Glassy-winged sharpshooter (Homalodisca vitripennis)
Compiled by the IUCN SSC Invasive Species Specialist Group
October 2009
1.0 PREVENTATIVE MEASURES
2.0 PHYSICAL CONTROL
3.0 CHEMICAL CONTROL
4.0 BIOLOGICAL CONTROL
5.0 BREEDING RESISTANCE
1.0 PREVENTATIVE MEASURES
The current strategy for containing the problem of GWSS and the disease causing Xylella fastidiosa bacterium it transmits is to keep the insect out of new areas. In the United States tighter inspections and regulations of nursery trade in infected areas have been imposed in an attempt to slow down the rapid spread of the pest.
Effective control of GWSS requires careful monitoring and detection (Varela et al. 2007). Monitoring of pest densities is highly important as it ensures control measures are used most effectively. In California large yellow sticky traps are commonly used in orchards to monitor for adults. Sweep nets are also used to monitor for GWSS in agricultural situations. GWSS infestations can also be determined by examining the underside of plant leaves for egg masses (Varela et al. 2007).
Other sampling methods include suction devices powered by gasoline motors, known as A-Vac and D- Vac and the hand-operated beat-net and pole bucket dislodgement devices. Castle and Naranjo (2008) evaluated these 4 methods and found that they detected similar temporal and spatial distribution profiles; although abundance was different. They authors conclude that the pole bucket was the best sampling method as it had the greatest sensitivity. The study also compared the relationship between pole-bucket samples and yellow sticky card samples, and found a strong relationship indicating that yellow sticky traps may provide a good relative index of GWSS infestation (Castle and Naranjo 2008).
In California citrus is an important host of GWSS. Vineyards that are near citrus are likely to have larger populations of GWSS. Proximity of vineyards to citrus was a major factor in the disease epidemic in Temecula Valley. In 2000 an area wide program begun to reduce populations of GWSS by treating citrus with a combination of knockdown insecticide, followed by systemic insecticide which was apparently successful. Citrus distribution and management is an important aspect of control programs for GWSS (Varela et al. 2007).
2.0 PHYSICAL CONTROL
In California physical control methods include removing infected grapevines immediately to prevent them becoming inoculation sources in vineyards (Miller 2006) and removing other host plants such as weeds and fruit trees from the area (Conklin and Mizell 2004).
3.0 CHEMICAL CONTROL
The main chemical used to protect Xylella-susceptible plants in both commercial agriculture and urban landscapes is imidacloprid, which is registered for home and professional landscape use on nonfood crops. It is sold in two formulations: one for soil application and one for foliar application. Foliar applications are effective for only a short period of time and may disrupt biological control agents. The soil-application formulation provides the most effective, long-lasting control and is less disruptive to most biological control agents (Varela et al. 2007). This is because it is systemic, meaning that it is absorbed by the plant roots and distributed throughout the plant via the xylem vascular system. Because GWSS feed exclusively on xylem fluid these treatments directly target the inset through feeding (Byrne and Toscano 2007). The toxic residues persist in plants for up to 6 months once the insecticide levels are established within the trees, although it has the disadvantage that it takes several weeks to become effective (Castle et al. 2005 in Byrne and Toscano 2007).
“In instances where the white excrement produced by this pest causes intolerable residues on cars or other surfaces, other insecticides can be applied to infested foliage to provide immediate relief. The least toxic and disruptive to biological control are insecticidal soaps and oils. Insecticidal soaps and oils are only effective in killing the soft-bodied nymphs of the glassy-winged sharpshooter and must directly contact the insect to kill it, so thorough coverage of the plant or tree foliage is essential. Applications of these materials need to be repeated at about 7- to 10-day intervals. Other insecticides are available for foliar applications. However, these materials are much more damaging to the parasitic wasps that are being introduced for long-term control” (Varela et al. 2007).
Although imidacloprid is effective, it has the potential to interfere with biological control programs. For a classical biological control program to be successful it is important that imidacloprid residues present in the trees at the time of releases/augmentations are not lethal to the parasitoids, or affect the parasitoids ability to search for hosts. While adult parasitoids foraging on the leaf surface are most likely safe from the insecticidal effects of imidacloprid, there may be impacts of systemic imidacloprid residues on the survivorship and emergence of the important egg parasitoid Gonatocerus ashmeadi. Byrne and Toscano (2007) investigated the effects of imidacloprid on the egg stage of GWSS and G. ashmeadi. During development in the egg for both insects, imidacloprid did not have a lethal effect, even at high concentrations. However, upon emergence 1st instar nymphs and parasitoid adults experienced high mortality, although the adult G. ashmeadi had greater tolerance than GWSS nymphs. It is important that the interaction between chemical and control management systems are fully understood so that the benefits of both systems can be maximized. The authors conclude that “our data show that there is great potential for the integration of the two systems—imidacloprid has no lethal effect on the sharpshooter embryo during its development within the egg, ensuring that the parasitoid will be able to develop to the adult stage. The greater tolerance of the parasitoid to the insecticide will further ensure that the leaf residues of insecticide will have a lesser impact on its survival during emergence than on the 1st instar nymph in unparasitized eggs” (Byrne and Toscano 2007).
4.0 BIOLOGICAL CONTROL
Parasitoid wasps are the most commonly used biocontrol agent against GWSS. Searches for parasitoid wasps have resulted in the collection of several mymarid and trichogrammatid species. In California species released include Gonatocerus ashmeadi, G. fasciatus, G. morrilli, and G. triguttatus, and most recently a strain of Anagrus epos from Minnesota (Morse et al. 2006). Research by Krugner et al. (2008) aims to determine host range and efficacy of A. epos against GWSS populations.
G. ashmeadi was released on Tahiti in May 2005 (Grandgirard et al. 2007 in Grandgirard et al. 2009). The parasitoid spread rapidly throughout Tahiti and self-introduced to the neighboring island of Moorea. G. ashmeadi proved to be highly successful, reducing GWSS abundance by more than 90% one year after release. Due to the success on these islands G. ashmeadi was released on eight new islands across three archipelagos of French Polynesia: the four main islands of the Leeward group in the Society Islands; Nuku Hiva in the Marquesas, and Tubuai and Rurutu in the Australs. Populations of GWSS were reduced by >95% on islands where the parasitoid was released, and is expected to be permanent (Grandgirard et al. 2009).
Other possibilities for control include entomopathogenic fungi. Kanga et al. (2004) identified two possible species for development in biocontrol: Pseudogibellula formicarum and Metarhizium anisopliae. Additional studies are needed to provide a better understanding of host-pathogen interactions and factors that enhance or limit disease in natural conditions. A project conducted by Boucias et al. (2007) to characterize fungi associated with GWSS resulted in the finding of a new species, Hirsutella homalodiscae nom. prov., which was determined to be the most dominant and widespread pathogen of GWSS in its native range. This species may have potential for use in control of GWSS, although more research on its biology and impact of GWSS populations is required (Boucias et al. 2007).
A single-stranded RNA virus with potential for biological control against GWSS was recently discovered in insects collected from California. This virus has been denoted Homalodisca coagulata virus-1 (HoCV- 1) and has been tentatively placed in the genus Cripavirus (family Dicistroviridae). The virus affects both sexes and all development stages of GWSS, and surveys indicate that this virus is widespread in the United States. Previous studies of related virus species have shown effects including delayed development and reduction in fecundity which are “factors vital to the establishment of viable, efficient biopesticides” (Hunnicutt et al. 2008).
Symbiotic control is a novel approach to prevent the spread of insect-transmitted pathogens. Genetically modified symbionts deliver antipathogenic gene products that reduce vector competence (Beard et al. 2002 in Ramirez et al. 2008). Current research is working to apply this management technique to reduce the spread of Xylella fastidiosa by making GWSS a less efficient vector for the bacterium using the transformed symbiotic bacterium Alcaligenes xylosoxidans variety denitrificans (Ramirez et al. 2008).
5.0 BREEDING RESISTANCE
Finding X. fastidiosa-resistant rootstocks may also be a possible way of limiting the affect of the disease, avoiding the problem of the leafhopper's presence altogether. While wine grapes (Vitis vinifera) are susceptible to Pierce's disease, Muscadine grapes are resistant, and may someday provide resistant rootstocks for the grape industry (Conklin and Mizell 2004).