Tree species richness around urban red maples reduces pest abundance but does not enhance biological control

Pest outbreaks are rare in forests where many tree species can host resources that support insect natural enemies and biological control services. In cities, trees are often planted in isolation or monoculture and pest outbreaks are common in such settings. Gloomy scale—Melanaspis tenebricosa (Comstock)—is a pest of urban red maple that is abundant where impervious surfaces are extensive. Planting more tree species around urban red maples may help regulate gloomy scales by supporting natural enemy populations. We investigated the effect that surrounding tree species richness and canopy cover had on gloomy scale abundance, natural enemy abundance, and biological control in red maple trees in Raleigh, NC. USA. We collected scales and natural enemies from 95 and 90 red maples respectively that spanned a gradient of tree species richness, canopy cover, and impervious surface values. We also measured gloomy scale parasitism and removal and predation of sentinel prey in a subset of these trees. Tree species richness and canopy cover interacted to negatively affect gloomy scale abundance, but these predictors were inversely related to impervious surface cover. Therefore, the negative influence of tree diversity on gloomy scales may be partially attributed to a lack of impervious surfaces around trees in diverse settings. We also found that natural enemies were less abundant in trees in diverse settings. Finally, tree species richness did not influence the outcome of any biological control experiment. Our findings suggest that tree diversity may reduce gloomy scale abundance, but not through natural enemy recruitment.