Grape Grower Magazine (March 2003)
by Devin Carroll
Mealybugs cause serious havoc in table grapes, and growers usually fight them with pesticides. Understanding mealybug biology is crucial to correct timing and choice of insecticide. Natural enemies are also important in an integrated control program, and can even provide effective biological control by themselves if properly encouraged. In this column, I will describe mealybug biology and the timing of pesticide applications. Next time I will discuss how to evaluate biological control.
Growers have at least three effective treatment options for mealybugs: Lorsban before budbreak, Admire through a drip system, and a July treatment with Lannate or Provado.
Grape mealybugs have two generations with a partial third. The first generation hatches under the bark during the winter, and a few crawlers may be found moving up to the canes on sunny winter days. Just before budbreak, the last eggs hatch and the majority of crawlers move upwards and congregate on spurs and canes in spots where the young bark has started to peel. There they find protection, and they can easily reach the cambium layer with their feeding stylets. If Lorsban is applied, it should be aimed at the spurs or canes.
Lorsban is hazardous and an environmental pollutant, and is being reviewed by regulatory agencies for possible banning or increased restrictions. Lorsban may also harm natural enemies of mealybugs and other pests such as mites and leafhoppers. Unfortunately no other registered chemical has been found that can be used before budbreak with the efficacy of Lorsban.
Although most of the crawlers are too hidden to be killed by most pesticides, Lorsban has a remarkable ability to reach them. However, a portion of the population always remains protected under the bark on the trunk. On some grape cultivars, such as Ribiers and Crimsons, the crawlers seem to have an easier time finding the cambium layer on the trunk, so that more remain there protected. Lorsban seems to be less effective on these varieties, especially if it has disrupted the natural enemies.
Some crawlers move from the spurs to leaves, but the majority stay put until the second molt, when many began moving downwards under the bark on the trunk. The tiny silk cocoons of the male mealybugs appear under bark around April 11-17 in the San Joaquin Valley. Meanwhile the females reach full size and start turning into eggsacs near May 11-14, and the second generation begins to hatch around June 4-6. This is the generation that causes the damage, because grape bunches are the preferred site for settling.
Admire applied through a drip system in mid to late May will reach the leaves in June at about the same time as the crawlers. Admire is very effective but expensive. It works best on growing leaves, not in the bunches or under bark.
The grower and PCA must decide, based on the number of mealybugs and the signs of biological control, whether to apply a summer spray. Timing is critical. Although the first crawlers typically appear on the leaves about June 11-14, many continue to emerge from under bark up to mid-July. New male cocoons appear around July 10-13. After that, the more advanced females become large enough to survive spray treatments. Thus the most effective treatment window is about July 10-15. Waiting this long also allows assessment of biological control. If enough is found, treatment will not be necessary.
In my opinion, the best material for a summer mealybug spray is Lannate. Not only is it more effective than alternatives such as Provado, but its short residual allows survival of parasites that are inside their mealybug mummies at spray time, then emerge later as adults. I have often seen good biological control following a well-timed Lannate spray. This is important, because even Lannate misses the many mealybugs that are protected under bunches or remain under the bark. Full strength Provado, which is required for mealybug control, leaves a residual that appears to be harmful to parasites.