Oecologia (1997) 109: 219–228 © Springer-Verlag 1997

Infection with an endoparasite frequently alters host behaviour. This study provides the first quantification of larval behaviour in a baculovirus / Lepidoptera system, and attempts to assess the ecological consequences of behavioural modification during infection. Larvae of the moth Mamestra brassicae (Lepidoptera: Noctuidae) exhibited higher rates of dispersal in the laboratory and field when infected with Mamestra brassicae nuclear polyhedrosis virus (MbNPV) than did uninfected larvae. They adopted positions at death which were not characteristic of healthy larvae, climbing higher on the foodplant and onto the top and edge of leaves. The horizontal and vertical distribution of virus following larval lysis and the effects of rainfall on this distribution were assessed for comparison with the distributions of healthy and infected larvae. Exposure to rainfall increased the infectivity of vegetation in bioassays. Alternative explanations for the evolutionary origins of behavioural modification are considered. I suggest that the behavioural changes observed are most likely to benefit the virus. In particular, climbing prior to death is likely to result in contamination of more foliage with virus particles than would otherwise occur by increasing exposure of cadavers to rainfall. Thus it may profoundly influence horizontal transmission and the dynamics of the host-virus interaction.