Aggressive fighting behavior in shelter-building caterpillars

Background/Question/Methods

The importance of competition in the evolutionary ecology of phytophagous insects is a highly debated topic. While many studies have shown that indirect competition mediated by plant chemical defenses or shared natural enemies is widespread in phytophagous insect communities, examples of direct competition are relatively scarce due to the fact that food resources are rarely limiting. However, competition may occur over other non-food resources such as shelters. Shelter-building is a widespread behavior in arthropod communities, particularly among Lepidoptera. Physical fighting behaviors and vibrational signaling among shelter-building caterpillars has recently begun to receive attention with reports of these behaviors coming from many diverse taxa. However, interspecific differences in levels of aggression in co-occurring caterpillars have yet to be explored. This study compares the behavior of four common, closely related species (Lepidoptera: Oecophoridae) that build shelters between overlapping leaves. By using clear acetate paper as one of the leaves, interactions within the shelter can be observed without disturbing the caterpillars. Caterpillars were assigned aggression scores based on the number of times they pushed or struck the other caterpillar and if they shared, usurped, or defended a shelter.

Results/Conclusions

The four species tested varied in their level of aggression. In order of decreasing level of aggression the four species are Psilicorsis reflexella, P. cryptolechiella, P. quercicella, and Pseudotelphusa sp. Notably, the two most aggressive species are much less common than the two least aggressive species. Thus, increased aggression may aid in the persistence of these species in the community even at low population levels. Conversely, the more common species may be less aggressive because they are found at high densities. These species frequently must share shelters with other caterpillars, particularly siblings, due to a limited number of available shelters. Interactions between these four species may influence their distribution among leaf shelters and consequently affect the composition of the shelter-inhabiting arthropod community.