Description of the Pest

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. Variegated leafhopper is also found in Napa County, although grape leafhopper tends to be more common. Virginia creeper leafhopper has been sporadically detected in vineyards in northern Sacramento Valley, northern Sierra foothill counties, and in Lake and Mendocino counties.

Leafhoppers overwinter as adults and are found in spring on basal grape leaves and weeds.

Grape Leafhopper

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 are laid singly in epidermal tissue on the underside of leaves and appear as a bean-shaped, blisterlike protuberance that is slightly less than 0.04 inch (1 mm) long. Eggs of the first brood are laid in April and May on basal leaves. Nymphs are white with six pale yellow spots on the thorax and clear eyes.

Variegated Leafhopper

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. Eggs are laid singly in the epidermal tissue of both upper and lower leaf surfaces. Eggs, 0.03 inch (0.8 mm) long, are inserted adjacent or within leaf veins and are imbedded deeply within the leaf tissue. This latter characteristic reduces the effectiveness of egg parasites against the variegated leafhopper.

Virginia Creeper Leafhopper

Adults have reddish-brown zigzag markings on the wings and reddish-brown eyes. Bean-shaped eggs are usually laid in groups of two to seven, but they can be laid singly as well. The female covers the eggs with a bluish-gray deposit. Young nymphs are pale yellow and have reddish-brown eyes, second and third stage nymphs have two pairs of orange spots on the thorax. Mature nymphs (fourth and fifth stage) have two pairs of dark reddish-brown spots on the thorax.

Damage

Nymphs and adults of all three 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.

Management

Although leafhoppers infest most vineyards in California, they may not require chemical treatment because vines can tolerate fairly numbers populations without harm, and predators and parasites may be able to maintain leafhop­per populations below tolerance levels. However, in coastal regions and the Central Valley, grape leafhopper populations may occasionally reach damaging levels and require treatment. If chemical control of leafhopper nymphs is necessary, if possible, delay application until the second (summer) brood to allow for egg parasitism to reduce populations. When Virginia creeper leafhopper is detected in a new region, their natural enemies might not be present to reduce populations and treating the first nymphal generation may be needed, especially in organic vineyards.

Biological Control

Many natural enemies help to provide control of leafhopper populations. The egg parasites, Anagrus erythroneurae and A. daanei, are the most common Anagrus spp. found in California vineyards during part of the season. These parasites may be more abundant in vineyards that are adjacent to prune, plum, and almond orchards, and near riparian areas where other leafhoppers, which overwinter in the egg stage, reside. Anagrus spp. can parasitize these eggs and survive the winter. After a leafhopper egg is parasitized it becomes visibly red. Unfortunately, these parasites are not as effective at controlling variegated leafhopper eggs as they are on those of the grape leafhopper. Eggs of Virginia creeper leafhopper are parasitized by A. tretiakovae and A. daanei in Washington State and British Columbia. Studies are underway to determine their presence and impact on California's Virginia creeper leafhopper populations. Sulfur sprays applied for fungal control may be toxic to Anagrus spp.

General predators of leafhoppers include spiders, green lacewings (Chrysopa spp.), minute pirate bugs (Orius spp.), lady beetles (Hippodamia spp.), black hunter thrips, and predatory mites. The predatory 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.

Cultural Control

Removing basal leaves or lateral shoots during berry set and the 2-week period following (before adult leafhoppers emerge), as recommended for Botrytis bunch rot management, will normally reduce peak leafhopper populations during the season by 30 to 50%. This coupled with Anagrus activity may preclude the need for insecticide treatment even when leafhoppers exceed the thresholds below. Time leaf removal to coincide with first generation nymphal development up to and including the 5th instar but just before adults are present. Also, leaf removal will improve coverage and efficacy of pesticides. In warmer growing areas, be careful not to remove excessive numbers of leaves, which can lead to sunburned fruit. Preventing overly vigorous vine growth will also help suppress leafhoppers.

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.

Biological and cultural control methods, including basal leaf removal, assist in control. Narrow range oils (mineral oil), neem oil, insecticidal soaps (potassium salts of fatty acids), PyGanic (pyrethrins), or kaolin clay may give partial control when nymphs are small. Soaps may spot table grapes and should only be used before bloom on this crop.

Monitoring and Treatment Decisions

About 4 weeks after budbreak, or whenever nymphs first appear, begin sampling for leafhoppers. Randomly select 20 vines in each block of the vineyard, each at least a few vines in from the end of the row.

How to monitor:

• First-generation nymphs—On each vine, choose one leaf at the 3rd or 4th node up from the basal node.
• Second- and third-generation nymphs—Choose young but fully expanded leaves in middle of canes.
• Count nymphs on underside of each leaf. Note which leafhopper species' are present.
• Check the leaves for red, parasitized eggs or eggs with emergence holes.
• Record observations on a monitoring form (example form).

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 and species; whether grapes are being grown for table, wine, or raisin use; canopy size; region; and degree of parasitization. A level of 10 to 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 or the Virginia creeper leafhopper, economic control by Anagrus spp. parasites may be 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.

Pesticides and Natural Enemy Releases

Important Links

• Insect and spider mite monitoring form (PDF)
• Photos of Leafhoppers