Mating disruption helps control codling moths in walnuts, but requires more vigilance
By Vicky Boyd
After three years, Chris Locke, a walnut grower near Lockeford, Calif., has brought his codling moth problem under control by trading harsher pesticides for pheromones and increased vigilance.
Although mating disruption, as the technique is called, isn’t necessarily a stand-alone control measure, it can be once codling moth populations are reduced to low levels, says Joe Grant, a University of California Cooperative Extension farm advisor in Stockton. In the interim, insecticides supplement mating disruption to help reduce codling moths.
And secondary pests, which may have been suppressed by codling moth sprays, could increase to troublesome levels, Grant says.
Nevertheless, Locke says the environ-mental benefits of mating disruption far outweigh any possible drawbacks. And that’s why he has spent nearly a decade working with Grant and other University of California researchers on the technology.
“We farm along the Mokelumne River, and water quality is very important,” says Locke, who has about 580 acres. “We have urban neighbors where our trees overlap their backyards. And then there are our employees, too.
“It all makes good sense to control insects without having to run an orchard sprayer out in the fields.”
How mating disruption works
Mating disruption involves flooding an area with synthetic female insect pheromones, so males can’t find a female with which to mate. If females don’t mate, they don’t lay eggs. Nut damage is reduced, and so are codling moth populations over time.
The technique works best in orchards of at least 40 acres, where a pheromone cloud can blanket a large area. It has been used successfully for several years in the Pacific Northwest and California to control codling moths in apple orchards and in California to control codling moths in pear orchards.
Early attempts at mating disruption involved applying dispensers that resembled bread bag twist ties or small pheromone-filled pouches to trees. Recommendations called for placing about 200 to 400 per acre.
The dispensers had to be hung high in trees, since that is where most of the codling moths are found. The canopies of older walnuts can exceed 40 feet, making applications of these types of dispensers too costly and cumbersome, Grant says.
More recently, companies—such as Suterra in Bend, Ore.—have developed puffers, which involve a cabinet, a battery-powered electronic timer and an aerosol can. The dispensers work much the same as automatic indoor air fresheners, releasing a puff of chemical at set intervals.
One can lasts the entire season, and growers only need to hang one puffer per 2 acres in walnuts, bringing the application costs down to a more reasonable level, Grant says.
Although the one-per-2-acre rate is a guideline, Grant says they have had to hang puffers slightly closer to each other on the upwind edges of orchards to prevent gaps in the pheromone cloud.
The puffers are hung the first week of April and removed shortly before harvest. Locke’s crew used pruning towers and ropes to hoist up the canister and housing. The ropes were left in the trees at the end of the season for the subsequent year.
During the past three years, Grant and Locke have experimented with the puffers on Locke’s entire 580 acres. Grant’s university colleague, Carolyn Pickel, has conducted a similar experiment on 180 acres of walnuts in Glenn County for the past three years. She added a second trial in Butte County in 2007.
This year, several more California walnut growers from Tehema County south to Kings and Tulare counties are expected to try the technology, says Steve Wulfert, a Suterra technical sales manager based in Chico, Calif.
“It’s moved beyond the trial stages. Now it’s into the implementation stage,” Wulfert says.
Changes in monitoring techniques
Locke and Grant also use the industry standard 1X codling moth lures in traps to ensure the pheromone is working. Typically, the pheromone shuts down the 1X traps because they are baited with the same synthetic scent. If something is wrong, then the traps will pick up moths.
In addition, Locke and Grant use “combo lure” traps that are baited with a combination of a plant volatile compound—kairomone—that isn’t shut down by the pheromone and the pheromone attractant.
Grant admits they’re still learning about the combo lure traps, especially how trap captures can be used as thresholds for deciding if supplemental insecticide treatments are needed.
Grant says they learned from a few mistakes made in 2005 and 2006 about when codling moth trap catches may indicate an insecticide treatment.
“In the early years, our decisions on the need for supplemental insecticide sprays were based mainly on the amount of nut damage that occurred by the end of the first and second generations,” he says. “We learned that by relying so heavily on these nut damage assessments to trigger a spray, we were spraying too late, allowing codling moth populations in the orchard to grow, rather than reducing them as we wanted.
“As we gained more experience with combo lure-baited traps, we learned how to use trap catches to tell us when we had a population that may be getting out of hand. We learned we had to spray based on population, not damage.”
Although watching for damage is still important, Grant says it needs to be combined with an awareness of population pressures in the orchard.
Of the 22 blocks, Grant says Locke treated 16 with Lorsban or Warrior in 2006. But he didn’t have to treat a single acre for codling moth in 2007.
“Over time with fewer sprays, Chris has reduced codling moth damage well below the levels he had with his previous conventional program and from the 2 to 2 1/2 percent range in 2005 to below 1/2 percent in all blocks in 2007,” Grant says.
Watch for secondary pests
Some products that control codling moth also have a side benefit of suppressing secondary pests, such as walnut husk fly, aphids and mites, Grant says. “When we spray less for control codling moth, these secondary pests need to be watched more closely,” he says.
Wulfert with Suterra agrees.
“Every orchard is unique in that respect, and you can’t really go into this with the assumption that secondaries will be a problem,” he says. “You have to be vigilant, since [mating disruption] isn’t broad spectrum. And when you choose your intervention methods, you have to be mindful that you don’t want to upset the balance. It’s going to require more knowledge than people needed in the past.”
Walnut husk flies definitely have become a problem in his orchards, Locke says, adding he uses a spinosad product to control them.
Aphids also have become more troublesome, he says.
“We’re still working on getting aphids under control without having to spray,” Locke says. “We’re trying to increase our beneficials by planting cover crops and hedgerows.”
Research is shedding new light on the effects of walnut insecticides on aphids and their natural enemies, Grant says.
“Some of the materials we use to control codling moth and other pests have adverse impacts on aphid natural enemies. Choosing the right products to treat these other pests can help reduce aphid problems,” he says.
Grant and Locke plan to continue the trials this year on Locke’s 580 acres. In addition, Grant says he plans to expand the program to contiguous landowners who have several hundred acres of walnuts.
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