Vine Mealybug Management
George Zhuang, UCCE Fresno County, Kent Daane, UC Berkeley
This past season, many growers and PCAs reported a tough battle suppressing vine mealybug populations in some Fresno County vineyards, leading them to question current management strategies to decrease mealybug densities and reduce their spread and damage in the vineyard.
The season typically starts with movement of vine mealybug from the lower vine sections underneath the bark of the trunk where they commonly reside during the dormant period. During early spring in Fresno County, new eggs hatch underneath the bark, and are not often seen by vineyard managers until the population increases and starts to move up from the trunk to the cordon and shoots. Rather than believing in a complete movement of the whole population up the trunk, it is better to consider that the population increases in spring and early summer and much of the next generation moves to the leaves and clusters as the vine matures (but there are always some mealybugs still on the vine trunk). Typically, growers notice the honey dew and sooty molds when the vine mealybug has already fed on the leaves and clusters, but in heavy populations the trunk can have a ‘wet’ appearance even before the population moves to the leaves. After harvest, the vine mealybug population again accumulates on the trunk and even the roots (just below the soil surface) for overwintering.
Growers need to control the population before it gets into the fruit, and in 2016 we saw some heavily infested grape clusters in some vineyards having irreversible damage to fruit yield and quality (Fig 1). For some wine grape growers, an additional concern is that vine mealybug is the vector of grapevine leafroll associated viruses (GLRaVs), which can reduce yield, delay fruit maturity, and reduce wine quality. Therefore, early scouting is critical to implement a proper management program with the best timing and selection of insecticide materials. However, if you had heavy mealybug populations and damage in 2016, you should be considering your strategy early in 2017 to prevent crop damage.
First, monitor and prevent the introduction of vine mealybug from infested vines or vineyards. This pest does not walk quickly or far, but it does move between vines and even vineyards.
Figure 1. A Thompson seedless cluster heavily invested with vine mealybug.
There is a natural spread from infested vines by wind and birds; the transportation of equipment and tools from infested vineyards (e.g., harvesters and shears from field crews) and the movement of contaminated fruit, pomace, canes and leaves which can all aid the dissemination of this pest.
Figure 2. Pheromone trap for vine mealybug. (Photo by UC IPM.)
Pheromone traps, such as a red delta trap (Fig. 2) baited with a synthetic vine mealybug sex pheromone, can be used to monitor vine mealybug. This technique is most useful when you don’t have any mealybug in the vineyard and you’re trying to monitor for new arrivals. Depending on the size of the location, two traps in a small vineyard or two traps per 80 - 120 acres can be placed on the edges of the block near the road or adjacent vineyard. In the San Joaquin Valley (SJV), the first significant flight of male vine mealybug is in June, and traps should be placed around this time of the year to monitor the counts of male adults on a biweekly basis. Peak flights occur from August to October in the SJV. The number of male mealybug catches will be important to identify the presence of vine mealybug, however, the count will be of less value in determining density of vine mealybug. Note however, that if you had considerable damage in 2016, using pheromone traps may not be as important as visually following the population development and increase in the block.
Figure 3. Dark and wet appearance of a grapevine trunk infected with vine mealybug.
Visual sampling of mealybug can be easy. Training a field crew to identify the signs of vine mealybug feeding is the most efficient way to make an early detection. The large amount of honey dew on the trunk and cordon (Fig. 3) will have a dark and wet appearance. The active movement of ants along the trunk is another good indicator of vine mealybug infestation. Having the field crew trained and scouting during the pruning, leaf removal and harvest stages can help to spot and mark the infestation areas and treat them after harvest or the next season. Vine mealybug identification posters are useful to train your field crew and they are available through http://cesanluisobispo.ucanr.edu/Viticulture/Vine_mealybug/ or contact your local farm advisor for availability. Once the presence and density of vine mealybug have been identified, certain strategies will be implemented to reduce the population. Here we will focus on conventional vineyards (for organic vineyards, the options will be more limited).
Ant control
Adult female vine mealybug is relative immobile and the large amount of honey dew it secretes attracts ants (e.g., native grey ant, the most common in the SJV). Ants are not only tending and protecting vine mealybug from predation and parasitism, but also moving vine mealybug along the grapevine. Controlling ants will expose the vine mealybug to the attack of parasitoids and predators, which could reduce its population density and slow its spread.
There are some ant controls, including baits and chemical sprays, although most methods are not a complete solution. Some growers have used bark stripping and the application of a sticky barrier to reduce ants moving up the trunk, but this can be very costly.
Mating disruption
Similar to the theory of pheromone traps, this method requires a much higher concentration of vine mealybug sex pheromone than pheromone traps in the vineyard to confuse the male adults to disrupt their successful mating. Synthetic sex pheromone is dispersed through the whole vineyard using dispensers or sprayable formulations. Mating disruption is most effective when the mealybug population is low.
Insecticide programs
There are a couple of insecticides which growers can apply to control vine mealybug. Based on their chemical characteristics, and the location of vine mealybug during the season, materials such as Lorsban®, Applaud®, Admire®, or Movento® can be applied as the season progresses. More information about the timing and rates of these insecticides can be found at http://ipm.ucanr.edu/PMG/r302301911.html.
Among all the insecticides, Movento® is most widely used on grapevines to control vine mealybug. The timing of spray is especially important to achieve the maximum effectiveness of control. Movento® is a foliar applied translaminar insecticide and it has to get into the vine through the active leaf first and then the metabolite changes to an ‘enol’ and it is this metabolite that helps kill the mealybug. The insecticide acts as a lipid biosynthesis inhibitor. It inhibits the metabolic processes that primarily deal with lipids and fats and the mealybug can’t move, molt, and feed. The mealybug will die off more slowly than with some contact insecticides, so managers need to be patient. Also, the enol will stay in the vine for a long time so it can kill mealybug long after the Movento® is sprayed on the vine (the Daane lab has found good levels of the enol in vines more than 150 days after application).
Recently, further studies have been carried out by the Daane lab to investigate the partitioning and translocation of Movento® through grapevines. The on-going study will help growers make the maximum use of their insecticides to control vine mealybug.