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Mitey Web of Woe - Spider Mites

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High springtime temperatures have brought early and, in some cases, serious outbreaks of webspinning mites in almond orchards in the warmer regions of the Central Valley.

"Mites are a perennial problem in Kern County," observes Mario Viveros, farm advisor. "But this year, a lot of growers are having trouble after just three hot days (in May)."

In the Sacramento Valley, Blue Diamond® field supervisor Daryl Brun reports early activity by brown mites, a cool season pest. He expected forecasted 100-degree days in mid-May would bring on early infestations of twospotted mites.

Mel Machado, Blue Diamond® field supervisor in northern Stanislaus, San Joaquin and Contra Costa counties, recalls a severe mite infestation in his area in 1995, "but nothing like the ugly outbreak last year. Entire orchards were defoliated in a week's time." With the big increase in mite populations last year, he worries that 1997 could be as bad or worse.

"I saw infestations at bloom this spring," Machado said, which is why he thinks it's a good idea for growers who see mite populations building in their orchards to consider buying and releasing some mite predators.

"If you go in with a high label rate application of miticide now, you kill off the predator species as well, which leaves you completely exposed to a subsequent infestation," he said.

A rapid increase in pest mite populations can result in defoliation, Viveros says, which creates a serious problem for the grower in the following year. "With defoliation, there're no carbohydrates for next year's crop," he explains.

There is no noticeable effect on tree growth and nut sizing in the year the infestation and defoliation occurs, he says, because the annual growth and nut sizing are usually complete by the time mite infestations become serious. The following year, however, nut set, leaf size, terminal growth, and crop size can be much reduced. According to the University of California's manual, IPM for Almonds, the crop can fall off by as much as 20 percent following a serious mite infestation and defoliation in the previous year.

The university lists six species of mites that can pose an economic threat to almond orchards in the Central Valley. The twospotted and Pacific spider mite, which are found throughout the valley, and the strawberry mite, which is occasionally found in almonds south of Merced, are generally referred to as webspinning mites due to their habit of covering tree terminals with copious amounts of webbing. These mites are generally found in the lower portion of the tree until the weather gets hot. Then their numbers grow rapidly and they disperse throughout the tree.

The other three troublesome species listed in IPM for Almonds are the European red mite, brown mite, and citrus red mite. The European red mite may occur anywhere in the Central Valley, but the brown mite is generally found in the northern San Joaquin and Sacramento valleys. The citrus red mite lives in the southern San Joaquin Valley and is found in almond orchards near citrus groves.

According to the university, the three webspinning mites reproduce rapidly in hot weather and commonly pose the greatest threat from June through August. If conditions are favorable, they can complete a generation every seven days, or less, and eight to 15 generations can occur in a single growing season.

While there's a high potential for mite damage in almonds, it occurs only when mite populations get out of hand. However, it is necessary to have a small resident population of mites in your orchard as a food source for predator insects, the university advises. Low levels (four to five mites per leaf for 20 to 25 days) of webspinning mites on well-irrigated trees have little effect on the tree.

Mite Control

According to the university, both the European red mite and the brown mite are most effectively controlled with a dormant spray of oil. If a dormant spray is used, in-season controls are generally not necessary. Low levels of both the brown and European red mites are desirable as food for predators.

Webspinning mites, however, cannot be controlled with dormant sprays. The best approach with these pests, the university advises, is good orchard management and biological control.

"Unfortunately, we have no good answers yet to the mite problem," Viveros observes. "But a grower can do several things to help prevent or at least minimize the problem."

First, he advises keeping your orchards as cool as possible with lush cover crops. "Don't mow too much," he says. "Don't worry about keeping the orchard floor looking good. Help your trees stay cool by letting the cover grow. That is one of the best things you can do."

Second, stay on top of the water needs of your trees. Above all, he says, avoid letting your trees stress; that's when mite damage becomes most serious. "In our area, when it gets real hot, growers need to irrigate every week to prevent their trees from stressing," he explains.

Pay close attention to water penetration, Viveros advises. "If you have a problem with water penetration, gypsum can help," he says. "Apply it every time you irrigate. You can inject it into your irrigation system."

Mite Predators

For biological control, the university lists the western predatory mite as one of the most effective predators. This mite can build up rapidly under optimal conditions and regulate webspinning mite populations. To be effective, however, it must be present in the orchard by May in order to be able to respond to pest mite buildups as hot weather arrives.

Maintaining viable populations of predators can be difficult when control of other almond pests is required. Insecticides applied to control navel orangeworm and other insect pests often kill the natural enemies of mites. The university notes, however, that strains of western predatory mites that are resistant to some organophosphate insecticides and carbaryl are commercially available.

Some growers and PCAs claim minimal success with released predator mites. They find more success with careful monitoring of mite and worm populations so that sprays can be timed to control both. The sprays that they use are also less harsh than some of the more popular materials in use today, so that less damage is done to resident predator populations.

No Predators? Tough Battle Ahead.

When predator populations fail to control pest mite populations, a grower may have to rely on a registered miticide such as Omite or Vendex.

Unfortunately, says Viveros, some of the growers in his area don't have viable populations of mite predators this year and as a result are faced with a tough and expensive battle to control exploding populations of the pest.

"If a grower doesn't have good populations of predator mites, he has to go with high label rates of Omite to get control and just hope for the return of cool weather and a buildup of predators," he says.

This year's heavy crop compounds the problem, he adds. "I don't have any data on this, but it has been my experience that when we have big crops, the trees need more water and there is more stress. When that happens, the temperature in the orchard and on the leaves goes up and higher temperatures hatch more mite eggs."

That scenario underscores Viveros' advice to pay close attention to your trees' water needs, your cover crop, dust control measures, and predator populations. Do these things and maybe you can avoid the crop crimping effects of a pest mite outbreak.