The feeding ecology of the carabid beetle Agonum dorsale in cereal crops

Previous field studies suggested that Agonum dorsale (Coleoptera:Carabidae) was the most promising polyphagous predators for the control of cereal aphids in Britain. The principal aim of this study was to assess this potential either for true biological or integrated control. In addition, the principles by which polyphagous invertebrates forage, and in particular how they select their pray, were studied. A. dorsale was nocturnally active but observation of the beetle was possible under red light. A. dorsale was robust experimentally, e.g. adults could be maintained at an immature reproductive stage by restricting their food supply and they could be maintained for long periods in simple laboratory conditions. Initial laboratory experiments suggested that A. dorsale had the attributes of an efficient cereal aphid predator; a type III functional response, area-restricted search, rapid passage of food through the gut. But both ambient temperature and reproductive maturity of the beetles were important in determining voracity. However, A. dorsale showed no specific adaptations for the detection of aphids in the field and when confined with mature wheat, climbed only occasionally; climbs being mostly confined to the lower leaves and stem. Of the prey selected by A. dorsale in wheat fields, micro-bomb calorimetry confirmed that cereal aphids had the highest calorific value. However, laboratory experiments showed that A. dorsale did not forage optimally. But differences between escape responses of the common prey types led to aphids being caught most easily for all likely field temperatures. Fieldwork supported laboratory findings: gut analysis showed that aphids could form a large part of the diet but suggested that the beetles foraged on the ground. A. dorsale rarely climbed wheat plants in the field even when aphid densities were relatively high but the proportion of aphids in their diet increased as aphid density increased. This was because as aphid density increased on the wheat the number of aphids on the ground (and hence vulnerable to predation) also increased. A computer model incorporating laboratory and field data was used to simulate the effect of predation by A. dorsale on a cereal aphid population. The simulations suggested that A. dorsale has little potential on its own, either in the biological, or the integrated control of cereal aphids. Aphid reproductive rate and the proportion of aphids reaching the ground mainly determined whether the beetles controlled the aphid population. Further simulations using predator density/voracity levels corresponding to the whole carabid population in a wheat field suggested that collectively they have an important role to play in the integrated control of cereal aphids.