Habitats and dispersal of the principal leafhopper vectors of Pierce's Disease in the San Joaquin Valley

Abstract

The green sharpshooter (DraeculaceXhala minerva [Ball]) and redheaded sharpshooter (Corneacephala fulgida [Nott.]), important leafhopper vectors of Pierce’s disease bacterium to grape and alfalfa, commonly inhabit permanent pastures and weeds alongside or in cropped fields. The species composition, stand, vigor, and persistence of grasses and other plants determine the extent to which sharpshooter populations can develop. Bermuda-grass (Cynodon dactylon) is a highly preferred host of both sharpshooters and watergrass (Echinochloa crusgalli) is a favorite host of D. minerva. Irrigation and weed control practices that produce succulent stands of preferred host plants thus increase sharpshooter populations. Conventional sweep net sampling was useful in phenological studies and for estimates of adult sharpshooter abundance in short, dry vegetation on warm days, but inadequate for estimates of nymphal densities. An analysis of the relationship of the average number of sharpshooters per sample and the number of samples to sampling reliability was made for both sharpshooter species. Discrete generations were determined most accurately by identifying the proportion of females with mature ovaries or the relative proportions of developmental stages, rather than by evaluating changes in the relative abundance of nymphs or adults. Adult sharpshooters disperse throughout the growing season from breeding habitats. Flights occur predominantly from 30 to 90 minutes after sunset during summer evenings when the sunset temperature is above 70°F (21°C). Draeculacephala minerva is an active flier from April through September; C. fulgida has peak flights from May through mid-September. The green sharpshooter is attracted to lights during early evening. Nearly equal numbers of males and females are usually found in breeding habitats, but female D. minerva are about five times more dispersive than males. Populations and aerial densities of both sharpshooters varied enormously according to site and year, but changes in C. fulgida populations were much more volatile than those of D. minerva. These studies suggest that the most productive strategy for preventing diseases spread by these sharpshooters is to eliminate and prevent habitats conducive to either sharpshooter, chiefly by weed control and irrigation practices.