Abstract
Natural or synthetic elicitors can affect plant physiology by stimulating direct and indirect defence responses to herbivores. For example, increased production of plant secondary metabolites, a direct response, can negatively affect herbivore survival, development and fecundity. Indirect responses include increased emission of plant volatiles that influence herbivore and natural enemy behaviour, and production of extrafloral nectar that serves as a food source for natural enemies after their arrival on induced plants. Therefore, the 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, an elicitor of plant defence responses, on three trophic levels: the plant, herbivores and natural enemies. We made exogenous applications of methyl jasmonate to transgenic cotton and measured volatile emission, extrafloral nectar production and plant performance (yield). We also assessed insect abundance, insect performance, and parasitism and predation of brown stink bug, Euschistus servus, eggs in methyl jasmonate-treated and untreated control plots. Application of methyl jasmonate increased emission of volatiles, in particular, (+)-limonene and (3E)-4,8-dimethyl-1,3,7-nonatriene, and production of extrafloral nectar, but not yield, compared with the control treatment. Despite increased volatile and extrafloral nectar production, methyl jasmonate 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, methyl jasmonate treatment affected cotton plant-induced responses, but not the insects that inhabit the plants. Our results were probably influenced by reduced natural enemy colonization of cotton from adjacent non-crop habitats, and subsequent low within-field population recruitment. Much remains to be learned about the effects of exogenous application of plant-produced 'enhancers' on the behaviour of natural enemies before crop physiology can be manipulated to enhance pest control.
Highlights
Conservation biological control (CBC) encompasses a variety of approaches that aim to preserve natural enemy populations and improve their efficacy through modification of the biotic environment and reduction of pesticide impacts (Ehler 1998)
Volatiles from the present study were not collected under field conditions, our results are consistent with those of a previous study (Rodriguez-Saona et al 2001) and show that cotton plants respond to methyl jasmonate (MeJA) by increasing volatile emissions
In addition to increasing herbivore-induced plant volatiles (HIPVs) production, we found that MeJA application led to an increase in cotton Extrafloral nectar (EFN) production on three of the five sample dates
Summary
Conservation biological control (CBC) encompasses a variety of approaches that aim to preserve natural enemy populations and improve their efficacy through modification of the biotic environment and reduction of pesticide impacts (Ehler 1998). In addition to the production and emission of volatiles, the attack–response signalling pathways regulate nectar production (Heil 2011) and expression of defensive substances against herbivores (Rodriguez-Saona and Thaler 2005; Schaller and Stintzi 2008). These plant defence responses can be induced by exogenous application of the synthetic elicitors, such as jasmonic acid or its methyl ester, methyl jasmonate (MeJA) (Thaler 2002; Wu et al 2008) or cisjasmone (Bruce et al 2003)
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