Efficacy of imidacloprid applications to manage Asian citrus psyllid infestations on residential and retail nursery citrus in California

The Asian citrus psyllid (ACP; Diaphorina citri) has emerged recently as a major threat to citrus in the Western U.S. This invasive insect, which transmits the bacterial pathogen that causes Huanglongbing disease, is largely an urban problem in Southern California.  Management efforts in the region consist of soil applications of the systemic insecticide imidacloprid to residential trees and in retail nurseries.  While soil applications of imidacloprid have been shown to provide effective control of ACP in experimental settings, their efficacy in residential and retail nursery settings remains unresolved.

We are investigating the efficacy of imidacloprid soil applications by analyzing the relationship between the presence of ACP adults and nymphs to imidacloprid concentrations in citrus leaf tissues, as well as the relationship between imidacloprid concentration and the time since application.  Preliminarily, we found a strong negative relationship between imidacloprid concentration and probability of ACP nymph presence on residential trees, but no clear relationship between imidacloprid concentration and probability of ACP adult presence.  In retail nurseries, ACP presence was extremely rare.  For both residential and nursery trees, while imidacloprid concentrations tended to be greater in more recently treated trees, concentrations were highly variable among trees.

Our preliminary results suggest that imidacloprid soil applications may be effective at reducing ACP infestations but not adult colonization of trees in residential areas.  The treatments appear to be highly effective at reducing infestations in retail nurseries.  However, high variability of imidacloprid concentrations among trees poses a challenge to predicting how long treatments will remain effective.