Generalist predators consume spider mites despite the presence of alternative prey

Abstract

The broad feeding habits of generalist predators can lead to uncertainty about their impacts on particular pests. Due to prey switching, consumption of one pest species by generalist predators might decrease as densities of attractive alternative prey increase. To test this, we used molecular tools to examine presence/absence of two-spotted spider mites (Tetranychus urticae) in the guts of predatory Nabis and Geocoris bugs, in potato (Solanum tuberosum) crops grown using either organic or conventional farming methods. Previous work suggested that organic potato fields house relatively high densities of alternative prey, which might disrupt predators’ consumption of spider mites. While densities of spider mites and of the attractive alternative prey green peach aphids (Myzus persicae) did not consistently differ between farming systems, their densities varied widely between years and among fields within each year. The likelihood of detecting spider mite predation was not correlated with spider mite densities, such that we often recorded mite consumption in fields where these pests were rare. Spider mite consumption also was not significantly altered when we detected predation of green peach aphids. However, we found consistently higher numbers of both predator taxa in organic fields than in conventional fields. Altogether, we found that generalist predators continue to consume spider mites against a background of changing numbers of mites and aphids. This suggests that diverse alternative prey availability may not weaken the impact of robust populations of natural enemies found on organic farms.