Prof studies grape vineyard's tiny predators

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Like the black widow spiders that Dr. Mark Mayse keeps in a jar in his campus lab, spider mites are classified as arachnids, not insects. Mayse and his students are trying to find ecological relationships between spider mite infestation and the health of grapevines.

by Mary Lisa Russell

They're small, no bigger than a poppy seed, but Dr. Mark Mayse says they can cause major damage to grape vineyards and are a constant nuisance to farmers.

Mayse, an entomologist in the Department of Plant Science at Fresno State, and his students, recently finished a study on spider mites, the tiny arachnids that settle onto the leaves and eat their way to vine destruction.

Mayse has field research plots in the Fetzer vineyards of Mendocino County that demonstrate a significant relationship between spider mite infestation and the health of the vine itself.

"We don't know yet if the mite simply causes the destruction of the vine, or if an unhealthy vine primarily promotes and attracts more mites," said Mayse. "Evidence clearly suggests that both types of effects are important in vineyards."

The study gives significant insight into ecological interrelationships, Mayse said. "The greater our understanding of these complex relationships, the better chance we have to effectively manage the mite populations."

Mayse said there is no simple or magic solution to mite infestation problems. "Effectively managing mite populations comes with lots of hard work, experience and luck," he said. Mayse explained that farmers are concerned about a variety of different pests and they cannot simply spray for each kind of bug.

"Mites are typically associated with vineyards that are stressed - whether it's due to moisture stress or poor fertilizer management. The mites feed on the leaves and decrease the effectiveness of the vine to produce the optimal amount and quality of fruit," Mayse said.

Since mites don't fly and are therefore likely to stay in one place, Mayse said it is imperative to keep a vineyard in top shape to keep their numbers down. That means maintaining a delicate balance, because the farmer is concerned with dozens of variables that affect the crop.

"Farmers do not like to use pesticides on the mites, but often prefer to rely on predator mites that reduce the spider mite population and work as an ally," Mayse said.

Spider mite populations tend to spread out slowly from spots within a vineyard, moving outward as their numbers increase. "The predators are often able to confine the mites to limited areas in the vineyard, and thus enable farmers to optimize grape production activities," Mayse said.

Mayse and his students plan to continue their efforts to develop better methods of allowing farmers to manage their vineyard agro-ecosystems.