Optimal Management of Lepidopteran Pests of Grapes

The primary objectives of this research project are: 1) To evaluate and refine monitoring programs for western grapeleaf skeletonizer (WGLS) and omnivorous leafroller (OLR) in grapes, and 2) To determine spatial and temporal patterns of WGLS and OLR distribution in order to develop a non-preventive, true IPM program for dealing with these two serious vineyard pests. Several significant results have already been produced during the two years of this project (1992, 1993). Sex pheromone bucket traps with insecticide strips caught more WGLS moths at peak flight than did traps with ethylene glycol. OLR bucket traps with insecticide strips were somewhat more efficient than sticky bottom pheromone traps. With respect to monitoring OLR larvae, the UC-recommended bunch count technique (developed mostly on Thompson seedless), appeared to have questionable applicability for detecting OLR in tight-clustered grape cultivars (e.g., Barbera). Thus, early season OLR infestations, readily evident before berry sizing in such cultivars, later became “invisible” as the clusters developed. Generally low population levels of both WGLS and OLR during 1993 made collection of sufficient numbers in field trials a major challenge. A study comparing releases of Trichogramma parasiotoids and applications of Bacillus thuringiensis microbial insecticide against OLR which involved careful examination of 600 clusters at harvest produced only 4 OLR larvae and 5 OLR pupae. However, these same counts also revealed from 1 to 6 clubionid spiders (prob. Chiracanthium inclusum) in each grape cluster, suggesting that these predators may have been a factor in the low OLR numbers. For all the commercial vineyard blocks which were sampled in 1992 and 1993, the growers felt that chemical control was unnecessary, based on the pest monitoring data which we compiled. Thus, we have clearly demonstrated that preventive treatment for these lepidopteran pests of grapes is by no means an essential activity in San Joaquin Valley vineyards.

Optimal Management of Lepidopteran Pests of Grapes

The primary objectives of this research project are: 1) To evaluate and refine monitoring programs for western grapeleaf skeletonizer (WGLS) and omnivorous leafroller (OLR) in grapes, and 2) To determine spatial and temporal patterns of WGLS and OLR distribution in order to develop a non-preventive, true IPM program for dealing with these two serious vineyard pests. During 1992 (first year of project), several significant results were produced. Sex pheromone bucket traps with insecticide strips caught more WGLS moths at peak flight than did traps with ehtylene glycol. OLR bucket traps with insecticide strips were somewhat more efficient than sticky bottom pheromone traps. With respect to monitoring OLR larvae, the UC recommended bunch count technique (developed mostly on Thompson seedless), appeared to have questionable applicability for detecting OLR in tight-clustered grape cultivars. Thus, early season OLR infestations, readily evident before berry sizing in such cultivars, later became “invisible” as the clusters developed. For all the commercial vineyard blocks which were sampled in 1992, the grower did not feel it was necessary to use chemical control at any time of the season, based on the pest monitoring data which we compiled. Thus, although there was no opportunity for comparing relative efficacies of alternative insecticide materials during 1992, we were certainly able to demonstrate that preventive treatment for these lepidopteran pests of grapes is by no means an essential activity in San Joaquin Valley vineyards.

Minimizing Fertilizer Inputs for Variegated Leafhopper Management in Grapes

This research investigated the relationship between host plant (i.e,, grapevine) nitrogen status and population dynamics of the variegated leafhopper (VLH) in a commercia1 vineyard for three years. During each year, replicated fieli plots received the following treatments: 1) control (no fertilizer added), .’) 75 lbs. N / acre (synthetic ammonium nitrate), 3) 150 lbs. N / acre (synthetic ammonium nitrate), and 4) two tons of compost / acre ( = 38 lbs. N / acre). The response of VLH to these treatments was investigated by monitoring nymph densities and conducting oviposition (egg-laying) and nymphal development rate tests. During all three years, VLH nymph populations reached a seasonal peak during the first generation. Nymph densities during the second and third generations were especially low in 1989 and 1990, and reached a moderate level during the second generation of 1991. Oviposition tests conducted in July 1989 and June 1990 demonstrated greater VLH egg-laying in the synthetic fertilizer plots. However, oviposistion during the July 1991 test was greatest in the compost plots. No significant differences in VLH nymphal development rates were observed among treatments.

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.