Since the early ’ 90s, grape growers in Riverside and San Diego counties have battled the glassy-winged sharpshooter (GWSS), an insect that spreads Pierce’s disease and causes millions in damages to vineyards. In a new study, scientists have discovered how to reduce the number of sharpshooters without harming non-target organisms, progress that moves one step closer to a solution in the fight against this prolific pest.
GWSS has been established in southern California since 1990 arriving from either Florida or Texas and has also successfully invaded French Polynesia, Hawaii, and Easter Island.
GWSS is a serious threat to California vineyards because it moves faster and flies greater distances into vineyards than the other species of native California sharpshooters. The insect occurs in unusually high numbers in citrus and on a variety of woody ornamentals. Crepe myrtle and sumac are especially preferred. The pest also feeds and reproduces on a wide range of trees in southern California including, eucalyptus, sycamores, peaches, pears, and coast live oaks.
Researchers from the University of California, the U.S. Department of Agriculture, and the California Department of Food and Agriculture have introduced parasitic wasps for the biological control of GWSS. Four parasitoids from the southeastern United States have been released and appear to be establishing in southern California. Parasitoids from Argentina are also being evaluated in quarantine but have not yet been released.
Entomologist Mark Hoddle from UC Riverside and his research team screened two species of wasps to see if they attack native California sharpshooter species and whether these exotic GWSS parasitoids would harm the parasitoids of native sharpshooters. “Selecting the right organisms to use as natural enemies is critical to the success of biological control efforts and for ensuring safety to other organisms,” explains Hoddle. “The ideal agent should establish stable populations in the field and reduce pest numbers to nondamaging levels without affecting nontarget organisms”
Parasitic wasps seek out sharpshooter eggs and lay their own eggs inside them. The developing wasps eat the contents of sharpshooter eggs before pupating and then chew circular holes through the sharpshooter eggshell to get out and disperse. In this way egg parasites can effectively reduce the number of sharpshooters that emerge during any given breeding cycle.
Hoddle says, “Our results show no unexpected harm to native non-target species, clearing the path for biological control of this pest. Results from the monitoring efforts over the next year will help in making management decisions regarding which species of parasitoid should be released when and where.”
The blue-green sharpshooter (BGSS) is considered to be the most important Pierce’s disease carrier for coastal California, from San Diego in the south through Mendocino counties to the north. Funded by the UC Exotic/Invasive Pests and Diseases Research Program, Hoddle and his research team first needed to determine where native sharpshooters prefer to lay their eggs. They found that females prefer to lay their eggs in new growth, primarily succulent young grape tendrils and canes.
Hoddle found two parasitoids that were reared from field collected blue-green sharpshooter eggs and constitute the first documented egg parasitoids of BGSS. The blue-green sharpshooter is also an unlikely host for any of the GWSS parasitoids.
In California, GWSS causes Pierce’s disease in grapes, almond leaf scorch, alfalfa dwarf disease and oleander leaf scorch. Oleander leaf scorch has caused more than $50 million damage on freeway median plantings in the state. The number and type of plant maladies caused by GWSS is likely to increase. Already, the bacterium has been found to cause two previously unrecognized diseases in olive trees and liquidambar, where it causes scorch-like symptoms.
There are currently more than 70 state and federal research programs studying GWSS. Researchers from several agencies are conducting studies to find biological control strategies to reduce GWSS populations in California. To this end, widespread releases of different parasitoid species that attack GWSS in its home range have been made in 13 California counties with high incidences of the pest.
Hoddle studied native sharpshooters to see if they are vulnerable to attack by parasitoids native to the southeastern United States, northeastern Mexico, and Argentina. To date, the only nontarget species susceptible to the introduced agents is a leafhopper species that exists primarily in desert habitats where parasitism by introduced GWSS parasitoids is generally low, and usually absent.
Hoddle’s research also indicates that females produce more males when they encounter other females of the same species. This finding has had important implications for efficient mass rearing of parasitoids where the objective is to produce as many reproductive females as cheaply as possible. Limiting female crowding in mass rearing cages encourages females to produce more daughters as opposed to sons.
Scientists also demonstrated that compared to plain water, providing honey-water and flowers of buckwheat significantly increased the longevity of both male and female natural enemies. This indicates that increasing the longevity of the female parasitoids may also enhance biological control of the GWSS because females that live longer may encounter more GWSS egg masses and consequently parasitize more eggs.
Another potential way to enhance parasitoid populations is to plant flowers that can provide nectar to parasitoids. Sowing flowering plants such as buckwheat, or alyssum could potentially increase GWSS biological control. “However, whether this cultural practice could benefit GWSS or Xylella is unknown and needs investigation. In citrus orchards nondamaging levels of brown softscale may also be beneficial for enhancing parasitoid survival and could also enhance biological control of GWSS because parasitoids benefit greatly from feeding on brown softscale honeydew,” says Hoddle.
Hoddle’s research indicates that the premier wine-growing areas of Napa, Sonoma, and Mendocino counties could be vulnerable to invasion by the GWSS. In contrast, states north of California that also have substantial grape industries may have winters that are too cold to harbor permanent populations of this pest.
“Biological control with host-specific egg parasitoids appears to be the only feasible control strategy for providing long-term, area-wide suppression of this pest,” says Hoddle. “Even excellent biological control of glassy-winged sharpshooter may not be sufficient to reduce substantial crop losses. In this instance, natural enemies, in particular egg parasitoids, must form the cornerstone of integrated pest management programs for glassy-winged sharpshooter.”