The Predatory Potential of Staphylinid Beetles in Cereals.
Aspects of the biology of staphylinids which limited their predatory potential were studied. Experiments investigated factors affecting overwintering survival, spring distribution and dispersal in relation to aphids in the crop, aphid consumption in the presence of a choice of food and their foraging behaviour on winter wheat with different levels of aphid and disease infestation. Two field trials isolated staphylinid species with aphids to evaluate their impact on aphid population development individually.
Variation in the numbers of staphylinids overwintering in similar field-margin sites was related to the quality of refugia which in turn were determined by their physical, vegetative and microclimatic characteristics. Vegetation cover, in particular the presence of tussock-forming grasses, improved the overwintering survival of Tachyporus hypnorum within a single post-and-wire-fenced grass boundary. Vacuum-net and quadrat surveys of summer, field populations of staphylinids showed strong correlations with food-rich areas of the crop but did not show temporal and spatial relationships between staphylinid and aphid densities in winter wheat. All the staphylinids studied consumed aphids as a high proportion of their own body weights/day. However, Philonthus cognatus consumed greater numbers/day than did all Tachyporus spp. Preferences for aphids were shown by T. hypnorum when offered a choice of arthropod prey at a range of prey frequencies. Petri dish experiments and observations of climbing behaviour showed that mycophagy made an important contribution to the diet of Tachyporus spp. but not to that of P. cognatus.
Spores of powdery mildew were preferred to aphids by T. hypnorum and Tachyporus spp. larvae but aphid preferences were higher for T. chrysomelinus and T. obtusus. Plant climbing and feeding by T. obtusus were higher on wheat stems and aphid contact was enhanced but occured on lower leaves for T. chrysomelinus and T. hypnorum with subsequently contact with fungal spores. Aphid populations within field enclosures were significantly reduced by field densities of P. cognatus, T. obtusus, and T. chrysomelinus, separately from <5 aphids/stem in June and by P. cognatus from >5 aphids/stem in July.
Fungi were important for sustaining populations of Tachyporus spp. in the absence of aphids. The apparent pattern of foraging and food preferences suggested that the presence of plant pathogenic fungi at the time of aphid infestation would reduce the impact of T. hypnorum, T.chrysomelinus and Tachyporus spp. larvae as aphid predators. T. obtusus and P. cognatus showed the highest aphid feeding potential but their dispersal into cereal fields was a limiting factor in their effectiveness under present cereal-growing methods.