Because shelters are frequently occupied by multiple arthropods (either concurrently or sequentially), there is considerable potential for both intraspecific and interspecific competition. Among herbivores sharing shelters, competition can be quite intense because the food resources available inside leaf shelters can be limited due to high densities of occupants. When food resources are exhausted, herbivores that have not completed development will be forced either to expand their shelter by pulling in additional foliage or to relocate and establish a new shelter on less-damaged foliage. However, studies have found that previously occupied (and damaged) leaf ties are as equally attractive to new herbivores as unoccupied ties, suggesting that food resources may not be the limiting factor in site selection (Lill 2004, Lill and Marquis 2004, Lill et al. 2007).
While the negative effects of resource competition within shelters remain to be examined, several studies have documented antagonistic behavioral interactions occurring among shelter occupants. For example, the leaf webs constructed by Depressaria pastinacella on parsnip are aggressively defended from being usurped by conspecifics displaced from their own webs (Berenbaum et al. 1993). Similarly, cherry leafroller caterpillars (Caloptilia serotinella) utilize vibrational signals (i.e., leaf scraping) in response to intruders in what are hypothesized to be territorial disputes (Fletcher et al. 2006). While the mechanism is unclear, one species of leaf-tying caterpillar (Psilocorsis quercicella) attained higher pupal mass when reared singly in a leaf shelter than when reared with two conspecifics when food was not limiting (Lill et al. 2007), suggesting that such negative behavioral interactions can influence fitness measures. The very existence of these aggressive behaviors underscores the prominent role competition likely has played in shaping the traits of shelter-dwellers.
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