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Costly vine mealybug traverses California in only eight years

For an insect pest that gets around on its own only by crawling, the vine mealybug sure gets around.

It has taken the tiny little pest only eight years to traverse California to every grape growing region. First officially identified in 1994 the desert table grape mecca of Coachella Valley in Southern California, the vine mealy but was identified for the first time last fall in vineyards in Sonoma and Napa counties; in Sacramento County in August of last year; in previously uninfested vineyards on the Central Coast. It has pretty well inundated the Coachella Valley and an estimated 1,000 acres are infested in the San Joaquin Valley.

Experts agree that man gave the exotic pest a ride from one end of the state to the other, and they also say the non-native pest without natural predators is here to stay. Eradication is not likely. All producers can hope for is contain its spread using sanitation and chemical control.

Ed Weber, Napa County University of California viticulture farm advisor, said vine mealybug was found for the first time in his county early last fall in two vineyards near St. Helena.

He is convinced the pest came in on nursery stock.

Most damaging

“The vine mealybug is much more damaging and difficult to control than the other mealybug species,” said Weber. Established populations require repeated insecticide treatments far in excess of the norm for Napa County vineyards, he said.

The vine mealybug reproduces at a higher rate than other species, enabling small numbers of mealybugs to reach damaging levels in a single season. Whereas the grape mealybug has only one generation per year, the vine mealybug has five to six generations in the San Joaquin Valley.

“The insect's behavior in the San Joaquin Valley may be altered in our cooler climate; however there will probably be enough similarities to warrant its place in the ‘very serious pest’ category,” said Rhonda Smith, viticulture farm advisor in Sonoma County where vine mealybugs were found in two vineyards about the same time they were identified in neighboring Napa.

Nursery stock also was implicated in the Sonoma County finds. However, movement by vineyard equipment has been and still remains the more common and much more l likely method of spread of vine mealybug within the San Joaquin Valley, said Smith.

In the San Joaquin, the first infested vineyards found had some association with Coachella Valley vineyards where the pest was well established, she noted.

“The female vine mealybug and all of the immature stages cannot fly, thus they must be transported,” she said. Only the male can fly.

The adult female insects are about three millimeters in length and all other stages are still smaller.

Easily transported

As a result the immature stages, especially the “crawlers,” are easily transported on machinery, she said.

Mechanical harvesters, tractors, ATVs, bin trailers, sprayers, leaf removers any machinery taken into a vineyards — will spread the crawlers down the vine rows. So will field workers who touch each vine during the season performing canopy management practices and at harvest because the crawlers are on their hands, clothing and shoes, gloves and picking pans In late fall, infested leaves will blow down the vine rows into uninfested areas and in the winter, the insects will be on the prunings.

It takes about two years for a new infestation to be noticed, she said. By that time, the masses of bright white adults an nymphs are relatively easy to find, depending on the severity of the infestation.

‘This means that there were several months when normal vineyard practices moved the insects from the initial site of infestation to other vines in the same block or to adjacent properties — or even further.

“That is why it is important for growers to take precautions now and not wait until an infestation is located before changing farming practices to prevent the movement of this pest,” she added.

Weber said the vine mealybug has overlapping generations so that all life stages are present year-round. This makes chemical control more difficult.

Vine mealybug It can be found on the leaves, in clusters, under the bark and even on the roots of grapevines.

Mealybugs are phloem feeders. They produce a sugary excretion (honeydew) that supports the growth of sooty mold. The vine mealybug excretes much more honeydew than other mealybug species.

The honeydew can cover leaves, canes, trunks and fruit, making entire vines a sticky mess.

“It often becomes so thick it resembles soft candle wax,” said Weber. “Fruit from heavily infested vines is not suitable harvest. The stickiness of all the plant parts also facilitates spread from vineyard to vineyard.”

Mealybugs damage vines by contaminating clusters with honeydew, sooty mold, egg sacs, and lots of mealybugs. In Southern California, severe vine mealybug infestations have also reduced vine growth and caused spur death.

Like other mealybugs, vine mealybugs can spread grapevine viruses from vine to vine.

Can limit spread

Weber recommended growers can limit vine mealybug spread by alert managers and harvest crews about the problem.

“Ask them to inform you if they find sticky fruit or sooty mold. Ants in the vines can also indicate a mealybug (or scale) infestation,” he noted.

When either native or non-native ants are associated with mealybug infestations, the populations of the mealybug are greater because the ants disrupt the biological control complex, said Smith. Ants tend to “farm” mealybugs to encourage the production of honeydew and in the process prevent parasitoids from getting close to the mealybugs.

Smith encouraged producers to find out what their nursery stock suppliers are doing to insure their plants are not infested. Insecticide treatments or hot water dips may be appropriate for plants originating from areas where vine mealybug is known to occur, such as Kern and Fresno counties, she said.

Plants produced in the southern San Joaquin Valley could be infested at low levels that are difficult to detect, then shipped throughout the state.

Vine mealybug is a B-rated pest, meaning the Agricultural Commissioner has authority to ensure that proper control measures are being implemented to reduce populations and limit its spread.

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