How to Manage Pests
UC Pest Management Guidelines
The grape leafhopper is a pest of grapes north of the Tehachapi Mountains, especially in the San Joaquin, Sacramento, and North Coast valleys. It is also a problem in warmer, interior Central Coastal valleys. The variegated leafhopper is the major pest of grapes in southern California and in the Central Valley as far north as San Joaquin County.
Leafhoppers overwinter as adults and are found in spring on basal grape leaves and weeds. The adult grape leafhopper is about 0.12 inch (3 mm) long and light to pale yellow with distinct dark brown and reddish markings. Eggs of the first brood are laid in epidermal tissue on the underside of the leaves in April and May and appear as a bean-shaped, blisterlike protuberance that is slightly less than 0.04 inch (1 mm) long. Although similar in size to the grape leafhopper, the variegated leafhopper is darker in color and distinctly mottled brown, green, and white with a reddish tinge. The nymphs are almost transparent when first emerged, becoming orange-brown to yellow-brown, in contrast to the white nymphs of the grape leafhopper. Eggs are similar in appearance to the grape leafhopper but laid deeper within the leaf tissue. This latter characteristic reduces the effectiveness of the egg parasite against variegated leafhopper.
Nymphs and adults of both species remove the contents of leaf cells, leaving behind empty cells that appear as pale yellow spots or stippling. If populations are high, the entire leaf may be pale yellow or white. Loss of leaf efficiency and leaf drop can occur when leafhopper densities are extremely high. This can result in fruit sunburn and may delay fruit ripening, especially in young vines. If there is a significant reduction in the overall photosynthetic capacity of the vine, young or stressed vines may have less shoot growth the following season.
The accumulation of small droplets of excrement on berries and the associated growth of sooty mold results in berry spotting that is a concern in table grapes. Adult leafhoppers are also a nuisance to workers when populations are high at harvest time. Their excrement appears as minute, sticky clumps that darken with age.
Although leafhoppers infest most vineyards in California, they may not require chemical treatment because vines can tolerate fairly high populations without harm, and predators and parasites may be able to maintain leafhopper populations below tolerance levels. In coastal regions and the Central Valley, however, grape leafhopper populations may occasionally reach damaging levels and require treatment. If chemical control of leafhopper is necessary, wait until the second (summer) generation, whenever possible, before treating.
General predators of grape leafhoppers include spiders, green lacewings (Chrysopa spp.), minute pirate bugs (Orius spp.), lady beetles (Hippodamia spp.), and predaceous mites. The predaceous mite, Anystis agilis, is an important predator of first instar nymphs especially in the North Coast. Although many growers have experimented with releases of lacewings for leafhoppers, control of economic populations has not been achieved in university field trials.
If the vineyard is accessible before budbreak and erosion is not a risk, remove weeds in vineyards and surrounding areas before vines start to grow in spring to reduce adult leafhopper populations that might disperse to new grape foliage.
Organically Acceptable Methods
Monitoring and Treatment Decisions
How to monitor:
Continue monitoring weekly until harvest. Starting at bloom, combine leafhopper monitoring with monitoring for spider mites and mealybugs see MONITORING INSECTS AND SPIDER MITES.
Treatment thresholds. Treatment thresholds vary according to leafhopper generation; whether grapes are being grown for table, wine, or raisin use; canopy size; region; and degree of parasitization. A level of 10-30% parasitism on eggs of the first generation may result in economic control of the grape leafhopper during the second and third generations. However, if the leafhopper population is made up primarily of the variegated leafhopper, economic control by this parasite is less likely, although a combination of parasite and predator activity can be effective. Use the general guidelines below to help determine treatment needs. If treatment is necessary, removing basal leaves will allow better spray coverage and thus improve pesticide efficacy.
Wine and raisin Thompson Seedless grapes. For the first generation, treatment is not necessary if 20 or fewer nymphs per leaf are found. If Anagrus is active on eggs of the first generation, it is best not to treat unless leafhopper numbers are well above 20 per leaf. Also helpful is the removal of basal leaves before adults of the first generation appear, as described under CULTURAL CONTROL, to allow better spray coverage and thus improve efficacy of the pesticide. If you have to treat, wait until more than half the nymphs are in the third instar; this allows sufficient time for most eggs to have hatched.
For the second or third generation on wine and raisin Thompson Seedless grapes, the treatment threshold is 15 to 20 nymphs per leaf. Generally lower populations do not need treatment. However, coastal wine grapes with a low incidence of parasitism and small canopies may have a threshold of 10 to 20 nymphs per leaf. Vigorously growing vines can support higher populations.
Table grapes. Treatment level is lower for table grapes because they need better fruit protection. For the first generation, treat if more than 15 leafhopper nymphs per leaf are found. In the second and third generations, early varieties (Flame Seedless) should not exceed 10 nymphs per leaf; midseason varieties (Thompson) 5 to 10 nymphs per leaf; and late varieties (Emperor) 5 to 8 nymphs per leaf. Large populations of adult leafhoppers in the fall are very annoying to workers who are hand harvesting grapes. A treatment just before harvest may be warranted if adult populations are high.
UC IPM Pest Management Guidelines: Grape
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