Volume59 Number1 January1991 L Aphelopzcs albopictzcs Ashmead (Hymenoptera: Dryinidae): Abundance, Parasitism, and Distribution in Relation to Leafhopper Hosts in Grapes L. T. Wilson, I. Carmean, and D. L. Flaherty ABSTRACT ApheZopus albopictus Ashmead (= A. comes; Fenton), a dryinid nymphal-adult parasite, has been reported as a major mortality agent for grape leafhopper, Erythroneura eZegantuZa Osborn, in the San Joaquin Valley of California. Parasitism levels varied widely among different vineyards. Our studies report up to 33% parasitism of adult grape leafhoppers, with levels considerably less during much of the season. In comparing the efficiency of the D-Vac and yellow sticky cards for monitoring the abundance of A. albopictus, parasitized grape leafhopper adults, parasitized variegated leafhopper adults, and nonparasitized adults of both leafhopper species, it appears that sticky yellow cards are less efficient at capturing A. albopictus parasitized grape leafhopper adults than adults of either leafhopper. The two sampling methods did not correlate well for adult A. albopictus. The within-vine distribution of A. albopictus was consistent with that reported in the literature for the nymphal stages of the variegated leafhopper, with the majority recorded in the more shaded areas within the canopy of the distal third of the canes. However, grape leafhopper and variegated leafhopper adults were not captured in greater frequency on shaded leaves. The variegated leafhopper, which was abundant at one of the study sites, is rarely parasitized by A. albopictus. Chemical intervention for control of this pest will likely increase, resulting in disruption of the normally high level of biological control directed against the grape leafhopper. THE AUTHORS: L. T. Wilson was Professor of Entomology, Texas A&M University, College Station, at the time of this research and was on leave as Professor of Entomology, University of California, Davis. I. Carmean, formerly Postgraduate Research Associate, University of California, Davis. D. L. Flaherty is Farm Advisor, University of California, Cooperative Extension, Tulare. L. T Wilson, I. Carmean, andD. L, Flaherty Aphelopus albopictus Ashmead (Hymenoptera: Dryinidae): Abundance, Parasitism, and Distribution in Relation to Leafhopper Hosts in Grapes1 INTRODUCTION Two IMPORTANT PESTS of grapes in California’s San Joaquin Valley are the grape leafhopper, Erythroneura elegantula Osborn (Homoptera: Cicadellidae), and the varie- gated grape leafhopper, E. uariabilis Beamer. The grape leafhopper is native to wild grapes in the San Joaquin Valley, while the variegated leafhopper has recently moved into the Valley (Kido et al. 1984),likely from either southern California or southwest- ern Arizona. Damage by both leafhoppers is caused by nymphs and adults feeding on the foliage and by fecal spotting of the grapes. High densities of these insects may also annoy vineyard workers during harvest (Flaherty et al. 1981 ). Two parasites are known to attack the grape leafhopper in San Joaquin Valley vineyards. The first is an egg parasite, Anagrus epos (Girault) (Hymenoptera: Mymaridae), and the second is a nymphal-adult parasite, Aphelopus albopictus Ashmead ( = A. comesi Fenton) (Hymenoptera: Dryinidae). Anagrus normally provides eco- nomic control of the grape leafhopper over much of California’s grape acreage. Control is particularly evident on grape cultivars grown near streams that contain habitats for overwintering hosts. Although extensive work has been conducted on parasitism by Anagrus (Flaherty et al. 1985; Settle et al. 1986; Pickett et al. 1987, 1989),little work has been directed at elucidating the role of A. albopictus in controlling the grape leafhopper. However, the potential for A. albopictus to exert a considerable degree of control was shown by Cate (1975) who found up to 77% parasitism of adult grape leafhoppers in the San Joaquin Valley. To better understand the role of A. albopictus in regulating grape leafhopper population density, several related studies were conducted in 1984 and in 1985. Specifically, our objectives were to ( 1)obtain basic information on the seasonality of A. albopictus and its two potential leafhopper hosts in different vineyard systems; (2) estimate parasitism rates by A. albopictus towards both leafhopper species under a range of environmental conditions; and ( 3) develop quantitative information on the distribution of A. albopictus in grapes, ’Accepted for publication January 15, 1991 2 Wilson, Carmean, and Flaherty: Aphelopus albopictus Ashmead Abundance.. METHODS AND MATERIALS D-Vac sampling During 1984, weekly D-Vac samples (Dietrick et al. 1959; Dietrick 1961) were taken at two test sites; one at the University of California, Kearney Agricultural Center (KAC), near Parlier, California, the other at a commercial table grape vineyard near Exeter, California. A mature ‘Thompson Seedless’vineyard was used at KAC. One-half of the vineyard was managed for raisin production, using flood irrigation and conven- tional clean cultivation, complete tillage, while the other half was managed for table grape production, using flood irrigation with a mowed weed cover crop. A mature ‘Emperor’ table grape vineyard was used at Exeter. This vineyard was furrow irrigated and had a mowed grass cover crop. For both sites, on each sampling date, 60 l-ft* (0.093 m2) suctions of the D-Vac, 30 from each side of the row, were taken from the more shaded areas of the vines. At the KAC vineyard, 60 D-Vac suctions were taken from the same areas of the vines from each treatment. Samples were returned to the laboratory and frozen for later counting under a low power dissecting microscope. The number of grape leafhopper adults, variegated leafhopper adults, parasitized adults for each species, and male and female A. albopictus was recorded. Yellow card sampling Stikem (Seabright Enterprises, Emeryville, California) treated 3-in. X 5-in. (7.6 cm X 12.7 cm) yellow cards were placed within the vineyards during 1984 and 1985. At all vineyards, during both years, yellow cards were changed weekly, with the same information recorded as with the D-Vac sampler. During 1984, sampling was con- ducted for each treatment within the KAC vineyard, Three yellow cards were placed in each of three randomly chosen vines, within both the clean cultivated and the mowed weed cover crop treatments. One card was placed within the canopy near the trunk, one within the canopy one-half of the way to the end of the cane, and one within the canopy of the most distal part of the cane; nearest where the canes from the adjoining vine meet (cane-pruned vines; see Flaherty et al. 1981). This method of card placement was also used at the Exeter site, except that four replicates were maintained. A second sampling method was also used at the Exeter site for 1 month, during which peak leafhopper activity had been predicted to occur (Flaherty et al. 1981).Ten yellow cards were placed within a single randomly chosen vine within each replicate. Five cards were placed in the trunk region, and five about two-thirds of the way towards the end of the canes. The five cards in each region were placed individually in each of five locations: ( 1) above the canopy (this area was usually sunny); (2) below the canopy (always shady); (3)in the center of the canopy (always shady); (4)on the north side of the vine (always shady); and (5)on the south side of the vine (usually sunny). During 1985, yellow cards were placed within each replicate of the Exeter vineyard. Three additional vineyards were also used: one at the KAC; one at the University of California, Westside Field Station (WSFS),near Five Points, California; and one in the Sierra Nevada foothills (about 2,000-foot elevation) east of Exeter. The Sierra foothill HILGARDIA Vol. 59 No. 1 January 1991 3 site consisted of native grapes (Vitis californica Bentham) growing alongside a small perennial stream. For each treatment or replicate within each of the four vineyards, a single yellow card was placed in full shade, near the distal third of the canes, on three randomly chosen vines. Leafhopper and parasite generation development Estimates of generation time for A. albopictus are based upon an accumulation of heat units above a lower developmental threshold of 10.28OC (50.5OF) estimated by Cate (1975) for E. elegantula. This threshold is very close to the 10.5OC estimated by Mochizuki (1984) for the variegated leafhopper. Cate (1975) chose to use a 12OC developmental threshold for A. albopictus, and for a second grape leafhopper parasite, Anagrus epos, based on Campbell et al. (1974), concluding that parasites often have developmental thresholds that are higher than that of their host(s). For the purpose of this study, and in the absence of developmental data for A. albopictus, the 10.28OC lower developmental threshold will be used for both A. albopictus and for the two leaf- hopper species, with degree-day (OD) accumulation beginning January 1 of each year. Data analysis The Chi-square goodness-of-fit-test statistic was used in analyzing A. albopictus sex ratio data. The detailed within-vine data from the yellow sticky cards collected during 1984 from the Exeter site were analyzed using two separate nonreplicated factorial analyses of variance (Zar 1974; Steel and Torrie 1980). For the first analysis, the effects of two main factors were examined: region (trunk vs. the distal third of canes) and location (above, below, center, north, and south as described previously). For the second analysis of variance, two factors were again examined: region and exposure (shade vs. exposed leaves). For this analysis, the center yellow card data were excluded to provide balance with respect to shaded and exposed leaves. Differences between means were tested using Duncan’s new multiple range test (Duncan 1955). The relative efficiency of the D-Vac and yellow sticky card sampling methods at capturing adult grape leafhoppers, adult variegated leafhoppers, A. albopictus adults, and parasitized grape leafhopper were compared using the 1984 KAC and Exeter data separately and combined.