By James A. Bethke.
The glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Figure 1), is an insect that was inadvertently introduced into southern California in the early 1990s. I remember seeing this insect on chrysanthemums that I was rearing for leafminer studies, but at the time I didn’t know it was an insect that would become notorious for disease transmission. GWSS is native to the southeastern United States, and it was most likely brought into California accidentally as egg masses in ornamental or agricultural plant foliage.
Sharpshooters and leafhoppers are in the insect family Cicadellidae. The GWSS is a large leafhopper (about 0.5 inches long) that uses its piercing sucking mouthparts to feed on the xylem (water and nutrient conducting tubes), but it rarely causes any physical plant damage. It does, however, excrete copious amounts of liquid that evaporates and make leaves and fruit appear whitewashed. Also, in high numbers, they can make it feel like mini rain, especially if you are sitting under an arbor that is covered in passion vine or wisteria. Because of my long forehead (baldness), I have a good mini-rain detector, and yes, we have an arbor with both passion vine and wisteria. While sitting under the arbor with my wife, I made the mistake of telling her exactly what we were experiencing.
You can imagine her reaction, “That’s bug poop?”
Recently, I saw the whitewash on my citrus, and upon inspection, I observed sharpshooter immatures on flush growth. They have the curious behavior of sliding around the opposite side of the stem from where you are, so sometimes to get a look at them you need to move around or move the stem so you can get a good look. If you threaten them, adults are good fliers and move to another branch or nearby tree. Nymphs cannot fly because they lack wings as an immature.
GWSS are not a problem on citrus, but if the citrus is next to grapes as is common in many agricultural areas, then there is a real problem. GWSS vectors the disease-causing bacterium Xylella fastidiosa, which proliferates in the xylem and plugs transport of water and nutrients. This bacterium is the causal agent of devastating plant diseases such as Pierce’s disease of grape, oleander leaf scorch, almond leaf scorch, mulberry leaf scorch, and many others. Other diseases to landscape plants in California include sweet gum dieback and cherry plum leaf scorch. Outside of California, other strains of X. fastidiosa cause phony peach disease, plum leaf scald, leaf scorches in sycamore, elm, maple, and oak. There is no cure for any of these diseases and the agricultural and landscape losses in California have been great. Lots of well-established ornamental olives have been lost in San Diego County due to this insect.
Because the insect is an efficient vector, the ornamental plant industry is not allowed to ship plants with any evidence of GWSS eggs in any plant tissues (Figure 2). Therefore, nurseries that are near citrus groves or any plants heavily infested with GWSS are always at threat of shipping disruption by the agricultural commissioner’s office. If GWSS are intercepted by a GWSS free county or state and traced back to a shipper in San Diego County, the shipping grower is inspected and may have to take significant control measures prior to making any new shipments.
So, if you find GWSS in your yard, what can you do to protect your plants? Honestly, not much. They are susceptible to many pesticides, but unless you are experiencing significant plant loss, it is probably not worth your while to try to control this pest. You can hang some yellow sticky cards to warn of their presence and maybe to remove some from the environment, but traps will not remove enough GWSS to make a difference because their numbers are usually staggering. In our studies, we used what is called a beat stick to remove adults and nymphs from tree branches and collect them in great numbers for study. Even so, we did not remove enough to make a real difference in the overall, local population.
Fortunately, there are several native parasitoids that attack GWSS egg masses and do a pretty good job of knocking the populations down toward the end of summer. Further, because there is a native sharpshooter of similar size, there is a native wasp that preys on the adults. GWSS can be a nuisance pest due to their numbers, but it is unlikely you will need to do any control.