Slaves of the environment: the movement of herbivorous insects in relationto their ecology and genotype

The majority of insect species do not show an innate behavioural migration, but rather populations expand into favourable new habitats or contract awa y from unfavourable ones by random changes of spatial scale. Over the past 50 years, the scientific fascination with dramatic long-distance and direct ed mass migratory events has overshadowed the more universal mode of popula tion movement, involving much smaller stochastic displacement during the li fetime of the insects concerned. This may be limiting our understanding of insect population dynamics. In the following synthesis, we provide an overview of how herbivorous insec t movement is governed by both abiotic and biotic factors, making these ani mals essentially 'slaves of their environment'. No displaced insect or inse ct population can leave a resource patch, migrate and flourish, leaving des cendants, unless suitable habitat and/or resources are reached during movem ent. This must have constrained insects over geological time, bringing abou t species-specific adaptation in behaviour and movements in relation to the ir environment at a micro- and macrogeographical scale. With insects that u ndergo long-range spatial displacements, e.g. aphids and locusts, there is presumably a selection against movement unless overruled by factors, such a s density-dependent triggering, which cause certain genotypes within the po pulation to migrate. However, for most insect species, spatial changes of s cale and range expansion are much slower and may occur over a much longer t ime-scale, and are not innate (nor directed). Ecologists may say that all animals and plants are figuratively speaking 's laves of their environments: in the sense that their distribution is define d by their ecology and genotype. But in the case of insects, a vast number must perish daily, either out at sea or over other hostile habitats, having failed to find suitable resources and/or a habitat on which to feed and re produce. Since many are blown by the vagaries of the wind, their chances of success are serendipitous in the extreme, especially over large distances. Hence, the strategies adopted by mass migratory species (innate pre-progra mmed flight behaviour, large population sizes and/or fast reproduction), wh ich improve the chances that some of these individuals will succeed. We als o emphasize the dearth of knowledge in the various interactions of insect m ovement and their environment, and describe how molecular markers (protein and DNA) may be used to examine the details of spatial scale over which mov ement occurs in relation to insect ecology and genotype.