Note by LL: Research in our lab does not only revolve around plants. Some people also study for example plant-animal interactions, and Triinu, who studies this stuff actually did her PhD on body size evolution in insects with different colouration strategies. So from time to time she publishes purely entomological research as well.
Text by Triinu Remmel
First citation: Molleman, F., Remmel, T., & Sam, K. (2015). Phenology of predation on insects in a tropical forest: temporal variation in attack rate on dummy caterpillars. Biotropica. (link to full text)
Second citation: Sam, K., Remmel, T., & Molleman, F. (2015). Material affects attack rates on dummy caterpillars in tropical forest where arthropod predators dominate: an experiment using clay and dough dummies with green colourants on various plant species. Entomologia Experimentalis et Applicata. (link to full text)
The numbers of herbivorous insects fluctuate with some regularity in tropical forests, yet almost nothing is known as to what keeps them fluctuating, which are the forces that regularly reduce the abundances. A natural guess is that it might have something to do with predation. We exposed artificial models of insect larvae on leaves of different understory plants in a lowland forest of Kibale, and inferred the abundance and identity of predators from marks left in the malleable materials of the larvae. This was repeated many times throughout a year. As it turns out, instead of driving the abundance fluctuations, predation pressure seems to dampen them: predation was highest during the peak abundances of natural tree-feeding caterpillars, indicating perhaps that predators shift their diets to more abundant food at these peaks, or that predators increase in numbers during the peaks. Interestingly, bird predation was almost absent and arthropods were to be blamed for most of the „mortality“. This is in sharp contrast to temperate and tropical montane forests, and can also account for the lack of effect of host plant species.
Abstract of the Biotropica paper:
In communities of tropical insects, adult abundance tends to fluctuate widely, perhaps in part owing to predator–prey dynamics. Yet, temporal patterns of attack rates in tropical forest habitats have not been studied systematically; the identity of predators of insects in tropical forests is poorly known; and their responses to temporal variation in prey abundance have rarely been explored. We recorded incidence and shape of marks of attacks on dummy caterpillars (proxy of predation rate) in a sub-montane tropical forest in Uganda during a yearlong experiment, and explored correlations with inferred caterpillar abundance. Applying the highest and lowest observed daily attack rates on clay dummies over a realistic duration of the larval stage of butterflies, indicates that the temporal variation in attack rate could cause more than 10-fold temporal variation in caterpillar survival. Inferred predators were almost exclusively invertebrates, and beak marks of birds were very scarce. Attack rates by wasps varied more over time than those of ants. Attack rates on dummies peaked during the two wet seasons, and appeared congruent with inferred peaks in caterpillar density. This suggests (1) a functional response (predators shifting to more abundant resource) or adaptive timed phenology (predators timing activity or breeding to coincide with seasonal peaks in prey abundance) of predators, rather than a numerical response (predator populations increasing following peaks in prey abundance); and (2) that predation would dampen abundance fluctuations of tropical Lepidoptera communities.
Abstract of the Entomologia Experimentalis et Applicata paper:
Predation can be one of the key factors that determine abundance in insect herbivore communities, and drive evolution of body size, and anti-predator traits, including crypsis. Population dynamics and selection pressures will depend on the identity of dominant predators in the system, and these may vary substantially among habitats. Arthropods emerge as chief predators on caterpillars in the understorey of non-montane tropical forest, whereas birds dominate elsewhere. In a tropical forest in Uganda, Africa, we evaluated marks on dummy caterpillars that differed in size, material (clay vs. dough), colourant, and plant species on which dummy caterpillars were exposed. We included live caterpillars to estimate the extent to which studies using artificial caterpillars reflect actual levels of predation. Ants and wasps were the most important damagers of dummy caterpillars, whereas bug and beetle damage was very rare, and no bird or small mammal damage was observed. Daily attack rates did not differ significantly from apparent mortality of live caterpillars (daily mortality = 12.1%), but dummy caterpillars made from dough were attacked more frequently (daily attack rate = 18.4%) than those from clay (daily attack rate = 6.9%). Caterpillars of different colour and size, and caterpillars exposed on different plant species had the same chances to be predated. This is in contrast to results from temperate area studies where birds dominate and are not affected by dummy caterpillar material, but prefer larger caterpillars. Our results are consistent with dominant predators on tropical forest caterpillars being invertebrates that are more chemically than visually oriented, so that: (1) material used for dummy caterpillars is important, (2) background matching is relatively unimportant, and (3) being large may have less of a cost. These patterns in predation might facilitate polyphagy and evolution of large body size in tropical Lepidoptera.