The Potential of Carabidae in the Control of Insect Pests of Winter Oilseed Rape

This thesis reports the first study in the UK to focus on the role of carabid beetles as bio-control agents of the four major pests of winter oilseed rape, namely, the cabbage stem flea beetle (Psylliodes chrysocephala (Linnaeus)), the pollen beetle (Meligethes aeneus (Fabricius)), the cabbage seed weevil (Ceutorhynchus assimilis (Paykull)) and the brassica pod midge (Dasineura brassicae (Winnertz)).

The spatio-temporal distributions of adult carabids active on the soil surface of winter oilseed rape crops at IACR-Rothamsted, Hertfordshire, was compared with those of female and larval P. chrysocephala in the autumn, and with the mature larvae of M. aeneus, C. assimilis and D. brassicae in the summer that dropped from the crop canopy to the soil surface to pupate. Insect samples were collected from spatially referenced sampling points across each crop using pitfall and water traps. Counts of insects were mapped, analysed and the degree of spatial association between predator and pest determined using Spatial Analysis by Distance IndicEs (SADIE).

Patterns of crop invasion and distribution of the crops by both pests and carabids were complex, not uniform, and dynamic, with invasions on multiple fronts and aggregations at the edges or centres of the crop depending on species. The ten most abundant and active carabid species that were temporally associated with the pests were identified. Of these, Pterostichus madidus (Fabricius) and Trechus quadristriatus (Schrank) were spatially associated with P. chrysocephala larvae during October. Agonum dorsale (Pont), Asaphidion flavipes (Linnaeus), Amara similata (Gyllenhal), Harpalus rufipes (Degeer), Loricera pilicornis (Fabricius), Nebria brevicollis (Fabricius) and Pterostichus melanarius (Fabricius) were spatially associated with one or more species of pest larvae in the summer. Bembidion lampros (Herbst) was not spatially associated with any pest species.

The predatory ability of some of the key carabids on the soil surface, beneath the soil surface and in the aerial parts of the crop was investigated. Feeding tests with exposed prey showed that carabid species varied in their voracity, which was reduced in the presence of alternative food items (Collembola and rape seeds) and large species did not feed on small prey such as eggs. Carabids did not appear able to kill pest larvae once these had burrowed beneath the soil surface although mortality of pupating larvae in test controls was also high. The carabid Pterostichus cupreus (Linnaeus) was capable of climbing to 20 mm to feed on Brevicoryne brassicae (Linnaeus).

A four-arm olfactometer with a novel automated image processing system was developed to investigate carabid responses to olfactory cues from M. aeneus larvae and from B. brassicae adults. No responses were detected indicating that the carabids tested may not use olfactory cues to locate these pest species.

The spatio-temporal and prey consumption data was used to rank the key carabid species in order of their importance as bio-control agents of the four main pests of winter oilseed rape. Integrated Pest Management strategies, such as the spatial targeting of insecticides, habitat management and inundative release to enhance the bio-control potential of carabids in winter oilseed rape are discussed.