In the Fall 2000 issue of the MEC Newsletter, Shepherd Bliss writes of wines and oaks, of a powerful industry, of problems he considers more important and immediate than the glassy-winged sharpshooter (GWSS). He is unremitting in his criticism of the wine industry for having too much power, for spreading fear, and for its agricultural practices of trying to combat the spread of this insect. He writes well, but unfortunately completely misses the main point of the GWSS presence here in California. He speaks of a "campaign of hatred being waged against this tiny insect" and says "the glassy-winged sharpshooter is not my enemy." Friend or enemy is irrelevant. Nature doesn't give a hoot about our precious feelings.
Before I go any further, I need to say I raise grapes. I am a member of CCOF and annually grow around 75 tons of certified organic wine grapes. I also have planted over 8,000 oaks and 40,000 conifers on our ranch, and have lectured on oak conservation.
The issues with the GWSS (a leafhopper with the scientific name of Homalodisca coagulata) are not enemy or friend, but rather first the role of an alien species and the balance of nature itself, and second the plant diseases carried by this insect. This leafhopper is a native of the southeastern United States, and lives and reproduces from Georgia and Florida westward to eastern Texas and up the Mississippi Valley to southern Missouri. Living where it does, its numbers are kept in check by natural mechanisms. Predation by spiders, birds and small mammals is important. There are several parasitic wasps in each of the Families Mymaridae and Dryinidae which effect the main control on the sharpshooter in its native habitat. These wasps lay their eggs on the egg masses and nymphs of the sharpshooter and the wasp larvae live on the developing insect, killing it. However, these parasites are restricted to areas where their hosts live, in the southern United States. The main factor in controlling the northern spread and distribution of the GWSS is climate, for it is restricted to warm climates such as its native habitat and much of California. It cannot tolerate the very low temperatures of the Midwest or New England.
In its natural location, the insect functions as part of the natural economy, living, breeding, dying, and ultimately providing its bodies and eggs as food for other animals. But in California (where it hitchhiked to in about 1990, probably on nursery stock), there are no natural enemies. Predators will take a few, but the specific parasitic wasps are unknown here. The opportunity has been given this insect to thrive without controls, to increase its population by the billions. Without predators or parasites to keep its numbers down, we cannot even estimate how many there will be. We know it will be beyond counting. Most ecologists will unequivocally disagree with Mr. Bliss when he says the GWSS "is nature's remedy for an unbalanced monoculture." When animals or plants are brought into new areas, nature becomes unbalanced, and the way to restore the balance is to eradicate or at least control the newcomer.
The GWSS is an extraordinary opportunist. It feeds and lays eggs on plants from acacia to yucca. It is a sap feeder, extracting nutrients from the xylem tubes of leaves and small stems. Among the favored foods of this insect are liquids from the leaves of fruit trees which we all enjoy: apricots, peaches, plums, apples, avocados, figs, grapes, citrus and pears. It is also partial to ornamentals such as roses, wisteria, sunflowers, hollyhocks, bougainvilleas and many cacti. Among trees we need to list ash, oaks, redbuds, cottonwoods, locusts and even some pine species. Mr. Bliss suggests the GWSS is innocuous and simply takes out a few grapevines, and that it "may be a gift horse in disguise." I take a different view, because this insect feeds on the xylem juices of over 100 known plants already and the lists grows longer as we gain knowledge. And it does so in California without natural controls on its population. Moreover, there are two generations per year. The GWSS is in the same league as yellow starthistle and gypsy moth, and we'd be better off without all of these invasive aliens.
The GWSS is not to be underestimated, especially because its deleterious and fatal effects are not from its dietary habits. When we factor in Pierce's Disease which it carries, this insect is frightening to all of California agriculture and not just to wine grape growers and wineries. This disease is the real menace of the GWSS, for the sharpshooter is a very efficient vector of this fatal bacterial disease of plants. The organism is called Xylella fastidiosa and kills grape vines within two or three years. The bacteria multiply and actually clog the xylem tubes preventing transport of liquids. Mr. Bliss discusses the loss in the Temecula area due to the GWSS of "only 25% of the vines" and apparently sees this as acceptable. He glosses over the connection of vine death to Pierce's Disease so that an uninformed reader could not possibly put the two together. It's true that grapes are a preferred target of the disease, but Mr. Bliss forgets that not all grapes are grown for wine. The California Department of Food and Agriculture published the following of California land use in 1999 with bearing acreage: raisin grapes (279,000 acres), table grapes (87,000 acres), wine grapes (424,000 acres). Fortunately, not all of these vines are at risk for Pierce's Disease (for reasons I won't go into here) but enough are at risk to cause alarm among all grape growers.
What makes the situation worse is that the identical strain of the bacterium causes almond leaf scorch and even some alfalfa diseases. Similar strains, also vectored by the GWSS, cause phony peach disease, fatal to peach trees. In much of Georgia, peaches can no longer be grown because of Xylella fastidiosa. There is a strain of the bacterium found in Brazil but not in California which kills citrus trees, and a different strain is also implicated in a condition called oleander leaf scorch, a lethal disease of roadside oleanders. In each of these cases, the GWSS is a major if not the prime vector. Fortunately for most of the 100 plants the GWSS feeds on, it appears they are immune to Pierce's Disease. In the Central Valley of California, the GWSS moves readily between citrus which it lives on but doesn't infect, and almonds or grapes which are infected and killed by the bacteria. If the citrus killing strain comes to California from South America, that whole industry becomes at risk. Were the GWSS established in Mendocino County, the insect would move between grapes and many host plants, such as willows, blackberries and coyote bush. These other plants serve as vector reservoirs, or asymptomatic plant hosts for the bacterium. That is the second reason the GWSS is so important, and we must recognize that it's important to more than just wine grapes, and act accordingly, recognizing that almonds, alfalfa, peaches and nectarines are all at serious risk and potentially all citrus as well.
So we see that this tiny insect is not merely an innocuous newcomer to California. It doesn't belong here and it carries plant pathogens fatal to fruit we all like. Mr. Bliss doesn't have to like the wine industry, but it's not just the wine industry which is threatened. The most recent issue of the CCOF newsletter has an excellent article on the GWSS and Pierce's Disease, and CCOF treats this as a grave matter. Quoting from the magazine, "CCOF is opposed to the mandatory treatment of organic farms with synthetic pesticides that are prohibited from use by CCOF standards." But at the same time it states, "This pest must be controlled."
The significance of the spread of Pierce's disease is on a par with Dutch elm disease which nearly exterminated the American elm, or the chestnut blight in the eastern states of the 1915-45 era when the blight got them all. Mr. Bliss is correct in worrying about the recently discovered Phytophthora fungus which is killing tanoak and members of the black oak group, but he is wrong to dismiss so easily the threat of the GWSS, both because it's an alien and because of Pierce's Disease. The GWSS and Pierce's Disease are not symptomatic of "the over planting of vineyards"; they constitute a serious problem for both agriculture and the environment.
Rudolph H. Light, Ph.D.
Redwood Valley
Copyright Mendocino Environmental Center 2001
Permission granted to excerpt or use this article if source is cited