Different roles for innate and learnt behavioral responses to odors in insect host location

Volatile chemical cues are used by herbivorous insects to locate and identify their host plants. Many species show a preference for volatiles experienced during development in the natal habitat. The reliability of this learnt information, however, may be limited. Many insects develop in restricted habitats, often on a single plant. Large between-plant variability in volatile emission, due to genetic differences and different exposure to biotic and abiotic factors, means that the volatile profile of a single plant may not be representative of the entire species. Insects must, therefore, balance the benefits of learning with the risks associated with its reliability. This is especially important for insects for which habitat exploration is costly. We hypothesize that information gained in the natal habitat is most likely to be utilized in situations where the cost of habitat exploration is lowest. To test this hypothesis, the black bean aphid, Aphis fabae, was reared on artificial diet while exposed to volatiles from its host, broad bean, and an unsuitable host, mustard. When offered the choice between bean and mustard leaves as adults, aphids showed a preference for the leaves whose odor they had experienced during development. When only exposed to volatiles from the two plants, in the absence of cues to indicate proximity or accessibility of the odor source, aphids preferred bean volatiles, regardless of experience. This suggests that information acquired from the natal habitat is only utilized when the perceived cost of habitat assessment is low, with innate preferences dominating otherwise.