On March 26 Agriculture Department inspection staff discovered a Glassy-winged sharpshooter egg mass on a nursery shipment. The Glassy-winged sharpshooter is a vector for Pierce’s Disease which can threaten the wine industry if the insect becomes established. Agriculture Department staff immediately quarantined the shipment, contacted the shipper, the Agriculture Commissioner from the origin county and the California Department of Food and Agriculture Pierce’s Disease Program. CDFA confirmed that the egg mass was non-viable. Although the egg mass was nonviable, the Agriculture Department insisted that the entire shipment be rejected and taken to another nursery outside of El Dorado County.
The situation
California's vineyards are facing a serious threat. The combination of a plant disease with no cure and a half-inch-long leafhopper called a glassy-winged sharpshooter has wrought millions of dollars of damage in just a few years.
Pierce's disease has existed for more than 100 years in the state, but until recently there was no carrier as effective in transmitting the bacteria more than a few feet and spreading the bacteria so rapidly.
The glassy-winged sharpshooter, first found in 1990 in Ventura County, has spread throughout Southern California. The insect is now moving northward.
Pierce's disease
Pierce's disease is caused by a bacterium,Xylella fastidiosa. The bacterium blocks the xylem, the water- and nutrient-conducting vessels of plants. The typical symptom is for leaves on the plant to begin to dry or to scorch. Infected vines can die in as little as one to two years.
X. fastidiosa also causes almond leaf scorch, phoney peach disease, alfalfa dwarf, oleander leaf scorch and citrus variegated chlorosis.
Pierce's disease decimated 40,000 acres of grapes in the Anaheim, Calif., area in the late 19th century. It was dubbed Anaheim disease, but the name was later changed because of Newton Pierce, who studied the infection. The incurable plant disease has appeared on and off ever since, but its spread was limited. The principal carrier, or vector, was the blue-green sharpshooter, a weak, small insect not able to fly much further than three feet.
The glassy-winged sharpshooter
The glassy-winged
sharpshooter, Homalodisca coagulata, is native Glassy-winged sharpshooter to the southeastern United States. It was first found in California in 1990. It is a large insect, almost a half-inch (12 mm) in length. It is a dark brown to black. Its head and back are stippled with either ivory or yellowish spots. It receives its name from its transparent wings.
The glassy-winged sharpshooter can fly up to one-quarter of a mile, and it frequently appears in high numbers. The insect is able to survive winter temperatures dipping as low as 20 degrees Fahrenheit.
The insect overwinters as an adult. It begins laying egg masses from late February through May. The year's first generation matures as adults from May through August. The year's second generation begins as egg masses laid from June through Sepetember. It is this generation that produces the next year's offspring.
How the glassy-winged sharpshooter spreads Pierce's disease
The glassy-winged sharpshooter is a voracious eater. It can consume 10 times its body weight in liquids per hour.
Sharpshooters can acquire the bacterium from infected plants and transmit it to healthy plants while feeding. If the adult stage of the insect has the bacterium that causes Pierce's disease, the bacterium remains in its mouthparts throughout its life, which can last over 6 months.
In some host plants, such as grape, the bacteria can spread systemically and cause disease. Once in the plant, the bacteria multiply and block the xylem, or water-conducting vessels of the plants. Plants eventually develop symptoms of dry or scorched leaves, particularly in mid-summer.
Once a plant is contaminated with Pierce's disease, the plant can act as a reservoir of the bacterium. Any sharpshooter that feeds on the plant can pass the infection to other plants. Once an adult sharpshooter acquires the bacterium, it may transmit the disease throughout the rest of its life.
Non-Pesticide Response
UCR scientists are pursuing non-pesticide biological control of the glassy-winged sharpshooter by introducing a natural enemy, a tiny stingless wasp, Gonatocerus triguttatus. The wasp is successfully reducing populations of the sharpshooter in Mexico and Texas.
A natural enemy of the glassy-winged sharpshooter, this parasitoid lays its eggs inside the eggs of the GWSS.
The first batch of wasps have been released by David Morgan, a post-doctoral research scientist at the University of California, Riverside.
Management
No precise treatment thresholds have been developed for the glassy-winged sharpshooter (GWSS). Because GWSS acts as a vector for, or transmits, the pathogen that causes Pierce’s disease (PD), treatment may be necessary when as few as one or two GWSS are found in a vineyard or orchard. Eggs are difficult to control with insecticides because they are laid in leaf tissue. Thus, adults and nymphs are easier to control with insecticides. Successful management of PD requires an integrated program that focuses on the reduction of GWSS populations by relying on chemical, cultural and biological control methods. While insecticides may be required in most vineyards and orchards where GWSS is found, they should be selected carefully and used when a monitoring program signals their need.
Ideally, grapes should not be planted next to citrus. In addition, GWSS is attracted to weeds in and around a vineyard. Cultivation and cleanup of adjacent weedy land in the fall will eliminate favorable overwintering sites in and near vineyards. It’s also important to remove nonessential fruit and nut trees.
The choice of ornamentals and landscape vegetation around vineyards should be given careful planning and consideration as well. When GWSS occurs, an application of a contact insecticide may be required. Furthermore, the use of a systemic insecticide in vineyards and adjacent citrus may prevent the buildup of large local populations. Systemic insecticides are recommended in vineyards next to citrus and eucalyptus.
Monitoring
Three monitoring methods can be used to detect GWSS: yellow sticky traps for flying adults beat sampling for adults and nymphs visual leaf inspection for eggs branch or vine to catch the insects that are dislodged. All stages of GWSS that fall into the sweep net or beating sheet can be counted. Then sampling moves on to the next vine or tree. Fifty trees or vines should be beaten in each vineyard or orchard. Beats into a 21-inch diameter sweep net have proven effective for monitoring adults and nymphs in citrus, especially in cooler temperatures.
Biological Control
Biological control is an option being pursued as a management strategy for GWSS. This is the intentional use of a pest’s natural enemies to suppress its population growth. These enemies are the pest’s naturally occurring predators, parasites and pathogens. GWSS is native to the southeast United States and northeast Mexico. In California, GWSS lacks several species of parasitoids that attack egg masses in its native range. This lack of specialized natural enemies that attack GWSS may be one reason why GWSS numbers are so high in California.
As part of a biological control program for GWSS, foreign exploration for egg parasitoids of GWSS in its home range is being conducted. Three genera of egg parasitoids, Gonatocerus (Mymaridae), Ufens and Zagella (the latter two are both Trichogrammatidae) parasitize GWSS egg masses. Several species in these genera are not present in California and need to be established here to help control GWSS. GWSS egg parasitoids lay their eggs inside individual eggs that constitute the GWSS egg mass. Upon hatching, larval parasitoids feed on the contents of GWSS eggs. Parasitoids complete their larval development and pupate inside GWSS eggs. Adult parasitoids escape from GWSS eggs by chewing circular exit holes in GWSS eggs and crawling out. Very little is known about predators, parasitoids, and pathogens that attack GWSS nymphs and adults. Greater research effort is needed here.
These methods should be used together in a comprehensive monitoring program to determine the need and timing of treatments and to evaluate your control program.
Yellow sticky traps catch adults when they are active or are dispersing to adjacent vineyards or orchards. Double-sided yellow sticky cards (7 x 9 inches) are the standard traps currently used to detect adult GWSS. Traps should be placed from the edge to within 30 feet of vineyards for early detection. Traps may be placed on trellises or above the canopy. Move trap placement as vine growth and development occurs to prevent traps from becoming obscured by foliage. One sticky trap per 10 acres is recommended, but placement in an area is dependent upon terrain and surrounding GWSS host plants. Because dusty conditions will reduce trap performance, traps should be replaced every two weeks.
For monitoring in citrus, traps may be placed on trees at shoulder height or on stakes next to trees. Traps should be used as an indication that sharpshooters are in the area and not for population size determinations. While sticky traps are useful for detection of GWSS infestations, the numbers of GWSS adults may not correlate closely with the number of eggs and nymphs on leaves, stems and/or canes. Sticky trap counts should be backed up with weekly foliage inspection or beat sampling in the vineyard or orchard
Visually searching leaf stems is another way to monitor for GWSS adults and nymphs. Visual inspections are especially good for detecting GWSS egg masses. Randomly conduct 50 leaf turns throughout a vineyard or orchard. Record the egg masses, nymphs and adults that are detected. Beat samplings can be done by hitting a tree branch or vine with a heavy stick, such as a broom handle. A sweep net or beating sheet is placed underneath the These vectors spread the PD bacterium from host plants in their habitat to vineyards, but not from infected to healthy grapevines within vineyards.
Epidemics of PD that are spread in this manner conform to a simple interest model, or a rate of disease spread that is independent of the number of infected grapevines in a vineyard. Furthermore, outbreaks of PD involving these vectors show an edge effect pattern because these native vectors do not fly far from their desired habitats. As a result, only the first several rows of vines near sharpshooter habitat typically are affected.
Epidemics of PD associated with GWSS differ from outbreaks associated with native California vectors in two important ways. First, GWSS spreads the disease far into vineyards. Although there is an association between proximity from GWSS habitat and incidence of PD incidence in vineyards, a high incidence of disease can be observed several hundred feet away from the edge of the vineyard.
Second, the rapid spread of the PD epidemic associated with GWSS conforms to a compound interest model. This means the rate of disease spread in a vineyard accelerates with increasing numbers of infected grapevines. This is likely due to the ability of GWSS to feed from woody tissue and inoculate grapevines on or near branches that would not be pruned off. Thus, while grape growing in areas with California vectors is possible and practical, growing in the presence of GWSS may require nontraditional disease management tactics.
Chemical Control
Insecticides are most effective against GWSS adults and nymphs, not against eggs laid inside leaves. Try to time treatments when monitoring results indicate that either the first or second GWSS generation has commenced. Good coverage is essential for effective use of insecticides against GWSS. Because of difficulties in obtaining complete coverage of a tree or vine, multiple insecticide applications may be required. Rotating between chemical classes when making multiple applications for GWSS is recommended to reduce the development of resistance. For best control of GWSS in vineyards, systemic insecticides are recommended.
Vector Relations
Xylella fastidiosa, the pathogen that induces Pierce’s disease (PD) and diseases of other plants, is spread among host plants by members of a group of leafhopper insects known as sharpshooters, which feed in the xylem.
Sharpshooters acquire the bacterium from feeding on infected plants. Once the bacterium is acquired, adult sharpshooters are infectious for life because the bacterium grows within their mouthparts. The bacterium attaches to areas of the mouthparts that are shed. Thus, sharpshooter nymphs that had acquired the bacterium lose it when they molt. Infectious sharpshooters inoculate healthy plants during xylem feeding, and can inoculate multiple plants. Therefore, it is important to removed vines infected with PD from the vineyard.
Several native sharpshooters traditionally have been important vectors of the bacterium in California. These include the bluegreen sharpshooter, associated with riparian habitats, and the green and redheaded sharpshooters, associated with grassy areas, especially irrigated pastures and weedy alfalfa fields.
Guidelines for
glassy-winged sharpshooter management1Fenpropathrin
2.4 EC 10 2/3 oz. fl. oz. 21(Danitol)
Comments:
Apply with air or ground equipment as a full coverage spray in sufficient waterfor thorough coverage. Spray may be tank-mixed and/or alternated with other commonly
used insecticides and miticides to comply with local IPM programs. Do not exceed two 2/3
pints of Danitol 2.4 EC spray (42 2/3 fl. oz., 0.8 lb. a.i.) per acre, per season. Do not apply
within twenty-one (21) days of harvest.
Imidacloprid
(Provado Solupak)
75 WP 0.75 oz. 0Comments:
Restricted entry interval: 12 HOURS. A foliar-applied product that gives a fastkill of sharpshooters but lasts only about two weeks. Do not apply more than 2 oz. of
product/year. Allow at least 14 days between applications.
.or.
(Admire)
2F 16-32 fl. oz. 30Comments:
Restricted entry interval: 12 HOURS. Use allowed under a Special Local NeedsRegistration. Soil-applied product that provides a slower kill of sharpshooters than foliarapplied
Provado but remains effective longer. Do not apply more than 32 fl. oz./acre per
crop season. Do not exceed 0.5 lb. a.i. imidacloprid (Admire and Provado) per acre, per year.
Dimethoate
400 2 qt. 28Comments:
Check with your county agricultural commissioner about the use of thismaterial under a Special Local Needs registration. If sharpshooters have migrated into the
vineyard and there are more than a couple inches of new shoot growth on the vines, treat
the first 200.300 feet in from the edge of the vineyard. It is best to treat during warm
weather and as soon as possible in the morning before the winds increase. Avoid drift into
water. Maximum of two applications/year in riparian areas.
Cyfluthrin
1.6 to 3.2 fl. oz. 7(Baythroid 2 . grapes: Section 18 No. 01.08)
. The United States Environmental Protection Agency (US EPA) has not established a
time-limited tolerance for this Section 18. EPA expects to be able to establish the
necessary time-limited tolerance for this use in the near future.
. Effective March 26, 2001, through Nov. 5, 2001.
. Riverside and Kern Counties ONLY on grapes for GWSS.
. Additional counties may be added only by approval of California.s Dept. of Pesticide
Regulation (DPR).
. Air or ground; maximum of 6.4 fl. oz. per acre, per growing season; 14-day interval; fiveday
REI. Application gpa are recommended at a minimum of 10 by ground or 25 by air.
1 . In general, the pyrethroids (Baythroid and Danitol) are more toxic to natural enemies than
the imidacloprid (Admire and Provado). In order of least toxic to most toxic to natural
enemies, they are ranked as follows:
Admire<Provado<Dimethoate<Baythroid<Danitol
Admire is least toxic because it is systemic.
Surprising Symbiosis: Glassy-Winged Sharpshooter Eats With Friends
ScienceDaily (Jun. 19, 2006) — Like a celebrity living on mineral water, the glassy-winged sharpshooter consumes only the dilute sap of woody plants—including grapevines in California , which is feverishly working to prevent the insect's flight into prized vineyards. Now, in a surprising study published in the June 6 issue of Public Library of Science Biology (PLoS Biology), researchers at The Institute for Genomic Research (TIGR), the University of Arizona , and their colleagues have discovered that the sharpshooter's deprivation diet is sneakily supplemented by not one, but two co-dependent bacteria living inside its cells.
See also:
Although insect-bacteria symbiosis is common, this is the first genomic analysis of three partners. In the study, a team of scientists led by TIGR microbiologist Jonathan Eisen, now at the University of California , Davis , uncovered an intimate metabolic co-dependency among the glassy-winged sharpshooter ( Homalodisca coagulata ) and two bacteria, Baumannia cicadellinicola and Sulcia muelleri . The sharpshooter channels the sweets from sap to the bacteria, which in turn feed the insect vitamins, cofactors, and essential amino acids.
“Much as mosquitoes transmit malaria, the sharpshooter transmits plant disease, including Pierce's disease, which threatens vineyards,” Eisen says. “In order to design methods to fight the insect, we've got to understand how it works and its weaknesses. We knew symbionts were doing something for this insect--but until this study, we had no clue what that was.”
In particular, in this case, the threesome came as a surprise. Many insects, such as aphids and cicadas, feed on the sap from a plant's xylem or phloem, pipes that transport water and food within a plant. These sap-feeders are often known to rely on resident bacteria for a balanced diet – especially the synthesis of the “essential” amino-acids that all animals, including humans, cannot make for themselves. But researchers had assumed that the sharpshooter needed just one bacterial symbiont (in this case, B. cicadellinicola ), as does the biologically similar aphid.
University of Arizona evolutionary biologist Nancy Moran, who has extensively studied the co-evolution of insects and their resident bacteria, recruited Eisen to the current project. “My initial interest in sharpshooter symbiosis was in the hope that we could find out exactly how xylem can be used as food,” Moran explains. “It's terribly poor in nutrients.”
In the study, the team first carried out a painstaking forensic type of DNA analysis known as “metagenomics,” in which they sequenced the B. cicadellinicola 's genome from material gathered via dissections of hundreds of insects. The scientists were dumbstruck to find no evidence of the biochemical pathways needed to synthesize amino acids. Could the plant be somehow providing amino acids to its insect predator? Unlikely. Was the sharpshooter somehow cranking out its own amino acids? Doubtful. Could there be something else, some other bacteria, adding these essential ingredients?
Yes. Realizing the amino acid pathways might be carried out by other bacteria living inside the insect, the team began picking through their forensic samples of DNA sequences, removing all the sequence reads that matched neither the insect nor its known symbiotic B. cicadellinicola bacteria. A large amount of the leftover DNA mapped to another bacterium, S. muelleri . Sure enough, when they pooled the bits of sample DNA that came from S. muelleri , the team found all the essential amino acid synthesis pathways.
“When doing this type of forensic metagenomics, some scientists suggest you can just analyze the whole system as one unit—a so-called ‘black-box' approach--without knowing which piece of DNA came from which organism,” Eisen says. “But this black-box ecology just does not work well. To really understand the system, you've got to assign the different bits of DNA to organisms. This study shows why.”
For this particular insect-bacteria trio, genome-based reconstructions of metabolic activity suggest that the two resident bacteria are close neighbors, residing next door within host tissues and feeding each other chemical precursors needed to make nutrients. In the future, Eisen says, the bacteria will likely evolve into organelles of the insect, losing their distinction as bacteria altogether.
Looking ahead, Eisen is continuing to explore the genomes of other animal-bacterial symbioses to understand how such systems originate and work. “Symbiosis is a pervasive strategy in biological systems and we still do not understand the rules of how it works.” “This is why this sharpshooter symbiosis is so important – it has given us a good model system with two co-dependent bacteria rather than just one.”
In addition to TIGR, teams at the University of Arizona 's Department of Ecology and Evolutionary Biology and at the J. Craig Venter Institute's Joint Technology Center contributed to this work. It was funded by a National Science Foundation grant to Nancy Moran.
Adapted from materials provided by The Institute for Genomic Research, via EurekAlert!, a service of AAAS.
