Seasonal movement and distribution of the grape mealybug (Homoptera : Pseudococcidae): Developing a sampling program for San Joaquin Valley vineyards

The grape mealybug, Pseudococcus maritimus (Ehrhorn), is an important pest of table grapes in California's San Joaquin Valley. The mealybug causes dir ect damage by infesting grape bunches, resulting in very low economic injur y levels. To develop a sampling program to help growers predict damage and make control decisions, we destructively sampled six entire grapevines each month to determine mealybug abundance and within-vine distribution. These absolute counts were then used to evaluate several relative sampling method s: sticky tape barriers on canes, excised spur samples, standard-sized piec es of bark, timed counts, and nondestructive counts on spurs. At midseason we sampled additional vines to correlate mealybug numbers with economic dam age at harvest. Finally, mealybug life stages and natural enemies were reco rded throughout the study. Timed 5-min counts show the strongest correlatio n with total mealybug numbers, and a simple count of mealybugs on three spu rs per vine at midseason is the best predictor of economic damage. Mealybug s completed approximate to2.5 generations in 1998. Comparison to data on me alybug development suggests that exceptionally long growing seasons could e xacerbate infestations by allowing the completion of a third generation. No mealybugs were found on bunches before early August, when second-generatio n crawlers moved out of the bark. Grape bunches that touched old wood had s ignificantly higher damage and mealybug densities. The majority of mealybug s were always found in protected locations (under the bark of the trunk, sp urs or canes), indicating the need for chemical or biological controls that can penetrate these refugia.