Use of Prune Refuges and Cultural Practices for Enhancing the Biological Control of the Grape Leafhopper
1. Impact of Prune refuges. Prune refuge management. Results of the analysis from WSFS indicate that significantly greater numbers of leafhopper eggs were laid on the modified pruning treatment during the season. The densities of eggs between treatments were highly variable early in the season, but favored the modified treatments during the later half of the season. We suspect the differences may be a response to the greater leaf densities in the modified treatments which may provide a larger number of potential oppositional sites and possibly a more favorable microhabitat. The results suggest the use of modified pruning practices may allow for greater population densities to be supported within a prune refuge as compared to conventional practices. The initial results of the irrigation study indicated no significant differences in nymph densities among the irrigation treatments, but significant differences were detected for sample date with no significant interaction among the factors. The analysis was repeated using a single factor analysis of variance for each sample date of the season (Table 1). On two dates, 19 August and 16 September, significant differences were found among the treatment means. Comparison of treatment means found trees in the highest irrigation regime had the largest nymph population (P < 0.05). We interpret these results as an indication that irrigation management practices may have an effect on the density of leafhopper nymphs supported by prune trees. The two dates where significant differences were detected correspond to the two sampling dates following cessation of irrigation treatments. Differences were probably not detected on prior dates due to insufficient time for water stress to have an effect on the leafhopper population. The lack of differences during the last sample date may be the result of water stress equally effecting all trees in the study. Longer term effects of irrigation management may be required to impact leafhopper abundance. Effect of irrigation-nitrogen management on the attractiveness of prune leaves to the prune leafhopper. This study addressed the optimum nitrogen and irrigation rates necessary to maximize the production of prune leafhoppers and Anagrus parasites. We evaluated combinations of four nitrogen rates and three irrigation rates. One year old twigs were pruned from each tree and brought to the laboratory and placed in sleeve cages containing adult prune leafhoppers. In the laboratory, the feeding preferences of the leafhoppers were recorded for each of 16 replications. Although the analyses are not yet completed, preliminary results suggest that the leafhoppers are more attracted to leaves having higher concentrations of water or nitrogen. We have not yet determined which factor has the greatest impact. Cultural practices involving cover crops. Effect of cover crops on grape yield. Results of yield comparisons among the three seasons found no differences in berry yield among the cover crop treatments. Overall berry yield varied significantly from year to year, but no significant interaction was detected between year and cover treatment. These results suggest the presence of a cover crop over several seasons does not appear to have a significant effect on yield. Effect of cover crops on Anagrus parasite and leafhopper populations. The presence of a cover crop was examined primarily for its effect on the density of variegated leafhopper (VLH) eggs and the proportion of these eggs which were parasitized. Results from the WSFS and Kearney vineyards suggest the presence of a cover crop had no effect on VLH egg densities as compared to the the non-cover treatment. Furthermore, no differences were detected in the proportion of these eggs which were parasitized. Results on the effect of the cover crop treatments on trap captures of adult insects suggests the presence of a cover resulted in lower parasite captures at WSFS and KAC. Furthermore, no significant differences in the number of grape leafhopper captures were detected among the treatments at Kearney or at WSFS. The effect on variegated leafhoppers were mixed, no differences were detected at the Kearney site while significantly greater numbers were found on vines associated with the non-cover crop treatment at WSFS. These results suggest the presence of cover crops in vine rows can have a significant effect on the number of adult parasites captured on sticky traps, although these differences do not appear to translate into differential parasitization rates among the cover crop treatments.