Abstract
Plants under herbivore attack emit volatile organic compounds (VOCs) that can serve as foraging cues for natural enemies. Adult females of Lepidoptera, when foraging for host plants to deposit eggs, are commonly repelled by herbivore-induced VOCs, probably to avoid competition and natural enemies. Their larval stages, on the other hand, have been shown to be attracted to inducible VOCs. We speculate that this contradicting behavior of lepidopteran larvae is due to a need to quickly find a new suitable host plant if they have fallen to the ground. However, once they are on a plant they might avoid the sites with fresh damage to limit competition and risk of cannibalism by conspecifics, as well as exposure to natural enemies. To test this we studied the effect of herbivore-induced VOCs on the attraction of larvae of the moth Spodoptera littoralis and on their feeding behavior. The experiments further considered the importance of previous feeding experience on the responses of the larvae. It was confirmed that herbivore-induced VOCs emitted by maize plants are attractive to the larvae, but exposure to the volatiles decreased the growth rate of caterpillars at early developmental stages. Larvae that had fed on maize previously were more attracted by VOCs of induced maize than larvae that had fed on artificial diet. At relatively high concentrations synthetic green leaf volatiles, indicative of fresh damage, also negatively affected the growth rate of caterpillars, but not at low concentrations. In all cases, feeding by the later stages of the larvae was not affected by the VOCs. The results are discussed in the context of larval foraging behavior under natural conditions, where there may be a trade-off between using available host plant signals and avoiding competitors and natural enemies.
Highlights
Maize plants attacked by herbivorous insects emit volatile organic compounds (VOCs) that attract natural enemies of herbivores (Dicke et al, 1990; Turlings et al, 1990; Turlings and Wäckers, 2004; Arimura et al, 2009)
They are emitted upon mechanical damage, immediately after feeding on the maize plant begins (Turlings et al, 1998), and have been considered important for the innate attraction of parasitoids, as they are emitted in higher amounts by freshly damaged plants than by plants with only old damage (Whitman and Eller, 1990; Hoballah and Turlings, 2005)
ATTRACTION OF FOURTH-INSTAR S. littoralis LARVAE TO INDUCED MAIZE PLANTS The larvae that had fed on maize and the larvae fed on artificial diet were both more attracted toward caterpillar-damaged maize plants than to intact plants (GLM P < 0.001 and P < 0.002, respectively; Figure 3)
Summary
Maize plants attacked by herbivorous insects emit volatile organic compounds (VOCs) that attract natural enemies of herbivores (Dicke et al, 1990; Turlings et al, 1990; Turlings and Wäckers, 2004; Arimura et al, 2009). Among the VOCs that have been identified in these blends, GLVs have received particular attention They are emitted upon mechanical damage, immediately after feeding on the maize plant begins (Turlings et al, 1998), and have been considered important for the innate attraction of parasitoids, as they are emitted in higher amounts by freshly damaged plants than by plants with only old damage (Whitman and Eller, 1990; Hoballah and Turlings, 2005). Insect herbivores are repelled by inducible plant volatiles (Bernasconi et al, 1998; De Moraes et al, 2001; Rostas and Hilker, 2002) This is evident for Lepidoptera (De Moraes et al, 2001), but this is not true for all herbivores. In particular coleopterans are known to be attracted to previously infested plants (Bolter et al, 1997; Landolt et al, 1999) and they may be attracted to GLVs as was found for scarab (Hansson et al, 1999) and buprestid beetles (de Groot et al, 2008), and flea beetles (Halitschke et al, 2008)
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