Can One Little Bug Slurp California’s Wine Economy Dry?

The glassy-winged sharpshooter is a leafhopper with piercing, sucking mouthparts and rows of spinelike setae (hairs) in its hind legs. Scientists have classified the glassy-winged sharpshooter (GWSS) as a true bug. Its scientific name is Homalodisca coagulate, but most grape growers in California simply call it a pest.

This leafhopper lives up to its name and moves from one plant to another. Vines, which produce grapes for wine and table consumption, are delicious targets for the xylem-sucking insect. The GWSS drills into the main rootstock of the vine and penetrates the nutrient canals of the xylem. While sucking out the plant’s fluid, insect deposits bacteria into the vine, causing Pierce’s disease.

Built like a stealth bomber, this sleek insect is about half an inch long with a mottled dark-brown body and yellow speckling about the head. It is twice as large as other common species of sharpshooters. The underside of its abdomen becomes charcoal in color as the insect ages.

Most leafhoppers are weak flyers, but the GWSS can travel miles during its life span. Other sharpshooters simply do not fly as far as the glassy-winged and seem to be more susceptible to temperature variation. The GWSS also appears in greater numbers since it has an extensive host plant range of 73 species of plants in 35 different families.

The glassy-winged sharpshooter has a voracious appetite. It can consume 100 times its body weight in plant material per day during its three- to nine-month life span. The GWSS produces two generations a year in southern California where the counties of Kern, San Bernardino, Santa Barbara, Venture, Los Angeles, Riverside, Orange, and San Diego have reached infestation levels.
In the case of the grapevine, the GWSS feeds on the plant by inserting its needle-like mouthparts into the plant’s xylem, sucking out the nutrients. If that were all it did, it would simply be a nuisance. Bacteria logged in its gullet are deposited into the central core artery system of the attacked vine. The plant’s response to this invasion is to choke off the arteries, which move the nutrients around the plant. The result is lethal: the plant’s main rootstalk dies in one to three years.
If California’s grape growing areas reach levels of infestation throughout the state in the future, the GWSS will have played a pivotal role in spreading Pierce’s disease throughout the vineyards of California, devastating the state’s $33 billion-a-year wine industry.

The Connection Between Xylella fastidiosa and Pierce’s Disease

The bacterium Xylella fastidiosa causes the deadly Pierce’s disease in grape rootstocks. Xylella fastidiosa is a rod-shaped, gram-negative bacterium which attacks the xylem, or water-conducting material, in a plant. Marcos A. Machado first described Xylella fastidiosa in the Genome Project report for the Instituto Agronomico de Campinas Centro de Citricultra Sylvio Moreira, August, 1997, “rod-shaped with distinctive rippled cell walls, non-flagellate, do not form spores, measure 0.3-0.5 millimeters in length. There is enough strain variability to justify taxonomic separation at the sub species or pathovar level.” He further stated that “DNA analysis suggests the existence of five groups of Xylella fastidiosa: the citrus group, the plum-elm group, the grape-ragweed group, the almond group, and the mulberry group.”

Xylella fastidiosa lives in the mouth and digestive tract of various sharpshooter vectors of which the glassy-winged sharpshooter is the most climate-tolerant. At almost twice the size of other sharpshooters, the GWSS is also able to fly the farthest, thereby penetrating the interior of vineyards and causing widespread destruction. It is a grim twist of fate that the Xylella fastidiosa bacterium was probably brought to California on rootstocks used to replant vineyards that had been attacked by the aphid-like insect Phyloxerra in the 1890s. The Xylella fastidiosa bacteria flourished in California because it accepted a wide range of host plants. According to the University of California Davis Research team, “the glassy-winged sharpshooter has an extensive host range, attacking at least 73 species of plants in 35 different families.”

According to U.C. Davis entomologist Bruce Kirkpatrick, “(the) bacteria Xylella fastidiosa has such a wide plant host range — in agricultural crops, ornamental plants, riparian plants, and many common weeds — I’m certain that there are plants infected with the bacteria in nearly every part of the state.” He further clarified, “The exceptions may be in counties that experience significantly cold winters, i.e., snow is on the ground for a month or so. We know that Xylella fastidiosa, the bacteria that causes Pierce’s disease, is killed in some plants that undergo a severe cold weather dormancy. That’s probably why Pierce’s disease has never been reported in Oregon, Washington, and New York states. In addition, we have never found Pierce’s disease in Sierra foothill vineyards above 2,000 or 3,000 feet. However, for all the rest of the state (California) where grapes are grown, I’m sure that the bacteria and GWSS can overwinter quite nicely.”

A vector, like the GWSS, which taps into an infected plant, spreads the bacterium from plant to plant. Xylella fastidiosa is able to hold fast to the inside surfaces of the insects’ throat. While the GWSS dines on the watery sap in the plants’ xylem, the Xylella fastidiosa is transmitted directly into the new plant where they “move around in the xylem of the grapevine,” says Alexander Purcell, a U.C. Berkeley entomologist.

The groundbreaking Genome Project, in August, 1997, stated that “Xylella fastidiosa is the casual agent of many economically important plant diseases including Pierce’s disease in grapevine (PD), alfalfa dwarf, phony peach disease (PPD), periwinkle wilt (PW), leaf scorch of plum, mulberry, pear, almond, elm, sycamore, oak, maple, coffee, and citrus variegated chlorosis (CVC).”

What is Pierce’s Disease?

A great deal of confusion has surrounded Pierce’s disease since it was first reported as a mysterious disease affecting grapes in 1892. The strain of the bacterium Xylella fastidiosa infecting grapes also causes disease problems on alfalfa (alfalfa dwarf) and almond (leaf scorch). The University of Nebraska-Lincoln, Department of Plant Pathology reports, “It was known as a mysterious disease, California vine disease, vine plague and Anaheim disease before it was named Pierce’s disease after N.B. Pierce, the USDA scientist who did much of the early investigative work on the disease. In 1953, researchers discovered that similar maladies of grape existing in Florida and Texas as well as an alfalfa disease were, in fact, Pierce’s disease. The true nature of the pathogen was not revealed until the 1970s.”

In all cases, the plant’s response to the introduction of Xylella fastidiosa is the same. The plant’s nutrient conducting xylem system is chocked off by gums and tyloses that plug the system. The xylem, as a water nutrient system, stretches from the smallest leaf to the main rootstock. Since nutrients and water are not reaching affected areas, one of the most common symptoms of the disease is leaf scorch or “sunburn” of the plant’s leaves. They turn yellow and progressively die.

The disease eventually eventually affects grapes on the vine. Small branches, as well as the main canes of the plant, become infected. Young plants usually die within the first year, while older plants may survive up to five years before eventually succumbing.

Reducing the number of vector glassy-winged sharpshooters from spreading Pierce’s disease is an important part in controlling the disease — perhaps equally as important is developing a variety of grape plant that is disease-resistant. This complex problem has drawn considerable scientific effort. International scientists, botanists, and researchers are working collaboratively to define and solve the problem. Hanging in the balance is the future of the grape and wine industries, and the hope of one day raising a glass to celebrate the defeat of a tiny “true bug.”