Vine mealybug (VMB) has become public enemy No. 1 for many California grape growers. The voracious pest can make a mess out of grape bunches, covering berries with sooty mold and spawning bunch rot organisms, ruining the fruit.
Fortunately, North Coast grape growers don't have to worry as much about fruit damage from VMB as their counterparts in California's warmer grape producing areas. The growing season is shorter there, and mealybugs do not have as long to make a menace of themselves.
However, the new California resident vine mealybug and many of its mealybug cousins and scales can have a longer term, more devastating impact on premium wine grape vineyards as vectors of grapevine leafroll, which causes poor color development and non-uniform grape maturation.
Kent Daane, University of California entomologist based at Kearney Ag Center in Parlier, told Sonoma County grape growers recently that the university is shifting its North Coast research focus, looking more closely at the impact of the mealybugs on the spread of the virus, than on controlling it as a pest of fruit.
Controlling it as a vector and not as a fruit damaging pest could represent two entirely different strategies, according to Daane.
The insect ecologist has studied mealybugs worldwide and compared the situation in Sonoma County to the cooler production areas of New Zealand and South Africa where the spread of leafroll virus via mealybugs forces growers to pull out vines every 15 to 25 years due to virus infections.
In South Africa, mealybugs have been recovered up to four feet below the ground living on the old roots of vines that have been pulled out. Growers there, he said, treat for this hidden pest by applying glyphosate to stumps to kill the old roots where the mealybugs are surviving to infect another day.
Daane told growers and PCAs at the Santa Rosa, Calif. grape day that the native grape mealybug and the newcomer vine mealybug are the most important vectors of grapevine leafroll virus (GLR) on the North Coast. The spread of the virus from an infected vineyard to an uninfected vineyard tracked closely with the movement of mealybugs from virus-infected to virus-clean vineyards, he said.
Daane and other UC entomologists have done extensive research on when the mealybugs pick up the virus and how long they can carry it.
A piece of good news from this study is that virus is not vectored by leafhoppers. Therefore, the glassy-winged sharpshooter may cause other problems, but it will not spread leafroll viruses.
However, Daane said phylloxera may be a leafroll virus vector. And that is not good news.
One of the important factors gleaned from studies is that the first mealybug in-stars are 20 times better able to transmit grapevine leafroll from source vines to recipient vines than adult mealybugs. This could have a significant impact on control measures to prevent virus spread which Daane and others are now developing.
Avoidance, said Daane, is the best way to protect a vineyard from infection. However, this is easier said than done.
Starting with clean, certified plant material is a good start. Nurseries in the Central Valley now hot water dip new cuttings and benchgrafts to prevent the spread of mealybugs and therefore the virus.
There is a wide selection of pesticides that control mealybugs, from powerful carbamates and organophosphates to insect growth regulators. The former are quick acting and knock out pests and predators alike. IGRs and other softer materials also control the pests, but it often takes hours or even days for the mealybugs to die. In the meantime, virus-laden vectors can continue to feed and spread the leafroll.
Daane and others are introducing parasites imported from areas of the world where vine mealybug is now established to see if they can find one to control the pests, but again this may be a slow process that will not quickly control the pest and stop virus spread. Daane also has worked with pheromone mating disruption technology. It looks promising, and he worked to get a Section 18 for use of pheromone puffers. Unfortunately, the cost of controlling mealybugs in high value wine grape vineyards would not support an emergency registration. Therefore, Daane is working on a full registration for this. He does not expect government approval until 2009 or 2010.
Pheromone traps are available to monitor mealybug population levels and even dogs have been trained to sniff out low populations of mealybugs.
It is difficult to detect low mealybug populations. Daane suggests the growers and PCAs look for past year damage, honeydew and sooty mold on vineyards at harvest time.
Mealybugs like to feed early on spurs and basal leaves before they go underneath the bark and that is another way to detect low populations. Once they get into the bark of vines, they are difficult to ferret out, Daane noted.
“Train harvest crews to look for sooty mold and small numbers of mealybugs to detect low populations early,” he said.
Another telltale clue of vine mealybug presence is ants moving along cordons. Mealybugs and soft scales feed on the phloem and excrete honeydew, which ants feed upon. Ants actually protect mealybugs from predators. The Argentine ant is the most common species in coastal vineyards, said Daane.
Liquid sugar baits have been used successfully to control ants and there are several commercial products and bait stations available.