EASTON, Calif. — You might imagine, given the attention to other insect pests since, the western grapeleaf skeletonizer that threatened San Joaquin Valley vineyards back in the mid-1970s had gone away.
No, WGLS is still there, sure enough, now challenging the granulosis virus that gave successful biological control for many years.
In vineyards where the virus is not abundant enough for adequate control, WGLS control has required spraying with organophosphate or carbamate insecticides applied to adequately reach the under surfaces of leaves.
In the 1980s, releases of natural-enemy insects gave minor control of WGLS, but in Kern County, development of high populations of the granulosis virus, spread by one generation of the pest to the next, proved to be the best way to avoid spraying. Bt sprays have also controlled it in organic grapes.
But David Haviland, Kern County farm advisor, says management approaches with new-chemistry insecticides look promising as an alternative.
His 2004 trials on Thompson Seedless near Mettler showed high knock-down performance against large WGLS larvae with Success, the spinosad product applied for thrips, and with Assail and Provado, the two neonicotinoids used for leafhoppers and glassy-winged-sharpshooters. "We smoked ‘em with these," he said.
The advantage of the new "reduced risk" products over the earlier "standards" is they eliminate the need to treat for small larvae in anticipation of damage that might, or might not, occur as they mature.
Instead, growers can wait until they see damage and large larvae before applying the new materials, if necessary. Some refinements in timing treatments may be needed for multiple pests.
Known in Southern California since the 1940s, WGLS spread its territory slowly because the adult forms fly only short distances. Although now widely distributed in the SJV, it does not occur in all grape-growing areas of the state.
The pest’s early instar larvae, appearing in early May, group side-by-side and chew up the lower surfaces of grape vine foliage leaving veins and upper cuticle, creating the characteristic papery, "skeletonized" appearance.
The older, yellow, black and purple-banded larvae, if abundant, can defoliate vines and bring on bunch rot in clusters. And the crawlers’ poisonous spines give field workers the same discomfort as burning nettles.
Haviland recently told grape growers at the San Joaquin Valley Grape Symposium in Easton that during the past three to four years growers in his county have reported seeing the defoliating pest again where virus populations were small.
Sign virus losing
In 2004, defoliation by second generation WGLS, rather than the more common damage by the third generation, was reported in some Kern County vineyards, a sign that the virus was losing ground.
In a research project funded by the California Table Grape Commission, he investigated applying new reduced-risk materials where the biological control was insufficient. The advent of new insecticides, Haviland said, opens the way to new approaches to WGLS management, including not spraying at all if the virus is doing the job, or perhaps controlling WGLS with applications made for other pests.
"A knock-down approach to skeletonizer is much better than a preventative one, and your PCA can now wait to see the level of defoliation before deciding on a treatment."
But he has some conditions for the multiple-pest, bloom-time sprays with Success. "One problem with this scenario is that overwintering pupae are just emerging into egg-laying adults when this application is made.
"Undoubtedly, there will be excellent control of any small larvae present, but it is uncertain if sufficient residual will be present to kill all larvae from all clusters of eggs that continue to be laid by first-generation adults during the bloom period."
The best timing for Assail and Provado, he added, "is when you see large larvae." Either product could be used just prior to harvest when leafhoppers and skeletonizer need to be removed out of courtesy to field workers."
Admire, the soil-applied product with the same active ingredient as Provado, will likely also give good control of WGLS populations, he said.
Make own spray
Haviland also pointed out that just like any other microbial, the granulosis virus is regulated by EPA. However, growers who want to augment the virus in their vineyards can collect infected WLGS larvae from their own property, process them into a sprayable liquid, and apply it to their own vines. He said he would describe the steps to accomplish this and publish them in an Extension newsletter.
Another speaker at the event sponsored by the University of California Cooperative Extension and the SJV Viticultural Technical Group was Fred Rinder, wildlife management supervisor with the Fresno County Agricultural Commissioner’s Office.
Rinder offered tips on control of vertebrate pests in vineyards, particularly gophers, which destroy vines and damage drip irrigation lines by gnawing and divert furrow irrigation by burrowing.
He said recent heavy rainfall suggests that gopher activity will be high this season in the moist soil.
Pocket gophers reside in intricate underground burrows and their activity can be detected by characteristic half-moon shaped mounds at each opening. As a non-game species, when damaging to crops, they can be taken at any time without a permit. No one control is completely successful, so several may be needed.
Traps, mainly the two-pronged pincher type and the box type, Rinder said, work well for small populations but even so are time-consuming to place and monitor.
In setting traps, locate a main burrow and with a shovel dig out a section large enough so traps can be placed within the burrow.
Traps should be placed in pairs facing opposite directions and wired to a stake for monitoring and retrieval. Place soil around sod, rock, or cardboard covering the opening to exclude light and encourage the gopher to continue using the tunnel.
Baits are better adapted to large populations and may be set out either by hand or by a tractor-mounted mechanical applicator. Strychnine and zinc phosphide baits require permits for purchase and use.
Rinder said it is important to place baits about 12 to 18 inches from a burrow entrance on either side. If too close to the entrance, they will encourage the gopher to seal the burrow.