Insect Frass as a Pathway for Transmission of Bacterial Wilt of Cucurbits

Insects that vector diseases of plants are of critical concern to agriculture, but relationships between the vectors and pathogens often are poorly understood. In this study, we present research on vector relationships between the striped cucumber beetle, Acalymma vittatum (F.) (Coleoptera: Chrysomelidae), and the pathogen that causes bacterial wilt of cucurbits, Erwinia tracheiphila (Smith) (Enterobacteriales: Enterobacteriaceae). We studied how the bacteria were retained in the gut of the beetle by developing a polymerase chain reaction (PCR)-based technique for extracting and identifying bacterial DNA in the frass. Bacterial DNA usually was present in the frass for 24 h after beetles had consumed inoculum but diminished quickly and was undetectable within 96 h. The amount of time that bacterial DNA could be detected in frass increased with the amount of inoculum and the length of time that beetles were exposed to inoculum and also varied with the strain of bacterium. Frass that tested positive for bacterial DNA also was infective to cucumber plants, confirming that DNA was indicative of viable bacteria and that frass could be a pathway for transmission of the pathogen. This research suggests that few cucumber beetles serve as long-term vectors of the pathogen and that aggregation of the beetle on host plants may be critical for initiating plant infections in spring.