The Gonatocerus wasp (top)
may soon play a more effective role in holding down the spread of a grape
disease harbored by the glassy-winged sharpshooter. The sharpshooter is roughly
15 to 20 times larger than its natural enemy the wasp. Click the images for
more information about them.
Advance May Help Mobilize More Wasps Against Grape
Pest By Jan
Suszkiw September 28, 2005
Gonatocerus wasps don't bug people. But these tiny parasites
can put a real hurt on glassy-winged sharpshooters, leaf-hopping insects that
pose a disease threat to California grape vineyards.
Now, an experimental method of refrigerating parasitized sharpshooter
eggs for up to 60 days may improve the artificial rearing of Gonatocerus
wasps for field release as biological control agents. Agricultural Research
Service (ARS) entomologist
Leopold is investigating the insect-storage method along with Marion Harris
and Wenlong Chen, both with North Dakota
State University in Fargo.
Gonatocerus wasp releases are part of a multipronged approach
California has taken to keep sharpshooters from spreading the bacterium
Xylella fastidiosa, which causes Pierce's disease in grapevines and
other host plants. Gonatocerus wasps reproduce by laying their eggs inside
those of sharpshooters. After hatching, Gonatocerus larvae eat their egg
hosts, develop and emerge 10 to 12 days later as adult wasps. In California,
Gonatocerus' spring emergence lags behind sharpshooters', so fewer are
around to parasitize the pests' eggs. Fall attacks, though, can inflict
sharpshooter losses of up to 90 percent.
Releasing insectary-reared wasps can help close that gap, but the
practice is expensive and time-consuming since live plants and sharpshooter
eggs must be used. Refrigerated storage could cut production costs and furnish
more time to amass the wasps for spring releases, notes Leopold, with ARS'
Research Laboratory, Fargo.
In studies there with G. asmeadi and G. triguttatus
wasps, Leopold's team stored parasitized sharpshooter eggs for 30 to 60 days by
adjusting three temperature settings in a stair-step fashion. Under one such
regimen--starting and ending with 4.5 and 7.5 degrees Celsius,
respectively--the wasp emergence rate was 60 percent. Importantly, their health
and longevity were similar to wasps from untreated eggs, reports Leopold.
His team plans on submitting a scientific paper describing the
results, including wasp-emergence rates from dead eggs. Leopold and Chen will
also present their work at the
Disease Research Symposium in San Diego this December.
ARS is the U.S. Department of
Agriculture's chief in-house scientific research agency.