Beetle spatial dynamics

Two projects were conducted with collaborators from Seale-Hayne College (later to become University of Plymouth) and Rothamsted Research funded by BBSRC.

  • 1.  Measurement and simulation of spatio-temporal dynamics to reduce pesticide inputs.

Cereal aphids occurred in small ephemeral patches throughout the field.  However, the beetle Pterostichus melanarius was found to dynamically respond to these ephemeral distributions, and orient towards aphid prey patches and away from low-density gaps. Consequently, we were able to confirm that P. melanarius, a generalist species with a varied dietary range, caused a measurable decrease in aphid growth rates and therefore within-field populations of cereal aphids and could act as viable pest control agent. A simple model of transient spatio-temporal predator-prey dynamics was derived that provided a good explanation for the structure observed in our empirical studies.

  • 2.  Individual-based spatio-temporal predator-prey dynamics.

This project was undertaken because we needed to understand the movement patterns of individual beetles to evaluate their value for aphid control. In the study, we used an automated laser, developed by a team at Seale-Hayne, that could mark each beetle with a unique three-digit code. We tracked the beetles by recapturing them in a grid of pitfall traps across a field and monitored the number of cereal aphids at each trapping location.

In 2003, we marked a total of 8,046 ground beetles (Poecilus cupreus), of which 2,270 were recaptured at least once. This information allowed us to determine that there were 17,199 beetles within the 2.75 hectare field. The field distribution of these beetles showed a high population activity-density in the field centre (Figure 1). In contrast, the distribution of cereal aphids was highest around the field edges over the main infestation period (Figure 2).

Comparing the two figures reveals that there was a clear relationship between the total number of beetles and maximum number of cereal aphids (Metopolophium dirhodum) and an obvious disassociation in their distribution. We doubt whether this one species of beetle was responsible for all the aphid control because many beetle species eat aphids. Nevertheless, our study clearly showed that predatory beetles can reduce cereal aphid numbers.

Figure 1: The field distribution of the beetle Poecilus cupreus. Contours were derived from the total number of beetles caught at each sampling position

The field distribution of the beetle Poecilus cupreus. Contours were derived from the total number of beetles caught at each sampling position

Figure 2: The field distribution of the aphid Metopolophium dirhodum. Contours were derived from the maximum number recorded at each sampling position

The field distribution of the aphid Metopolophium dirhodum. Contours were derived from the maximum number recorded at each sampling position

In 2004, we tested the extent to which the beetles respond to aphid patches. We did this by spraying parts of the field with an aphicide to create aphid-free gaps. We then established two sampling grids with differently sized patches in a field of winter wheat and released laser-marked beetles into it. We tracked their movement in relation to the aphids over the following two weeks and found that they moved only short distances regardless of the aphid densities around them.

We learnt that these beetles do not alter the way that they search for aphids as aphid abundance changes, unlike aphid-feeding specialists, which move less when they find an aphid patch. Aphids are not, therefore, an especially sought-after food item. Nevertheless, of the 6,792 beetles that we released, 31% were recaptured and two-thirds of these had eaten aphids.

Our study confirms that beetles reduce cereal aphid numbers, probably in conjunction with a large number of other insects and spiders and explains the patchiness of aphid distribution. As large numbers of beetles over-winter in the soil as well as the margins, less disruptive soil tillage should improve their survival and boost numbers for aphid control.

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