Serological Estimates of the Seasonal Dynamics of Erwinia tracheiphila in Acalymma vittata (Coleoptera: Chrysomelidae)

Serological assays were used to estimate the proportion of Acalymma vittata (F.) that harbored Erwinia tracheiphila (E. F. Smith) Holland, the causal agent of bacterial wilt in cucurbits. These proportions were related to the proportion of A. vittata that transmitted disease in single beetle caged bioassays. The serological assays classified beetles as harboring the bacteria when the titer was above ≈10⁵ cells per beetle. From 7.1 to 10.7% of the A. vittata captured as they emerged from soil that had been in cucurbits the previous year tested positive for the presence of E. tracheiphila. Also, from 0 to 8.3% of beetles captured on squash traps during this time of beetle emergence tested positive. This provides strong serological evidence for A. vittata serving as the primary overwintering reservoir for E. tracheiphila. During the growing season, the proportion of beetles testing positive with serological assays varied and ranged up to ≈53, 78, and 39% in 1995, 1996, and 1997, respectively. These serological proportions were 3.6-5.1 times higher than the proportion of beetles that transmitted disease in single beetle caged bioassays, and explained 44-49% of the variation in the proportion of beetles that were able to transmit disease in the caged bioassays. We suggest the proportion of A. vittata that harbored at least some E. tracheiphila cells may be >5 times higher than the proportion of beetles that can, alone, transmit disease in a short time. We discuss these data as supporting the hypothesis of beetle aggregation behavior as an important component of bacterial wilt epidemiology.