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Project description

Identification of the parasitoid fauna associated with California sharpshooters and host specificity testing of exotic mymarid parasitoids released for classical biological control of glassy-winged sharpshooter. (03XA003)
Program Exotic Pests and Diseases Research Program
Principal
investigator
M.S. Hoddle, Entomology, UC Riverside
Host/habitat Grapes; Citrus; Landscape Ornamentals
Pest Glassy-Winged Sharpshooter Homalodisca coagulata; Blue-Green Sharpshooter Graphocephala atropunctata; Green Sharpshooter Draeculaocephala minerva; Red-Headed Sharpshooter Xyphon fulgida; Smoke Tree Sharpshooter Homalodisca liturata; Pierce's Disease Xylella fastidiosa
Discipline Entomology
Review
panel
Agricultural Systems
Start year (duration)  2003 (Three Years)
Objectives Identify and catalogue the native California parasitoid fauna attacking native California sharpshooter species belonging to the tribes Cicadellini and Proconiini.

Determine the risk posed to native California sharpshooters and their respective indigenous parasitoids posed by the introduction and establishment of exotic mymarid parasitoids that have been released for the classical biological control of glassy-winged sharpshooter.

Produce a colored pictorial guide of all economically important California sharpshooters (native and exotic) and their associated hymenopterous parasitoids.

Project
Summary
Glassy-winged sharpshooter (GWSS) is an exotic pest in California (CA) that is the focus of a major classical biological control program with mymarid egg parasitoids from southeastern USA and northeastern Mexico. In addition to GWSS, CA has 10 native sharpshooter species in two tribes that are confirmed or potential vectors of a xylem-inhabiting bacterium, Xylella fastidiosa, which causes lethal scorch-like disease symptoms in a variety of economically important crops, including grapes. The parasitoid fauna (mymarid and trichogrammatid) associated with CA native sharpshooters (excluding the native Homalodisca liturata) is totally unknown despite the economic importance of these insects in the vineyards of northern CA. The purpose of work proposed in this grant is to document for the first time the native parasitoid fauna attacking native sharpshooters in CA and to conduct retroactive host specificity tests to gauge the potential impact exotic parasitoids released for GWSS control may have on native sharpshooters and to verify laboratory host range predictions with field-collected data.
Final report A rigorous host specificity testing protocol was evaluated for estimating potential physiological and ecological risk to nontarget species posed by exotic parasitoids introduced for use in the exotic glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae; Fig. 1A) classical biological control program in southern California (SoCal), U.S.A. Physiological and ecological host ranges were estimated on multiple plant species for two GWSS egg-parasitoids, the solitary Gonatocerus ashmeadi Girault (Fig. 1B) and gregarious G. fasciatus Girault (Fig. 1C) (Hymenoptera: Mymaridae), on three nontarget indigenous sharpshooters, smoke-tree sharpshooter (STSS), Homalodisca liturata Ball, blue-green sharpshooter (BGSS), Graphocephala atropunctata (Signoret), and green sharpshooter (GSS), Draeculacephala minerva Ball (Hemiptera: Cicadellidae; Fig. 2A-C).

Host specificity testing results were further evaluated by comparing observed laboratory and field nontarget impacts. For G. ashmeadi, significant laboratory and field nontarget impacts were documented for STSS eggs on both the native plant jojoba and Eureka lemons. BGSS and GSS eggs on the different plant species were not acceptable hosts for G. ashmeadi. For G. fasciatus significant laboratory impacts for STSS eggs, minor impacts for GSS, and no impact for BGSS eggs was documented. However, nontarget impacts as predicted in the laboratory for G. fasciatus were not revealed in field surveys. Field collected sharpshooter eggs, reciprocal sentinel plant deployments, and laboratory tests confirmed Gonatocerus latipennis Girault and Polynema sp. (Hymenoptera: Mymaridae) as egg parasitoids of the BGSS (Fig. 3); Paracentrobia auriscutellum (Girault), Paracentrobia sp. (Hymenoptera: Trichogrammatidae), and Gonatocerus mexicanus Perkins (Hymenoptera: Mymaridae; Fig. 4) were confirmed as egg parasitoids of the GSS. Through the course of this work, a novel testing cage was designed for standardizing biological studies on arthropods; the oviposition preferences determined and a SoCal day degree model were constructed for BGSS.

Second-year
progress
To determine the oviposition preference of female blue-green sharpshooters (BGSS), Graphocephala atropunctata (Signoret) (Hemiptera: Cicadellidae), a survey was conducted on southern California wild grapes, Vitis girdiana Munson (Vitaceae), growing near Temecula, Calif., in August 2003 and 2004 where populations of BGSS were known to occur.

Female BGSS oviposited into new growth, primarily the succulent tendrils and stems. The undersides of small leaves and petioles were also used for oviposition, but to a lesser extent. Mature stems, large- and medium-sized leaves and petioles were not used for oviposition. Two parasitoids, Gonatocerus latipennis Girault and a Polynema sp. (Hymenoptera: Mymaridae), were reared from BGSS eggs. Literature reviews revealed a deficiency of known natural enemies for G. atropunctata. Deployment of sentinel plants and reciprocal tests were implemented to further confirm the parasitization of BGSS eggs by these parasitoids.

Collectively, the Polynema sp. and Gonatocerus latipennis constitute the first documented parasitic natural enemies of BGSS eggs. A further examination, starting in January 2004, of the activity of BGSS and its parasitoids in southern California is under way. Blue-green sharpshooter adult activity reached its peak in July while biweekly samples of wild grape canes and tendrils revealed that peak emergence of blue-green nymphs and parasitoids occurred from mid-July to mid-August. No-choice tests with Gonatocerus ashmeadi Girault, a parasitoid of the glassy-winged sharpshooter, Homalodisca coagulata, and BGSS eggs as part of a nontarget impact assessment have indicated that BGSS eggs are not used by G. ashmeadi.

More testing is planned for 2005 to confirm this. No-choice tests with G. ashmeadi and the native smoke-tree sharpshooter (STSS), Homalodisca liturata Ball, a native congener to GWSS, yielded no significant differences in percent parasitism of eggs when compared to the glassy-winged sharpshooter control.

First-year
progress
A survey was conducted in late summer 2003 to determine the oviposition preferences of female blue-green sharpshooter (BGSS), Graphocephala atropunctata (Signoret) (Hemiptera: Cicadellidae), on Southern California wild grape, Vitis californica Benth (Vitaceae). Female BGSS oviposited into the new growth, primarily the succulent tendrils and stems. The under sides of small leaves and petioles were also used for oviposition, but to a lesser extent. Mature stems, large and medium sized leaves and petioles were not utilized for oviposition. Two parasitoids, Gonatocerus latipennis Girault and a Polynema sp. (Hymenoptera: Mymaridae) were reared from BGSS eggs. Literature reviews revealed a deficiency of known natural enemies for Graphocephala atropunctata. A sentinel plant study was conducted to further confirm the parasitization of BGSS eggs by these parasitoids. Collectively the Polynema sp. and Gonatocerus latipennis constitute the first documented parasitic natural enemies of the BGSS.

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