Search Behaviour: a Study of Three Caterpillar Species

The searching behaviour of three species of caterpillar-Pievis rapae L., Plusia californica Speyer, and Plutella maculipennis (Curt.)-has been described and the descriptions incorporated into simulation models. 2. Canadian P. rapae were studied in most detail. Their behaviour changes with hunger. When replete, a larva moves slowly, turns often, and 'head-waves' frequently. As it becomes hungrier, it speeds up, straightens out, and stops head-waving. At the same time, the distance from which it can perceive a host plant decreases. All these changes can be temporarily reversed by allowing the caterpillar to contact (but not necessarily feed on) a host plant. The rate at which the changes occur is temperature-dependent. 3. Simulation of these search patterns shows that the replete behaviour (called 'conservative search') is appropriate to searching within a small clump of plants, whereas the later behaviour ('radical search') is appropriate to random or uniform distributions and low plant densities. 4. Australian P. rapae showed the same pattern of behaviour, but the changes were less pronounced. They neither began their search as conservatively, nor adopted such radical search patterns later. 5. Neither Plusia californica nor Plutella maculipennis show substantial changes in behaviour as they starve. Plusia always travels rapidly and with a moderate amount of head-waving. It turns infrequently and has a tendency to zig-zag. Thus it uses radical search from the beginning. Plutella is just the opposite. It moves slowly, head-waves frequently, and turns often; it maintains this conservative search pattern throughout. 6. The relationship of these species' behaviour to the distribution of their host plants is discussed. P. rapae and Plutella both feed on cruciferous plants, which tend to occur in small clumps where conservative search is appropriate behaviour. (Plutella's failure to change its behaviour if unsuccessful may be related to its relatively low voracity and to the timing of its life cycle.) Plusia is polyphagous, so its resources are distributed less contagiously, and radical search is appropriate behaviour. 7. Other circumstances where the different types of search are appropriate behaviour are discussed.