Nutritional conditionality of an ant-lycaenid interaction

Mutualistic relationships entail both costs and benefits for the interacting species, and the net effects of the interaction are often dependent upon variation in environmental conditions and partner quality. The larvae of many lycaenid butterflies have facultative relationships with ants in which ants protect them from predators and parasitoids in exchange for receiving a sugary solution that the larvae produce from a dorsal nectary gland. Although these relationships usually have positive effects for both partners, there may be conditions under which the costs of the interaction exceed the benefits. In this study, I tested the effects of nutritional status of ants (the Argentine ant, Linepithema humile) on their tending of the larvae of a common lycaenid (the ceraunus blue, Hemiargus ceraunus) and the subsequent functional consequences for both partners. Ants that did not receive sugar increased tending of lycaenid larvae over the course of the study, but protein treatment had no effect on tending behavior. Although the number of ants tending had a significant negative effect on the length of pupation, neither nutrient status of the ants nor the number of ants tending influenced the age at pupation or pupal mass of H. ceraunus larvae. In fact, only maternal identity significantly explained variation in age at pupation and no variables tested in this experiment explained significant variation in pupal mass. These results suggest that variation in environmental conditions may lead to behavioral changes in ant-lycaenid interaction, but that such changes may not have functional consequences for the lycaenid larvae.