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

Abstract

The majority of insect species do not show an innate behavioural migration, but rather populations expand into favourable new habitats or contract away from unfavourable ones by random changes of spatial scale. Over the past 50 years, the scienti¢c fascination with dramatic long-distance and directed mass migratory events has overshadowed the more universal mode of population movement, involving much smaller stochastic displacement during the lifetime 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 insect movement is governed by both abiotic and biotic factors, making these animals essentially`slaves of their environment'. No displaced insect or insect population can leave a resource patch, migrate and £ourish, leaving descendants, unless suitable habitat and/or resources are reached during movement. This must have constrained insects over geological time, bringing about species-speci¢c adaptation in behaviour and movements in relation to their environment at a micro-and macrogeographical scale. With insects that undergo longrange spatial displacements, e.g. aphids and locusts, there is presumably a selection against movement unless overruled by factors, such as density-dependent triggering, which cause certain genotypes within the population to migrate. However, for most insect species, spatial changes of scale and range expansion are much slower and may occur over a much longer time-scale, and are not innate (nor directed).Ecologists may say that all animals and plants are ¢guratively speaking`slaves of their environments', in the sense that their distribution is de¢ned 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 ¢nd suitable resources and/or a habitat on which to feed and reproduce. 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-programmed £ight behaviour, large population sizes and/or fast reproduction), which improve the chances that some of these individuals will succeed. We also emphasize the dearth of knowledge in the various interactions of insect movement and their environment, and describe how molecular markers (protein and DNA) may be used to examine the details of spatial scale over which movement occurs in relation to insect ecology and genotype.

Spatial and temporal genetic variability in French populations of the peach–potato aphid, Myzus persicae

“…Genetic differentiation, even at small geographical distances, is not rare in aphids (Loxdale, 1990;Loxdale et al, 1993;Martínez-Torres et al, 1997;Sunnucks et al, 1997;Simon et al, 1999) and consequently aphid migration may be more limited than previously thought (Van Emdem et al, 1969;Taylor, 1977;Robert, 1988;Hales et al, 1997). However, it should be noted that local differentiation does not mean that intense migration and long-distance flights do not occur in aphids, but rather that long-distance migration may be rare or that the success rate of migration may be low (Loxdale et al, 1993;Loxdale and Lushai, 1999). The degree of geographical differentiation may depend on the species considered: the differentiation observed in M. persicae appears to be larger than that found in other aphid species.…”

Once a Batesian mimic, not always a Batesian mimic: mimic reverts back to ancestral phenotype when the model is absent

“…Butterfly species, like many phytophagous insects, are limited by the distribution of their larval host plants (Bernays & Chapman 1994;Loxdale & Lushai 1999). Host acceptance and selection are determined by a wide variety of ecological and physiological factors (e.g.…”

“…Unlike other well-studied butterfly mimicry systems, such as Heliconius, these three mimetic species also have different larval host plant requirements than their respective models (Opler et al 2004). Since the geographical distribution of the host species plays a large role in determining the geographical distribution of a phytophagous insect species (Bernays & Chapman 1994;Loxdale & Lushai 1999), disjunctions in the geographical distributions of model and mimic phytophagous insects may arise due to differences in larval host plant requirements. Mimetic herbivorous insects may exist in locations without their respective model, if hosts are available for the mimetic but not the model species.…”

Effects of drought and heat on forest insect populations in relation to the 2003 drought in Western Europe

“…The direct effects of temperature and drought on insects are likely to vary among species depending on the sensitivity of individual growth rates to increase temperatures [7], the life-stage at which they experience high temperatures and drought [3,26,42], their adaptation to warm, dry environments and climate extremes [27,67], and their capacity to escape unfavourable environmental conditions [58].…”