When a tree is damaged by a herbivore (such as an insect), it will generally secrete volatile gases known as herbivore-induced plant volatiles (HIPVs). These volatiles may serve a couple of purposes, with the two principal ones being to deter further defoliation by the insect, and to attract predators and parasitoids of the defoliator in an attempt to reduce defoliation severity (by increasing defoliator mortality). Of course, beyond the release of HIPVs, phenols and other compounds may also be deposited within the leaf structure, and foliar re-growth may contain greater concentrations of herbivore-dissuading compounds. Such additional aspects of defence against herbivory are however beyond the scope of this post, which will focus specifically upon how HIPVs fare in terms of attracting predation of an insect defoliator by birds.
The authors of this study, to assess whether HIPVs did increase predation (by birds, upon caterpillars), used small 1.5m tall apple trees (Malus sylvestris), and ‘infected’ certain individuals with the winter moth (Operophtera brumata). Following infection of certain individuals, the research sought to determine whether great tits (Parus major) opted to frequent the apple trees where the defoliating caterpillars were still present, where they had been removed, or where both the damaged leaves and caterpillars had been removed. An aim of recognising whether the great tits responded most optimally to chemical (HIPVs), visual (birds can see the caterpillars and damged foliage), or a mixture of both cues, was also investigated. To ensure the test had reduced bias, only a sensible number of caterpillars were placed upon particular apple trees (a number that would mimic a naturally-occurring defoliation event), and the great tits used for the study were naive (they had been bred in captivity, and thus not exposed to the caterpillars on apple trees before). Specific means of data capture for each research aim can be read in the journal article linked at the end of this post.
Following on from the study period, it was observed that great tits visited infested trees with a higher significance than those uninfested by caterpillars – both in terms of the trees they would visit first, and the trees they would visit over a longer time period. Interestingly, it mattered not whether the infested trees still had the caterpillars on them, or even had the damaged leaves retained after HIPV emission but prior to visitation by birds (meaning that birds were not drawn to the trees because they could see the larvae or damaged leaves). The volatiles emitted by damaged leaves were also significantly different to those emitted by in-tact leaves (though both leaf types did emit volatile mixtures). For example, α-Farnesene, a chemical compound found in apple pomes and associated with herbivore attraction (so the fruits are eaten and seeds dispersed), was emitted at much higher levels following foliar herbivory by the caterpillars, and therefore may be a principal reason for why birds were attracted to the tree (though likely not the only reason). In summation, chemical cues emitted from damaged leaves (HIPVs) can be seen as a significant factor in determining visitation by birds.
However, the fact that the remnant structure of defoliated leaves visually differed to in-tact leaves (principally in the level of reflectance of light), visual cues may perhaps play somewhat of a role, though this role is unlikely to be major as light reflectance may vary for other reasons as well (including overall light availability).
To conclude, what this study shows is that chemical cues can be considered as important factors when it comes to both the defence of the tree, and the locating of food by insectivorous birds (the two are perhaps associated with one another, to quite a marked degree). This is important, as it ensures that a tree does not succumb to massive defoliation events, though it does rely upon the defoliator being palatable by any visiting birds. For this reason, invasive insect species may (or may not), whilst inducing HIPV emission, not be predated upon as much as a native insect defoliator (at least, to begin with). Granted, native insect species with high levels of toxins (such as the caterpillar of the cinnabar moth Tyria jacobaeae, which accumulates toxic alkaloids from consuming its host plant ragwort Senecio jacobaea) that build up over instar stages, may also reduce predation levels by birds.
Source: Amo, L., Jansen, J., Dam, N., Dicke, M., & Visser, M. (2013) Birds exploit herbivore‐induced plant volatiles to locate herbivorous prey. Ecology Letters. 16 (11). p1348-1355.
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