The role of landscape context in biological control of cereal aphids

Producing enough food to sustainably meet the demands of a growing global population is one of the greatest challenges we face. In wheat, 8% of yield is lost to insect herbivores before harvest, so improving pest control would contribute significantly to food security. Given the negative effects of chemical insecticides, managing habitat to boost numbers of pest natural enemies offers a promising alternative. Recent studies highlight the importance of wider landscape context for natural enemy management, but there is uncertainty over which landscape characteristics are most important for different natural enemies, how this varies temporally, and which management strategies are worthwhile.

In this thesis novel analytical approaches using random forests were used to explore temporal and inter-specific variation in the influence of landscape context on species of aphid and hymenopterous parasitoid in winter wheat fields in the UK, and to produce models predicting the abundance of aphids, parasitoids and syrphid larvae as functional groups.

Aphid and parasitoid numbers responded strongly to the spatial configuration of vegetation parcels, both being more common in more fine-grained landscapes. Syrphid larvae were more abundant when arable land was rare within 1500 metres. Seasonal variation in landscape influence was more important than annual or inter-species differences for both aphids and parasitoids.

Map-based simulations were then performed to predict the outcome of hypothetical land-use scenarios, using a novel method based on statistical models. Displacement of non-crop vegetation by arable land, alongside increased aggregation, produced consistently undesirable results.

Estimates of the economic value of natural enemies to farmers were made, showing the potential to reduce yield loss and insecticide cost by at least £55 per hectare through beneficial habitat management. At high aphid densities, natural enemies were more valuable under insecticide free management.

Future work and implications of these results are discussed in chapter 6.