Studies on the Population Biology of the Tropical Butterfly Mechanitis isthmia in Costa Rica

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Abstract
The mortality of eggs and larvae of the tropical butterfly, M. isthmia (Bates) (Lepidoptera:Nymphalidae:Ithomiinae) was studied in Costa Rica. This butterfly deposits large masses of eggs on species of Solanum (Solanaceae) possessing thick, fleshy leaves covered with defensive hairs (trichomes) and spines. The larvae from an egg mass stay together as one or more groups on silken mats, and feeding, resting, molting and movement are communal activities. Sometimes individual larvae on the periphery of a group become isolated and are quickly taken by wasps and spiders. Such mortality is high for 1st- and 2nd-instar larvae. Older larvae survive well. More than 80% of the egg masses in an area are destroyed primarily by crickets, parasitic wasps and pathogenic microorganisms, with considerable plant-to-plant patchiness. The communal feeding allows young larvae to collectively clear away trichomes, but there is no apparent communal defense against predators. Older larvae, within a group, interlock their lateral tubercles, providing some form of communication among individuals. Most species of Ithomiinae are forest dwellers and deposit eggs singly on food plants. These solanums generally have smooth papery nonhairy leaves. Trichome-covered solanums are abundant in secondary habitats, and Mechanitis, in utilizing these plants as larval food, has evolved cluster egg-laying and gregarious forms of larval behavior.

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