Sharing the burden: Cabbage stem flea beetle pest pressure and crop damage are lower in rapeseed fields surrounded by other rapeseed crops

The cabbage stem flea beetle (Psylliodes chrysocephala) is a significant pest of rapeseed (Brassica napus). Feeding by adult P. chrysocephala can cause severe leaf damage and larval infestation can reduce stem strength, both of which impact crop growth and development, causing substantial yield losses and economic damage. The structure of the agricultural landscape can regulate herbivorous pest populations through top-down and bottom-up processes. This has shown promise in regulating the populations of other herbivorous pests, but remains relatively unexplored for P. chrysocephala. Here we investigate how the structure of the agricultural landscape influences P. chrysocephala abundance (pest pressure) and associated crop damage. We also examine the effect of the landscape on natural enemies and their ability to regulate P. chrysocephala populations. We show that P. chrysocephala populations are primarily regulated through bottom-up processes. We identify adjacency to another rapeseed crop and the total proportion of rapeseed grown in the landscape as key factors influencing beetle pressure, crop damage, and larval infestation, but find no effect of host crop proportions grown in the previous year at the examined scales up to 1 km surrounding focal crops. We also observe positive effects of crop heterogeneity and semi-natural habitat proportions on natural enemy abundance and diversity; however, these increases had no direct impact on P. chrysocephala. Bottom-up processes appear to contribute to herbivorous pest regulation by diluting beetles in the landscape, and could represent an important mechanism for sustainably managing pest populations by adapting the proportions and neighbourhoods of rapeseed crops at small to large spatial scales

Low prevalence of secondary endosymbionts in aphids sampled from rapeseed crops in Germany

Peach-potato aphids, Myzus persicae Sulzer (Hemiptera:Aphididae), and cabbage aphids, Brevicoryne brassicae Linnaeus (Hemiptera:Aphididae), are herbivorous insects of significant agricultural importance. Aphids can harbour a range of non-essential (facultative) endosymbiotic bacteria that confer multiple costs and benefits to the host aphid. A key endosymbiont-derived phenotype is protection against parasitoid wasps, and this protective phenotype has been associated with several defensive enodsymbionts. In recent years greater emphasis has been placed on developing alternative pest management strategies, including the increased use of natural enemies such as parasitoids wasps. For the success of aphid control strategies to be estimated the presence of defensive endosymbionts that can potentially disrupt the success of biocontrol agents needs to be determined in natural aphid populations. Here, we sampled aphids and mummies (parasitised aphids) from an important rapeseed production region in Germany and used multiplex PCR assays to characterise the endosymbiont communities. We found that aphids rarely harboured facultative endosymbionts, with 3.6% of M. persicae and 0% of B. brassicae populations forming facultative endosymbiont associations. This is comparable with endosymbiont prevalence described for M. persicae populations surveyed in Australia, Europe, Chile, and USA where endosymbiont infection frequencies range form 0-2%, but is in contrast with observations from China where M. persicae populations have more abundant and diverse endosymbiotic communities (endosymbionts present in over 50% of aphid populations)