They are the Dirty Half Dozen — Vine, Obscure, Grape, Striped, Long-Tailed and Pink Hibiscus. They are an unsavory lot, all from the mealybug family.
Mealybugs feeding on plants, particularly grapevines, can reduce vine vitality and transmit grape viruses. But what makes them a particularly slimy lot is that they can excrete great quantities of honeydew, rendering the fruit non-harvestable and covered with sooty mold.
Like any outlaw gang, there are the mean ones and the not-so-mean hombres in the Dirty Half Dozen, which rode across the Colorado River into California about the mid-1990s.
Vine and Grape are the Jesse and Frank James of this gang. Vine especially. It is the real bad one of the bunch. Pink Hibiscus is the wimp. Obscure and Long-Tailed can be mean if given half a chance. Striped is starting to exert its evil side, looking for trouble.
There is a big posse out looking for this bad bunch using whatever means they can; gunning them down; luring them into an ambush or just trying to find their hideout and keep them there before they can cause trouble.
Part of the posse, including University of California entomologists and farm advisors, Napa County ag commissioner and growers, was at a recent Bayer CropScience grape workshop to explain what to look for from the bad hombres and how to keep them at bay.
No need to worry about pink hibiscus, which was found hiding out in Imperial Valley in 1999. Parasites have successfully corralled this member of the gang. Pink Mealybug is generally found on ornamentals, anyway.
Striped mealy bug is moving rapidly through the Central Valley. It is being found in almonds and pistachios, according to "Marshall" Walt Bentley, UC Area IPM advisor, an expert on mealybugs stationed at the UC Kearny Ag Center in Parlier, Calif. Obscure and Long-Tailed have a relatively narrow temperature tolerance, but a wide host range. They are far more important pests in greenhouses and indoor plantscapes. Infestations in vineyards have been isolated to a small number in the Central Coast region.
Grape mealybug, according to Bentley, produces two generations per year and is not as prevalent on leaves as vine mealybug. However, it can produce a lot of honeydew.
While post harvest pesticides applications can control Vine Mealybug, Bentley said grape mealybugs go deep into barks in the fall through December.
"You will not kill them with any kind of pesticides with 400 gallons of water and 250 PSI at that time of year," Bentley said.
Crawlers start emerging in the spring and can be controlled with pesticides, but if it turns cold, they high-tail-it back into the bark where pesticide sprays will not touch them.
Sample in June
It is best to sample for Grape Mealybug in June at the base of the spurs. That sampling period may be later in cooler, coastal areas. Persistent sampling can detect spot infestations that can be treated early to prevent spreading throughout the vineyard.
Pulling spurs in winter is another way of detecting infestations. Crawlers move from the spurs to leaves in warmer weather where they begin to cause damage.
There is no pheromone yet for grape mealybug, but there is one for its meaner cousin, Vine Mealybug.
Vine Mealybugs produce five to six overlapping generations per year and is much more voracious than any of the other gang members. It overwinters in all stages on all parts of the vine and in roots. Vine Mealybugs are found on the top of leaves and heavy infestations can leave grapes covered with honeydew with the consistency of candle wax.
Growers can get "pretty good control" in the central valley with a post harvest treatment of Lorsban or Imidan, he said. Applaud works well in immature stages early, he added.
The first line of defense with most grape growers, especially table grape producers who can tolerate zero honeydew damage, is Admire, the systemic pesticide that can be applied through drip systems at rates of 32 ounces in one application or split applications of 16 ounces each. Growers under heavy vine mealybug pressure also do a post harvest application to prevent Vine Mealybug overwintering.
Ants guard mealybugs
Like any outlaw gang, the Dirty Half Dozen have accomplices. Ants are the protectors of mealybug. Ants feed on the honeydew and protect the mealybug from predators. Ants even carry them from vine to vine.
According to another posse member of the posse, UC entomologist, Kent Daane, also based at the Kearney Ag Center, ants can increase honeydew damage from mealybugs. The more ants feed on the honeydew, the more honeydew mealybugs secrete. Ants are so protective, they drive away mealybug parasites and Daane has remarkable video that shows that.
Daane is on a variety of mealybug trials, including one working with San Luis Obispo County farm advisor Mark Battany and UC Riverside urban entomologist Mike Rusk to develop attract and kill techniques to bring ants under control.
The Argentine ant is a major urban ant pest and as the coastal areas continue to urbanize, Argentine ants are moving from homes to vineyards.
The ants can be killed by pesticides, but these are often disruptive to predators. That is why Daane is working on attract-and-kill techniques.
Grey ants and fire ants are associated with mealybugs in the central valley and they are protein feeders. The Argentine ant feeds on protein as well, but it also thrives on carbohydrates: honeydew.
Daane has discovered the ants can be controlled by attracting to a sugar water bait laced with small amounts of pesticides in an attract-and-kill control strategy. However, the labor of servicing many traps is prohibitive. He hopes to develop an expensive capsule or disposable tube technology to make attract and kill economical.
In Napa County, "Mealybug Sheriff" and county agricultural commissioner David Whitmire and Napa growers are attempting to slow down mealybugs using an extensive trapping program to monitor vine mealybug infestations and hopefully slow down the spread with regulations. Infestation sites have gone from 19 in 2003 to 30 last season.
Good, bad news
"That is a good news-bad news scenario. There are more Vine Mealybug infestations than before, but the good news it is not everywhere," he said.
"We cannot eradicate it, but we can try to prevent its spread and from becoming established," said Whitmire. "I think we are making a difference."
Napa has taken this approach because of what Whitmire called "economical, environmental and social" reasons. There is a big vineyard sustainability push in the Napa Valley, almost bordering on organic. Widespread Vine Mealybug infestations would mean more pesticide use.
Whitmire said there are regulations to prevent the spread of Vine Mealybug in moving equipment from infested areas to non-infested areas. Similar restrictions are in place for movement of green waste.
All posse members stressed good sanitation measures are the first line of defense to keep members of the Dirty Half Dozen out of vineyards.
Infested leaves, bunches and bits of cane can travel on farm equipment from one site to another. It is extremely important that all farm equipment used in an infested vineyard is cleaned before it leaves the site. Vine mealybug can also be transported on clothing. Therefore vineyard workers should not move from an infested vineyard to a non-infested one on the same day.
The posse also warned that mealybugs can be moved on nursery stock and on budwood used in grafting.