Ant-caterpillar interactions in a neotropical savanna: Defensive strategies against ant predation in an ant-rich environment

Background/Question/Methods

Ants are dominant organisms in tropical ecosystems. As generalist predators on foliage, ants frequently reduce infestation by herbivorous insects. Lepidoptera is the second insect order in diversity and their larvae are among the most important herbivores in many ecosystems. Due to their low mobility, caterpillar vulnerability to natural enemies is very high. Here, we investigate the effectiveness of caterpillar defense strategies against ant predation, and identify which morphological and behavioral defense sets are better suited to survive ant encounters. We predicted that the best defensive set should be better suited for caterpillar feeding on plants highly visited by ants.  We conducted a survey of caterpillars in a ‘cerrado’ savanna in Southeast Brazil. We also examined the efficacy of the caterpillars’ defensive sets against predation by two common foliage-dwelling ant species in cerrado, Camponotus crassus and C. renggeri. We set up a series of laboratory encounters between larvae of different lepidopteran species and foraging ants from captive colonies. Trials were performed on simulated host-plants inside a foraging arena. We recorded and described the behavioral responses of the larvae during encounters with ants, and how these were related with ant aggressiveness. Larval mortality after encounters was also registered. 

Results/Conclusions

We found that shelter construction is a very common species-specific trait in caterpillars, and is also the most efficient strategy to avoid encounters with ant foragers on foliage. In general, caterpillar size is a good predictor of attack by ants and of larval mortality. The magnitude of this effect, however, depends on the ant species. Larval morphological traits such as long setae and/or scoli are weak predictors of attack by ants, and their defensive efficacy varies with the ant species. They are, however, very important to circumvent ant predation. Smooth-bodied caterpillars that do not build shelters, and those constructing shelters made solely of silk, are the ones most vulnerable to ant attacks and tend to use plants with decreased ant visitation. We suggest that the effectiveness of caterpillar defensive strategies may mediate the use plants that are highly visited by ants.  Thus the efficiency of the behavioral and morphological defensive sets against predation by foliage-dwelling ants can be a crucial factor determining caterpillar survival and host-plant use in the ant-rich cerrado savanna. (Supported by FAPESP, CNPq, FAEPEX/UNICAMP).