Arthropod Predation in Brassica Agroecosystems: Effects of Latitude, Community Composition, and Diet Breadth

Abstract

Arthropod predators provide crucial pest management services by consuming herbivore prey in agroecosystems. Yet, variation in arthropod predation strength among cropping systems and regions can prevent farmers from taking advantage of this alternative pest management strategy. This research examines underexplored potential causes of variation in arthropod predation in agroecosystems. Arthropod predation increases at lower latitudes. However, it is unknown whether this gradient extends to agroecosystems. Diet breadth of an arthropod predator can influence whether a predator can adequately control resident herbivore populations and can impact predation in a community context by determining whether predatory taxa will compete over shared prey, attack each other as intraguild prey, or partition herbivore taxa into distinct prey niches. To assess the effect of latitude, I first compared predation rates on live and artificial sentinel prey in Brassica agroecosystems between the tropical Federal District, Brazil and temperate Minnesota, United States. Contrary to expectations, I found that predation rates on all bait types were similar between the two localities and that reduced predation rates in the Federal District may be related to higher prey densities. Next, to further explore latitudinal effects, I assessed predation rates in Brassica agroecosystems across 15 degrees of latitude in the United States and 21 degrees of latitude in Brazil. Surprisingly, my results revealed a reverse predation gradient whereby arthropod predation increased with latitude in both countries. To examine the role of diet breadth, I first tested methods to improve molecular gut content analysis of arthropod predators by verifying broad metabarcoding results with species-specific melting curve analysis. This study documented common false positive and false negative taxonomic results and suggested that a species-specific verification step is necessary to ensure accurate depictions of arthropod trophic interactions. Lastly, I use the results of the gut content analysis to characterize the diet breadth of three coccinellid predator species (Coleomegilla maculata, Hippodamia convergens, and Harmonia axyridis) collected from a Brassica agroecosystem in Minnesota. I found that both herbivore and intraguild prey consumption were common among coccinellid species, but that C. maculata was least likely to engage in intraguild predation of fellow coccinellids.