Plant morphology alters the population spread rates of herbivorous insects

We demonstrate that measured differences in plant morphology can affect the spread of insects in herbaceous canopies, such as agricultural fields. We test the hypothesis that small-scale spatial structure may affect aspects of population dynamics (the diffusion-limitation hypothesis) using the pea aphid (Acyrthosiphon pisum (Harris)) and three near isogenic morphs of peas (Pisum sativum). Normal type peas are compared to two mutants that both lack tendrils, but which differ in leaf number and size. We characterize plant morphology using the fractal dimension and surface area. Digital data collection techniques were used to measure the spread rates of aphids through canopies composed of a single pea mutant. We combined the results of these greenhouse experiments with measures of aphid reproductive rates on the different pea morphs in order to calculate insect diffusion rates in the different canopy types. Our results suggest that although plant morphology may not alter reproductive rates, it significantly affects aphid movement rates. These data have implications for the applied issue of biological control. More generally, this finding provides support for the diffusion-limitation hypothesis which claims that small-scale spatial structure which affects animal movement rates can translate into larger scale alterations of population characteristics.