Evaluation of the possibility for mechanical transmission of powdery mildew conidia by a mycophagous coccinellid

The coccinellid tribe Halyziini consists entirely of obligate mycophages. The adults and immature stages of all described species are known to feed only on the hyphae and conidia of various powdery mildew (PM) fungi (Erysiphales). While there is definite promise in the utilization of these insects for augmentative biological control, disease indication and decision support, there are also questions regarding the potential for mechanical disease transmission. Microscope observations reveal that individuals removed from feeding sites tend to harbor asexual spores, or conidia, of PM on their bodies, and since adult beetles readily disperse among plants through flight it is feasible that disease inoculum could be moved from infected plants to uninfected. To test this possibility, several growth chamber experiments were initiated. First, plots of Zinnia elegans with similar heterogeneous disease severity were monitored for disease transmission rate and severity over time either in the presence or absence of adult Psyllobora vigintimaculata (Coleoptera: Coccinellidae). Next, plots of uninfected zinnia were similarly monitored for disease incidence, severity and transmission rate after either being inoculated through the release of spore-laden adult P. vigintimaculata or by aerial spore introduction. The results directly address the phenomenon of manual transmission in this system and raise implications for successful biological control.