Dynamics of short- and long-term association between a bacterial plant pathogen and its arthropod vector

Abstract

The dynamics of association between pathogens and vectors can strongly influence epidemiology. It has been proposed that wilt disease epidemics in cucurbit populations are sustained by persistent colonization of beetle vectors (Acalymma vittatum) by the bacterial phytopathogen Erwinia tracheiphila. We developed a qPCR method to quantify E. tracheiphila in whole beetles and frass and used it to assess pathogen acquisition and retention following variable exposure to infected plants. We found that (i) E. tracheiphila is present in frass in as little as three hours after feeding on infected plants and can be transmitted with no incubation period by vectors given brief exposure to infected plants, but also by persistently colonized vectors several weeks following exposure; (ii) duration of exposure influences rates of long-term colonization; (iii) frass infectivity (assessed via inoculation experiments) reflects bacterial levels in frass samples across time; and (iv) vectors rarely clear E. tracheiphila infections, but suffer no apparent loss of fitness. These results describe a pattern conducive to the effective maintenance of E. tracheiphila within cucurbit populations.

Figure 1. Frass deposition on leaves of C. pepo ssp. texana after cucumber beetle feeding.

Figure 2. Percentages of frass samples and whole beetles that were positive for Erwinia tracheiphila immediately (0 days), 5 days, and 28 days post exposure.

(2a) Frass samples following 3 h exposure; (2b) Frass samples following 24 h exposure; (2c) Whole-beetle samples following 24 h exposure.

Figure 3. Distribution (variance) of relative E. tracheiphila/totalDNA ratios in frass (3a) and whole beetles (3b) immediately (0 days), 5 days, and 28 days post exposure.

Figure 4. Relative ratios of EtOMP/18S for whole beetles (left) and beetle frass (right) immediately (0 days), 5 days, and 28 days post exposure.

Figure 5. Occurrence and progression of bacterial wilt symptoms in C. pepo ssp. texana inoculated with frass collected from beetles immediately (0 days), 5 days, and 28 days post exposure.