Impact of agricultural adjuvants on the toxicity of the diamide insecticides chlorantraniliprole and flubendiamide toward different life stages of navel orangeworm (Amyelois transitella) (Lepidoptera: Pyralidae)

Authors: DEMKOVICH, MARK - University Of Illinois; Siegel, Joel; Walse, Spencer; BERENBAUM, MAY - University Of Illinois

Submitted to: Journal of Pest Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2018
Publication Date: 2/7/2018
Citation: Demkovich, M., Siegel, J.P., Walse, S.S., Berenbaum, M. 2018. Impact of agricultural adjuvants on the toxicity of the diamide insecticides chlorantraniliprole and flubendiamide toward different life stages of navel orangeworm (Amyelois transitella) (Lepidoptera: Pyralidae). Journal of Pest Science. 91(3):1127-1136. https://doi.org/10.1007/s10340-018-0959-z.
DOI: https://doi.org/10.1007/s10340-018-0959-z

Interpretive Summary: In California orchards the navel orangeworm, Amyelois transitella is the primary pest of almonds and pistachios as well as a serious pest of walnuts, figs, and pomegranates. Larvae cause direct damage when they consume the nutmeat and generate large quantities of frass and webbing, and cause indirect damage by leaving nuts susceptible to infection by fungi that produce aflatoxins. Control is extremely challenging and requires a strategy to manage immigrating pests as well as an internal population. Insecticides play a major role in the control of this pest and these are mixed into orchard sprayers with adjuvants. An important class of narrow spectrum insecticides used to this pest is known as the diamides. These formulated insecticides are mixed with adjuvants, which are products designed to modify the physical properties of the spray mixture; the effect of adjuvants on the toxicity of diamides is unknown. Our laboratory study investigated the intrinsic toxicity of five adjuvants as well as the toxicity resulting from the combination of unformulated diamides (purity of 90% or higher) with adjuvant. Our targets were navel orangeworm adults and eggs. We then validated the laboratory study by assessing two adjuvants in the field using formulated insecticides, and found that the toxicity of the two adjuvants was dependent on the diamide insecticide used. We noted considerable variability in the response of this insect, which could be due to inherent differences between unformulated and formulated insecticide as well as differences in insecticide coverage between laboratory and field. This latter problem is well known and research on improving insecticide coverage is ongoing. We view our laboratory data as demonstrating potential benefits of certain classes of adjuvants but this must be validated in the field.

Technical Abstract: Five adjuvants, alone and in combination with two diamide insecticides (chlorantraniliprole and flubendiamide), were assessed for activity against the adults and eggs of navel orangeworm, Amyelois transitella (Walker). Laboratory studies utilized a spray tower for application while field studies used orchard air-blast sprayers. Laboratory exposure of both adults and eggs demonstrated that the adjuvants were intrinsically toxic to A. transitella and that their toxicity varied between lifestages; the most toxic adjuvant to adults was the least toxic to eggs. When the outcome was expanded to include both dead and moribund adults (incapacitance) additional adjuvants were more effective. Two adjuvants caused the highest neonate mortality but all adjuvants were toxic, and all increased stunting. In the laboratory, the addition of chlorantraniliprole to adjuvants increased incapacitance. The results for flubendiamide were variable and only one adjuvant had the same high impact for both diamides. For eggs exposed to these diamides, overall mortality was similar. Field studies only tested two adjuvants and their adult toxicity varied. They only incapacitated a maximum of 66.8% of the adults exposed. In contact assays both adjuvants were similar for Belt but differed for Altacor. This departure from our laboratory data may reflect differences in coverage as well as differences between unformulated and formulated insecticide. We conclude that adjuvants are intrinsically toxic to A. transitella in both the laboratory and field. Further studies are needed to attain the toxicity realized in the laboratory in field applications, and also to determine whether our findings can be generalized to other classes of insecticide.