Submitted to: Journal of Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2016
Publication Date: 9/12/2016
Citation: Bagchi, V., Siegel, J.P., Demkovich, M., Zehr, L., Berenbaum, M. 2016. Impact of pesticide resistance on toxicity and tolerance of hostplant phytochemicals in Amyelois transitella (Lepidoptera: Pyralidae). Journal of Insect Science. 16(1):62. doi: 10.1093/jisesa/iew063. Interpretive Summary: In California crops from the old and new world are grown close together and a pest’s ability to switch crops can have a major economic impact. The factors that enable this to occur are not well described and we used the navel orangeworm (Amyelois transitella), an important California pest of almonds, pistachios, walnuts and figs to explore this question. Resistance of navel orangeworm to the pyrethroid family of insecticides has been recently described in Kern County and we thought resistant insects could attack new crops. We report the variability in the response to pyrethroids of three lines of navel orangeworm, a lab strain (CPQ, unexposed to pyrethroids) and two strains isolated from the field in almonds (R347, resistant to pyrethroid) and figs (FIG). We challenged larvae with a pyrethroid insecticide, three chemicals that plants use for defense against insects (allelochemicals) and two novel hosts that occur in California but are not used by navel orangeworm (mesquite and wisteria). As expected, R347 was more tolerant to pyrethroids than CPQ, but surprisingly, FIG was equally as tolerant as R347, even though pyrethroids are not used in figs. The field strains were also more tolerant to allelochemicals than CPQ. Surprisingly, R347 did not do well on wisteria while FIG did the best on this plant. We had expected R347 to do well in all challenges because of its insecticide resistance, but FIG outperformed it. FIG may have a superior ability to detoxify chemicals because its host contains high levels of allelochemicals. R347 may have sacrificed its ability to handle the defensive chemicals of wisteria when it became resistant to pyrethroids.
Technical Abstract: For some polyphagous insects adaptation to phytochemically novel plants confers enhanced resistance to insecticides, but whether insecticide resistance enhances tolerance to novel phytochemicals has not been assessed. We used Amyelois transitella Walker (navel orangeworm), an economically important polyphagous pest of tree nut crops in California, to address this question. Our study used three strains of A. transitella--pyrethroid-resistant R347 (maintained in the laboratory for ~10 generations), fig-derived FIG, (maintained in the laboratory for ~25 generations) and CPQ (derived from almonds and maintained in the laboratory for 40+ years) to compare the toxicity of the pyrethroid bifenthrin and three phytochemicals (chlorogenic acid and the furanocoumarins xanthotoxin and bergapten), as well as survival on diets containing seeds of Wisteria sinensis (Wisteria) and Prosopis pallida (Mesquite), two Fabaceae species not reported as hosts. Only R347 had pyrethroid exposure, but all three strains may have been exposed to chlorogenic acid, which is found in both almonds and figs. Only FIG was exposed to furanocoumarins. Both the R347 and FIG strains exhibited greater tolerance to bifenthrin than did CPQ (4.95-fold and 18.95-fold, respectively) and more than two-fold greater resistance to the three phytochemicals compared to CPQ. Incorporation of the synergist piperonyl butoxide into the diet consistently increased mortality only for CPQ, implicating alternate detoxification mechanisms for both R347 and FIG. Our results suggest that, while P450-mediated pesticide resistance may enhance tolerance of certain phytochemicals in this species, it is only one of multiple biochemical adaptations associated with acquisition of new hostplant species.