Effects of Cold-Acclimation, Pathogen Infection, and Varying Temperatures on Insecticide Susceptibility, Feeding, and Detoxifying Enzyme Levels in Diaphorina citri (Hemiptera: Liviidae)

Abstract

Infection of Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), with 'Candidatus' Liberibacter asiaticus (Las), the causal pathogen of citrus greening disease or huanglongbing, increases psyllid susceptibility to insecticides. Diaphorina citri populations in citrus occur year-round in tropical and sub-tropical habitats, and thus insecticide applications for managing this plant disease vector occur over a wide temperature range (10-40°C). During the winter season, D. citri is occasionally exposed to periods of freezing temperatures, when temperatures fall below -6.5°C. In this investigation, we compared insecticide susceptibility of uninfected and Las-infected D. citri at various temperatures (20-37°C). Cold-acclimated (6 ± 1°C) D. citri adults were less susceptible to neonicotinoid insecticides as compared with non-acclimated controls, but this trend was not observed for other insecticides tested. A positive correlation between temperature and percentage mortality caused by chlorpyriphos, imidacloprid, spinetoram, and thiamethoxam was found irrespective of infection status when evaluated at temperatures ranging between 20 and 37°C. In contrast, a negative correlation between temperature and percentage mortality was observed for fenpropathrin for both infected and uninfected psyllids. Glutathione S-transferase levels were negatively correlated with temperature, whereas levels of cytochrome P450 and general esterase were not correlated with temperature fluctuations. These results indicate that altered insecticide susceptibility due to temperature may not be related to glutathione S-transferase, cytochrome P450, and general esterase levels. Diaphorina citri adults that carried the Las bacterium had reduced CYP4 transcript and protein levels, and ingested less than uninfected counterparts, as measured by the production of honeydew. Diaphorina citri adult feeding was greatest at 32°C within the temperature range tested. Overall, annual temperature fluctuation does not appear to be a major factor impacting management of D. citri.