioactivity and contact activity is greater for novaluron, susceptibility of TPB to this newer chemistry is very similar to that of diflubenzuron. The very low slopes for all of these models indicate high levels of heterogeneity in response to novaluron and diflubenzuron. Our primary objective for doing these bioassays was to estimate the toxicity of novaluron and diflubenzuron to eggs of TPB that had experienced little or no prior selection pressure from this insecticide. We chose to test novaluron and diflubenzurone as ovicides rather than nymphicides because reduction in egg hatch could be directly correlated to reduction in nymph hatching. Eggs are also readily accessible in large numbers with a lab or field population and are easy to measure age rather than nymphs that can vary in size. This research was supported in part by industry gifts of pesticide and research funding.

Reference Cited