CAUSES AND CONSEQUENCES OF SPATIAL AGGREGATION IN THE PHYTOPHAGOUS BEETLE ALTICA-TOMBACINA

1. Utilizing population censuses, field experiments, and population models, we evaluated the causes of spatial aggregation and its consequences for intraspecific competition in an insect herbivore. 2. Eggs, larvae and adults of the flea beetle Altica tombacina were consistently aggregated among stems of fireweed (Epilobium angustifolium) in a 3-year census of a field population. 3. Aggregation was due in part to the fact that flea beetle eggs are laid in clutches. Furthermore, adult beetles remained longer on fireweed clones they shared with conspecifics, suggesting that aggregation is enhanced by deliberate congregation of adults. Aggregation apparently does not result from any ovipositional preference for high quality stems, nor from any tendency for adults to alter their movement or oviposition behaviour in response to local host density. 4. Repeated manipulation of flea beetle density in field cages demonstrated that, in two years, larval survival declined as density increased, as did the mean dry weight of surviving adults in one year. 5. Using our data on the effect of density on larval survival, we estimated that overall larval survival in the aggregated field population was only 10.5% lower in 1988 and 5.2% lower in 1989 than it would have been were larvae randomly distributed among stems. In contrast, mean survival was 36% lower in 1988, when mean density was high, than it was in 1989. 6. Aggregation did not strongly alter the average intensity of intraspecific competition within years because the recruitment curve relating adult emergence to larval density lacked strong non-linearities over the range of densities observed in the field.