Methyl jasmonate-induction of cotton: a field test of the “attract and reward” strategy of conservation biological control

Authors: Williams, Livy; RODRIGUEZ-SAONA, CESAR - Rutgers University; CASTLE DEL CONTE, SANDRA - University Of Idaho

Submitted to: AoB Plants
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2017
Publication Date: 7/17/2017
Citation: Williams III, L.H., Rodriguez-Saona, C., Castle Del Conte, S.C. 2017. Methyl jasmonate-induction of cotton: a field test of the “attract and reward” strategy of conservation biological control. AoB Plants. 9(5):plx032. https://doi:10.1093/aobpla/plx032.
DOI: https://doi.org/10.1093/aobpla/plx032

Interpretive Summary: Beneficial insects are useful in killing insect crop pests, but the potential for utilizing beneficial insects remains untapped, partly due to the lack of experiments evaluating proposed control approaches. The “attract and reward” strategy takes advantage of a pest-damaged plant’s ability to produce odors as ‘distress signals’ to attract beneficial insects, and then reward these insects with excessive nectar production. Although this hypothesis sounds convincing, it has not been rigorously evaluated in crop fields. We conducted a field study to investigate the effects of a plant-produced chemical that initiates plant defense responses, and evaluated the effect on three levels: the crop plant, insect pests, and beneficial insects. We made applications of the chemical to cotton plants and measured odor emission, nectar production, and yield. We also assessed insect abundance, insect performance, and attack of pest insect eggs by beneficial insects. Our results showed that application of the chemical increased emission of several odors and production of nectar, but not yield. Despite increased odor and nectar production, application of the chemical did not affect pest performance, or death of pest eggs due to beneficial insects, and only marginally influenced insect abundance. Overall, treatment of cotton with the plant chemical affected the plants, but not the pest or beneficial insects. Our findings indicate that much remains to be learned before crop physiology can be manipulated to reliably and consistently enhance pest control by beneficial insects.

Technical Abstract: Natural or synthetic elicitors can affect plant physiology by stimulating direct and indirect defense responses to herbivores. For example, increased production of plant secondary metabolites, a direct response, can negatively impact herbivore survival, development, and fecundity. Indirect responses include increased emission of plant volatiles that influence herbivore and natural enemy behavior, and production of extrafloral nectar (EFN) that serves as a food source for natural enemies after their arrival on induced plants. Therefore, use of elicitors has potential for the study of basic aspects of tritrophic interactions, as well as application in biorational pest control, i.e., an “attract and reward” strategy. We conducted a field study to investigate the effects of methyl jasmonate (MeJA), an elicitor of plant defense responses, on three trophic levels: the plant, herbivores, and natural enemies. We made exogenous applications of MeJA to transgenic cotton and measured volatile emission, EFN production, and plant performance (yield). We also assessed insect abundance, insect performance, and parasitism and predation of brown stink bug, Euschistus servus (Say), eggs in MeJA-treated and untreated control plots. Application of MeJA increased emission of several volatiles and production of EFN, but not yield, compared to the control treatment. Despite increased volatile and EFN production, MeJA application did not affect plant bug performance, or mortality of E. servus egg masses, and only marginally influenced insect abundance. Mortality of E. servus eggs varied over the course of the study. Overall, MeJA treatment affected cotton plant-induced responses, but not the insects that inhabit the plants. Factors that may have influenced our results include spatially and temporally variable patterns of volatile diffusion and the subsequent effect on natural enemy behavior; and density dependent interactions between natural enemies and their hosts. Much remains to be learned before crop physiology can be manipulated to enhance pest control.