Distance and Sex Determine Host Plant Choice by Herbivorous Beetles

Abstract

BackgroundPlants respond to herbivore damage with the release of volatile organic compounds (VOCs). This indirect defense can cause ecological costs when herbivores themselves use VOCs as cues to localize suitable host plants. Can VOCs reliably indicate food plant quality to herbivores?MethodologyWe determined the choice behavior of herbivorous beetles (Chrysomelidae: Gynandrobrotica guerreroensis and Cerotoma ruficornis) when facing lima bean plants (Fabaceae: Phaseolus lunatus) with different cyanogenic potential, which is an important constitutive direct defense. Expression of inducible indirect defenses was experimentally manipulated by jasmonic acid treatment at different concentrations. The long-distance responses of male and female beetles to the resulting induced plant volatiles were investigated in olfactometer and free-flight experiments and compared to the short-distance decisions of the same beetles in feeding trials.ConclusionFemale beetles of both species were repelled by VOCs released from all induced plants independent of the level of induction. In contrast, male beetles were repelled by strongly induced plants, showed no significant differences in choice behavior towards moderately induced plants, but responded positively to VOCs released from little induced plants. Thus, beetle sex and plant VOCs had a significant effect on host searching behavior. By contrast, feeding behavior of both sexes was strongly determined by the cyanogenic potential of leaves, although females again responded more sensitively than males. Apparently, VOCs mainly provide information to these beetles that are not directly related to food quality. Being induced by herbivory and involved in indirect plant defense, such VOCs might indicate the presence of competitors and predators to herbivores. We conclude that plant quality as a food source and finding a potentially enemy-free space is more important for female than for male insect herbivores, whereas the presence of a slightly damaged plant can help males to localize putative mating partners.

Integration of Plant Defense Traits with Biological Control of Arthropod Pests: Challenges and Opportunities

“…Releasing HIPV technology without examining the ecological factors present may render the technology ineffective. Several studies have shown that application of synthetic elicitors such as methyl jasmonate (MeJA) to induce elevated plant volatile production can also attract herbivores (Ballhorn et al, 2013) as well as hyperparasitoids (Kaplan, 2012; Heil, 2014), both outcomes that would be counterproductive to the potential for increased rates of parasitism by primary parasitoids. Additional spatio-temporal considerations must be understood to apply this technology in a large field setting.…”

“…Yet, HIPV production may also influence the plant’s attractiveness to herbivores. In an interesting study of two chrysomelid beetles ( Gynandrobrotica guerreroensis (Jacoby) and Cerotoma ruficornis Olivier) attacking wild lima beans [ Phaseolus lunatus L. (Fabales: Fabaceae)], female beetles were repelled by HIPVs produced by induced plants regardless of level of induction (possibly reflecting competition and a lack of enemy-free space) whereas males were attracted by weakly induced plants (possibly indicating the presence of a mate) but repelled by strongly induced plants (Ballhorn et al, 2013). The effect of such changes in herbivore densities on parasitoid foraging decisions is unexplored.…”

Aboveground endophyte affects root volatile emission and host plant selection of a belowground insect

“…Qualitative and quantitative differences in VOC emission can result from herbivore and microbe activity or from abiotic stress factors. Changes in the VOC composition can therefore be indicators of competition, induced defences or poor-quality host tissue (Ballhorn et al 2013a;De Moraes et al 2001;Kalberer et al 2001;Mayer et al 2011;Winter and Rostás 2010). In contrast to AG herbivores, very little is known about the mechanisms of host selection in soil insects and the role of root volatiles other than unspecific CO 2 (Johnson and Gregory 2006; Johnson and Nielsen 2012;Wenke et al 2010).…”

How useful are olfactometer experiments in chemical ecology research?

“…We found that herbivores do indeed use HI-VOCs as orientation cues, but that chemical short-distance cues of the plants overruled the effect of long-distance cues. Specifically, we could show that: (1) HI-VOCs released from wild lima bean plants had very different effects on chrysomelid herbivores depending on the degree of induction, that is the amount of HI-VOCs released, and the sex of the beetle; and that, (2) chemical defensive leaf traits (cyanogenic glucosides) overruled the long-distance signaling provided by volatiles 8 . These results reveal that using olfactometer trials alone without experiments on additional factors driving insect choice behavior, such as quantitative variation of the emitted signal, sex of the insects tested and feeding behavior on the actual plants, are not sufficient to draw profound conclusions.…”