Abstract
Herbivorous arthropods use various cues to choose suitable host plants. We investigated whether three species of herbivores use cues associated with their omnivorous predator Macrolophus pygmaeus to select host plants. Earlier, we found that this omnivore induces plant defences which decreased the performance of two of the herbivores, i.e. the spider mite Tetranychus urticae and the western flower thrips Frankliniella occidentalis, whereas the green peach aphid Myzus persicae was not affected. Hence, the spider mite and thrips were expected to avoid plants that were previously exposed to M. pygmaeus because of their lower quality, and the aphid was not expected to avoid exposed plants because they were of equal quality as unexposed plants. However, the cues left behind by M. pygmaeus may also be indicative of predation risk, in which case all three herbivores were expected to avoid exposed plants. Spider mites and western flower thrips preferred clean plants over plants that had previously been exposed to M. pygmaeus, but no longer harboured this omnivore. Aphids showed no preference, in agreement with their performance, but not in agreement with reducing predation risk. We furthermore showed that the preference of spider mites and thrips for clean plants increased through time. Higher proportions of aphids left plants previously exposed to M. pygmaeus than clean plants through time. Hence, omnivorous predators can decrease herbivore densities on plants not only by killing them but also by indirectly affecting herbivore host plant selection.
Highlights
To find a suitable host plant for feeding and oviposition, herbivores use cues associated with the host plant quality (Dicke 2000), the likelihood of food competition (Bernays and Chapman 1994; Pallini et al 1997) and the risk of predation (Lima and Dill 1990; Janssen et al 1998; Grostal and Dicke 1999)
We showed that feeding of Macrolophus pygmaeus on sweet pepper plants induced direct plant defences, resulting in reduced performance of two-spotted spider mites (Tetranychus urticae) and western flower thrips (Frankliniella occidentalis) on these plants, but the performance of green peach aphids (Myzus persicae) was unaffected by previous omnivore plant feeding (Zhang et al 2018)
When thrips were recaptured within 1 h of arriving on the plants, the cumulative numbers of thrips found on clean plants increased faster through time than those on M. pygmaeus-exposed plants (Fig. 3a), resulting in a significant effect of the interaction between treatment and time (LME: χ2 = 14.1, df = 4, P = 0.0285)
Summary
To find a suitable host plant for feeding and oviposition, herbivores use cues associated with the host plant quality (Dicke 2000), the likelihood of food competition (Bernays and Chapman 1994; Pallini et al 1997) and the risk of predation (Lima and Dill 1990; Janssen et al 1998; Grostal and Dicke 1999). Plants produce volatiles that are often used by the natural enemies of the herbivores to locate their prey (Dicke and Sabelis 1988; Turlings et al 1990; Takabayashi et al 2006), and this information is picked up by other herbivores that can use it to locate suitable host plants (Shiojiri et al 2002; Sabelis et al 2007). Some herbivores are repelled by volatiles from plants with conspecifics or heterospecifics to avoid competition and predation risk (Pallini et al 1997; De Moraes et al 2001; Agut et al 2005), but there are examples of herbivores that are attracted by the volatiles from plants with conspecifics (Harari et al 1994; Loughrin et al 1995; Bolter et al 1997; Agut et al 2005; Sarmento et al 2011)
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