By Lucia G. Varela, Rhonda J. Smith and Glenn T. McGourty, University of California Cooperative Extension
In the North Coast, the native Western grape leafhopper, Erythroneura elegantula, has historically been a pest of grapes that occasionally requires insecticide control. More recently, Virginia creeper leafhopper, E. ziczac, has been reported from northern California vineyards and, where abundant, it can cause significant damage if not controlled. Native to the northern Midwest, it has spread throughout Canada’s grape-growing regions and to Washington State. It was first reported in Butte County, California in 1984. Since then it has been detected in some northern California vineyards from the Oregon border to Sacramento, Solano and Yolo Counties in northern Sacramento Valley, in northern Sierra foothills and most recently in Lake and Mendocino Counties in the North Coast. It is also reported in large numbers on Virginia creeper and Boston ivy.
The biology and life cycle of Virginia creeper leafhopper is similar to that of Western grape leafhopper with two generations a year in the North Coast, and a possible partial third generation in warm years or locations. In most areas of the North Coast, Western grape leafhopper is controlled by its natural enemies, the most effective of which are a few species of Anagrus, a tiny wasp, about one-tenth as long as the leafhopper.
Anagrus lays a single egg inside Western grape leafhopper eggs. The wasp larva develops within the leafhopper egg. Signs of parasitism include the egg turning red during larva development and an exit hole when the adult wasp emerges. These parasitic wasps are extremely efficient due to their ability to locate and attack Western grape leafhopper eggs, therefore preventing increases in leafhopper population. In addition, the wasp’s short life cycle permits its populations to increase more rapidly than those of the leafhopper.
Given that leafhopper pests of grapes overwinter as adults, thus leafhopper eggs are not available for Anagrus to overwinter in, the tiny wasp overwinters inside eggs of a number of other insect hosts. Some of these overwintering hosts include blackberry, prune and apple leafhoppers eggs and possibly many others. In the spring Anagrus females disperse from the overwintering hosts as Western grape leafhopper females begin to lay eggs in grape leaves. Signs of parasitism on Western grape leafhopper eggs can be seen in early May in vineyards located within a 5- to 10- miles of Anagrus overwintering hosts. More distant vineyards may not show parasitism until midsummer.
While many Western grape leafhopper eggs are parasitized, very few Virginia creeper leafhopper eggs observed last year and so far this year appear to be parasitized in the Mendocino and Lake populations. Thus, if not controlled with an insecticide application, high populations by the end of the season may defoliate the vineyard.
Due to the highly efficient Anagrus, the Western grape leafhopper rarely requires chemical control. If parasitized eggs are observed during the first generation, there is a high likelihood that populations will remain within tolerance levels. As a result, the decision to treat the first generation can be delayed until the second to give Anagrus an opportunity for control.
At the moment, given that levels of parasitism appear to be very low for Virginia creeper leafhopper, controlling the first generation with insecticide is advisable where this insect is found. For this reason, it is important to be able to distinguish the two species. For guidance on identifying the two leafhopper species, view the video at http://ucanr.edu/vineyardleafhoppers
Both leafhopper species are small insects with adults about 3 mm (1/8th of an inch) long. Western grape leafhopper adults are pale yellow with orange, red and brown markings. As the scientific name implies, Virginia creeper leafhopper adults have a reddish-brown zigzag marking on each front wing on a pale white to yellow background and reddish-brown eyes.
In both species kidney-shaped eggs are deposited just beneath the epidermis on the underside of fully expanded leaves. Western grape leafhopper lays eggs singly while Virginia creeper leafhopper lays eggs side-by-side in groups of two to seven and far less frequently singly. The latter covers her eggs with a bluish-grey deposit called brochosomes, which makes the eggs more noticeable.
Leafhopper nymphs can be seen moving sideways primarily on the underside of leaves. The first-stage nymphs of both species are colorless, but older nymphs can be distinguished by the markings on the mid-part of the body (thorax) and eye color. Western grape leafhopper nymphs have six yellow markings on the thorax and white eyes that can be seen with a hand lens. Virginia creeper leafhopper nymphs have four dark reddish-brown spots on the thorax, readily seen with the naked eye, and reddish-brown eyes.
Virginia creeper leafhopper has not yet been spotted in Sonoma County as of this time. The closest location with populations in vineyards is Hopland in Mendocino County. Early identification is critical to prevent high populations that may cause damage by harvest.