Love thy neighbors? Beneficial and pest arthropod populations in a pear and cherry orchard landscape

Abstract

Neighboring vegetation in an agricultural landscape can impact pest and beneficial arthropod populations in tree fruit orchards. In a pear monocrop region (such as Wenatchee WA, northwestern United States), the concern is that specialist pests can migrate from orchard to orchard and maintain an area-wide high population. In a mixed landscape region (such as Hood River OR, NW USA), the concern is that chemical control measures against invasive pests in nearby cherry orchards can disrupt biological control in pear orchards by killing beneficial arthropods. Specifically, we asked whether surrounding pear and cherry area cover (% in 1 km2), presence of adjacent cherry orchards, and pesticide impact scores influenced the abundance and richness of common pests (pear psylla and two-spotted spider TSS mites) and beneficial arthropods (the parasitoid wasp Trechnites insidiosus and generalist predators) in pear orchards in OR and WA. Pests and beneficial arthropods were collected in pear plots in OR and WA in 2016–2018 using a combination of methods (sticky cards, beat trays, leaf samples). A generalized linear mixed model revealed that surrounding pear cover area did not affect any of the outcome variables. Surrounding cherry cover had variable effects on pest and beneficial abundance in different regions and years, such as a negative correlation with predator abundance (OR 2016), a positive correlation with parasitoid abundance (WA 2017), and higher pear psylla abundance in cherry-adjacent pear plots (OR 2018). Furthermore, a quantile regression showed that pear orchards with low surrounding cherry cover had high variance in parasitoid abundance, while pear orchards with high surrounding cherry cover had consistently low parasitoid abundance. Pesticide impact scores negatively influenced predator and parasitoid abundance and beneficial richness in WA, but not in OR. Pear plots with higher pesticide impact scores had higher TSS mite abundance (in both OR and WA 2018), but did not affect pear psylla abundance. A redundancy analysis (RDA) showed that beneficial population assemblages tended to cluster away from pear plots with high pesticide impact scores. This study shows how landscape and chemical control practices can have variable impacts on pear orchards located in different regions.