Neonicotinoid-Induced Mortality of Diaphorina Citri (Hemiptera: Liviidae) is Affected by Route of Exposure

The use of neonicotinoids in citrus (Rutaceae) has increased substantially to help manage the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), a vector of the devastating citrus disease, huanglongbing (HLB). In citrus pest management programs, neonicotinoids are most often applied to the soil as a drench and move through xylem channels from the roots into the foliage. We developed a novel assay to quantify the dose required to kill D. citri following ingestion and compare it with the dose required to kill by contact. The LC50 of the laboratory strain for ingestion of imidacloprid, thiamethoxam, and clothianidin were each approximately 10-fold greater than the respective LC50 by contact exposure. Four field populations were tested to validate comparative exposure of the laboratory strain to imidacloprid and determine the relative susceptibility of field populations to imidacloprid by exposure through ingestion and contact. The contact assay exhibited low (<10) RR50 values for the Vero Beach and Labelle populations when compared to the ingestion assay method. High (>10) RR50 values were observed for the Lake Placid and Lake Alfred populations using the contact and the ingestion method. This research demonstrates that the ingestion assay method described herein is more sensitive in detection of low-level resistance and should be the standard methodology used in monitoring for resistance to systemic insecticides for this global pest. We found D. citri populations with a lower than expected susceptibility to neonicotinoids in the field, which warrants the implementation of resistance management practices to preserve the utility of soil-applied neonicotinoids in citrus.