Abstract

There is considerable potential to manage field margins to increase pest control by natural control agents and, in addition, to enhance biodiversity. This project aimed to develop management strategies for enhancing biological control of aphid pests in field crops, allowing farmers to fulfil their environmental commitments without jeopardising profitable crop production.

Strategies for the manipulation of aphid parasitoids, using aphid pheromones, and of hoverflies, by establishing wild flowers in field margins, were developed and tested on commercial cereal fields at four sites, with pilot trials in several vegetable crops in the final year. Data from cereal trials clearly demonstrated the importance of early parasitoid activity for summer aphid control. Use of an aphid pheromone stimulated rapid spread of parasitoids into cereal crops in spring to coincide with aphid invasion, significantly reducing aphid numbers. Flower-rich margins also significantly reduced cereal aphid numbers in many site/years, providing essential food for female aphidophagous hoverflies, especially Episyrphus balteatus, which then layed their eggs in the crop near aphid colonies. Hoverflies played an important role in maintaining control of pest aphid numbers, the effect being greatest after the impact of parasitoids (an early season control agent) began to wane in mid-summer. Thus, the effects of parasitoids and hoverflies were comlementary and together significantly reduced aphid population growth rates. Pitfall trap catches of the carabid beetle Harpalus rufipes appeared to be increased by the aphid pheromone in some site/years. There was no apparent effect of the pheromone on parasitoid activity or aphid populations in any of the vegetable crops investigated, although parasitoid numbers were very low in some of these trials. Further trials using pheromones more closely matched to those produced by the main vegetable aphid species are recommended. Flower-rich margins appeared to increase parasitoid impact on aphids on organic broccoli.

The foraging and oviposition behaviour of the hoverfly Episyrphus balteatus was also studied in the laboratory. The attractiveness of flowering plants to hoverflies was positively associated with the number of eggs that females subsequently produced. A range of UK native plant species were found to be equally or more attractive to hoverflies when compared to the non-native Phacelia tanacetifolia that is widely quoted in the literature as promoting hoverfly populations near arable crops. In particular, a range of umbellifer species, yarrow and white campion were highly attractive to E. balteatus. Provision of these species in managed field margins would provide a plentiful supply of high quality pollen and nectar at the critical point in hoverfly life cycles. E. balteatus females were attracted to aphid-infested wheat plants for oviposition, their searching behaviour resulting in a preference for larger plants, similar to those on which damaging aphid populations periodically occur in the summer.

Large-scale, spatio-temporal dynamics and movement of beneficial insects was investigated, including the influence of some biotic factors. Beneficial invertebrates were sampled using pitfall traps, in conjunction with measurements of plant cover and soil moisture, to investigate within-year and between-year changes in spatial distribution. The spatial distribution of most ground-dwelling predators was significantly clustered into patches and for some species these extended across field boundaries. For most species the location of patches and gaps remained consistent within the same year but was less consistent between years. Numbers of predatory invertebrates peaked in early July and then started to decline, but in July were more abundant in peas than in cereal crops. Many species of ground-dwelling predators were positively associated with weed cover but there was an optimum level of weed cover beyond which predator numbers declined. Soil moisture strongly influenced the survival of beetle larvae overwintering within fields and an optimum level was found. Measurements of beetle emergence highlighted the importance of arable soils as an overwintering site. Within one field the average density was 157 predatory beetles m-2.

Large scale mark-release-recapture experiments with several carabid beetles showed that although they could move between fields the majority remained within the field where they emerged. Field margins/boundaries containing tussocky grasses encouraged predatory beetle species that overwinter as adults, and their early spread into the crop complemented the initial impact of parasitoids on colonising aphid populations. Set-aside margin strips, although not sown with a plant mixture designed to encourage beneficial invertebrates, reduced the abundance of cereal aphids in one of two years. They had almost no effect on the invertebrates within the crop, but for some groups their numbers varied with distance from the field edge. There is potential to develop plant mixes for set-aside that will improve biocontrol. A margin cost calculator was developed that will allow farmers to calculate the cost of establishing different types of margins on their farms based upon income foregone and agri-environment payments. The distribution of pea aphids was highly ephemeral but predatory beetles contributed to their control.

A molecular PCR test was developed to detect aphid remains in the guts of polyphagous predators. Aphid-specific bands were still detectable in spiders 8 hours after they had fed on an aphid. Analysis of field-collected spiders revealed that they fed on aphids with equal efficiency up to 100m into the crop. Around 15-25% of money spiders collected in cereal crops had fed on aphids, whilst as much as 88% of those collected from a pea crop had fed on pea aphids. 21% of large carabid beetles (Pterostichus spp.) collected in cereal fields had consumed aphids; 23% collected from fields with a set-aside strip and 18% from fields without a set aside strip. The proportion of beetles that had consumed aphids was not significantly affected by distance from the margin, at least up to 100m, regardless of the presence of a set-aside strip.

HGCA Project Number: 2238
Price: £14.00