Researchers seek new answers to old strawberry pests

By Bob Johnson
Reprinted with permission

Frank Zalom checks on clear plastic mulch installed to provide weed control in a strawberry field trial at the UC Watsonville Strawberry Research Center.

A handful of the rows at the University of California Watsonville Strawberry Research Facilty test field stand out with their shimmering reflective mulch.

Strawberries are usually grown with clear plastic mulch over the bed, but many organic growers prefer black plastic mulch because it gives greater weed control. And now, reflective mulch is being tested to see if it helps with lygus control. Those rows will also be monitored to learn if the reflective mulch affects yields.

This is one of the many new approaches researchers are examining in order to cope with the threat insect pests including lygus bugs pose to strawberries.

Strawberry growers are finding control of numerous insect pests more complicated than ever because insecticides that have been used for years are no longer registered or have become less effective against pests. Thrips, whiteflies, mites and lygus can all cause serious damage to strawberries. And all of them have the potential of developing resistance to the remaining materials allowed on strawberries.

Lygus bugs, in particular, pose a growing threat. They puncture the strawberry seeds, which leads to distorted berries that cannot be marketed. Lygus can find haven in many weed crops, as well as in alfalfa and other cash crops commonly grown near strawberries. And the available insecticides are only effective against the young nymph stage.

Many growers have switched to pyrethroids for lygus control, but lygus bugs are beginning to escape those widely used materials.

“Lygus tolerance of pyrethroids in an entire area is increasing a lot. I have seen second-year strawberry fields, with repeated pyrethroid applications, where resistance is strong enough that you could not kill them with many times the label rate,” said Frank Zalom, [professor of entomology at UC Davis and] UC Cooperative Extension entomologist. He talked about lygus control strategies with the growers at the annual UC Watsonville Strawberry Field Day in May.

Monitoring and using degree-day models is particularly important in controlling lygus, because most registered lygus materials only work on the nymphs.

Zalom and UC Cooperative Extension farm advisors on the coast from Watsonville to Ventura are working in a far ranging study to fine-tune the weather model for lygus development.

They are closely monitoring the temperature and lygus populations in and near selected strawberry fields throughout the Central Coast region. Lygus development depends largely on the temperature, and the researchers are trying to come up with precise predictions on when the pest will be in the nymph stage, and vulnerable to insecticides. The study aims to make it easier to manage resistance through more precise timing of applications.

Work is also under way through the IR-4 program to get registration for other materials to manage lygus in strawberries. Beleaf is a material developed to control the adults, while Clutch is a neonicitinoid for lygus control. When these materials are registered, they may be helpful additions to a resistance-management program because they have different modes of action than anything currently available.

Although some studies showing that these materials work have been completed, and more studies are already under way, there is no date set for registration of either of these materials.

“It takes time to move these things through the process,” Zalom said.

Zalom is also studying the application of Admire through the drip line to control lygus in fields where pressure is likely to reach unmanageable levels.

“It’s expensive, but it may be a way to knock down populations in second-year fields where you already have lygus,” Zalom said.

The elimination of the organophosphate Lannate in strawberries has also complicated control of thrips.

Resistance may also make it more difficult to control thrips in a low-impact program of reduced risk materials. Dow Agrochemical has seen some thrips resistance to Spinosad-based insecticides, according to Zalom. The firm is considering a change in the label to allow fewer applications on strawberries. It is particularly important to maintain Spinosad because it is a low impact material, and there is even an organic formulation.

Spinosad is the second most widely used insecticide in strawberries, and figured to become even more widely used with the loss of Lannate.

Mites also became more of a problem in many strawberry areas around two years ago, and Zalom said he is still not sure why.

“When someone tells me they have mite problems, the first thing I ask is, what are they doing different? Are they spraying something different? Have they changed their fertilization or irrigation programs? All of these things can affect mites,” he said.

But Zalom had a stroke of good fortune in his ongoing trials of miticides. He noticed a reduction in both thrips and whiteflies as a side effect in trials of applying the miticide Oberon in trials. He is setting up trials to systematically study whether this miticide can be useful in managing these other pests.

[“AgAlert” is a California Farm Bureau Federation publication. For more information, please visit http://www.cfbf.com/agalert/]
 

• Bob Johnson, reporter, bjohn11135@aol.com
• Steve Adler, editor, AgAlert, sadler@cfbf.com