Using pheromones for mating disruption of codling moth is so widely accepted in the tree fruit industry that it’s become automatic for most orchardists.
But imagine where the industry would be without the technology.
In the nearly 25 years since mating disruption was first used in Washington apple and pear orchards for codling moth, pheromones have become the foundation of pest management programs and have allowed biological control to become feasible. Passive and active release pheromone dispensers have been developed and research has fine-tuned pheromone technology to be both cost effective and efficacious.
“It’s been a paradigm shift,” said Donald Thomson, director of research and development for Pacific BioControl Corp. of Vancouver, Washington.
“Ninety percent of all apples and pears in Washington State are now treated with mating disruption of one formulation or another. That’s quite a remarkable achievement.”
It took many hands—from growers to pest control advisors to university researchers—to move the tree fruit industry away from broad spectrum, organophosphate pesticides to mating disruption, he says.
Use of pheromones for pest control goes back more than a century, according to Thomson.
French entomologist Jean-Henri Fabre is credited with discovering in the 1870s that the giant peacock moth communicated with chemicals. Nearly 100 years later, in 1959, Adolf Butenandt of Germany determined the chemical structure of silkworm pheromone.
In 1967, University of California’s Dr. Harry Shorey (who would later help develop aerosol pheromone emitters) conducted some of the first mating disruption trials with cabbage loopers. The U.S. Environmental Protection Agency approved the first registered pheromone—which was for pink bollworm control in cotton—in 1978.
“We learned early to ‘hang ’em high and hang ’em early (in the season),’” he said. Another initial learning was to think big—treat big orchard blocks and not small ones. Because mating disruption was found to work best in large areas, several area-wide programs were established.
Also, using mating disruption under high codling moth population pressure required supplemental sprays. “Without access to Guthion (azinphos-methyl) in the early years of mating disruption, we wouldn’t have had the success we did,” said Thomson.
Experience also taught growers to pay attention to abandoned orchards and bin sanitation, both of which could harbor codling moth larvae, and to stay off steep slopes because the terrain interfered with the pheromone plume.
In recent years, scientists have applied new information about codling moth phenology to make mating disruption more effective. (See “Mating disruption tips.”)
Codling moth, which has successfully established itself in major apple producing regions of the world, likely originated near Kazakhstan, home of the apple, he said. “It was likely spread along the silk trade routes and went wherever apples were sent.”
So it’s not surprising that codling moth—coming from the desert climate of Central Asia—like it warm. Scientists have found that flying activity of male and female moths slows down and even stops when temperatures drop below 60°.
“If they’re not flying, that means there’s no sexual activity,” said Thomson, adding that more importantly, eggs are not as viable in cool, spring temperatures compared to the heat of summer.
Spring temperatures also impact how fast codling moth populations build within an orchard. At warm temperatures, populations can double within 30 days, but at cool temperatures, it could take 90 days.
“The lesson here is that during years with wet, cool springs, codling moth risk goes way down compared to those with warm, dry springs,” he said.
Thomson said that many researchers contributed through the years to codling moth biology.
U.S. Department of Agriculture entomologist Dr. Alan Knight, based in Yakima, Washington, showed that as female moths age, their egg production drops. Dr. Vince Jones of Washington State University showed that at day one of a female moth’s life, 50 percent of her eggs hatched, but at day six, only seven percent hatched.
Scientists also noticed that codling moth like to aggregate within an orchard and create hot spots. Research showed that the more traps placed in orchards, the better.
“More traps provide more information, especially in orchard ‘hot spots’ and allow growers to make better decisions,” he said.
Scientists have studied pheromones in the orchards but are not exactly sure how they work. Some think that the plume from pheromone dispensers creates a false trail, causing males to fly to the dispenser instead of to females. Others say the pheromone masks the odor trail of female moths so that the males can’t find them.
Dr. Larry Gut, Michigan State University entomologist, found that “competitive attraction” plays an important role. Males follow the pheromone plume to the dispenser and find the dispenser instead of a female, explained Thomson. The male olfactory sensors, while at the dispenser, become overloaded with scent and desensitized.
Thomson noted that scientists have used titanium smoke to track the pheromone plumes of passive and active pheromone release dispensers.
They’ve found that for passive dispensers, such as hand-applied ties, the size and reach of the plume stays the same regardless of the concentration of pheromone in the dispenser. However, active pheromone aerosol dispensers work by creating a big burst of small particles (70 to 80 micron size) that travel long distances of 400 to 500 feet.
What is known is that the pheromone plume makes it harder for males to find females and delays mating.
When the males do find their target, the females are older and not as reproductive, all of which help keeps population numbers lower.
Thomson spoke during the annual meeting of the Washington State Horticultural Association. •
Mating disruption tips
Entomologists and the tree fruit industry have learned a thing or two in working with mating disruption to control codling moth during the last 20-plus years.
Pacific BioControl’s research and development director Donald Thomson summarized years of research and knowledge into the following take-home summary for growers:
1. Biology is important. Risk goes down when you have cold springs because codling moth populations don’t grow as fast as in warm springs when females lay more and stronger eggs.
2. Codling moths aggregate, creating hot spots. You need plenty of traps to really know what’s happening in your orchards, especially if populations are high.
3. Point sources are important when using hand-applied dispensers. The more dispensers the better—100 dispensers are better than 50, and 200 better than 100, though not twice as better, he said. “If you want optimum control, use as many dispensers as you can. And if you have high populations, higher numbers of dispensers will give you better control.”
4. Pay attention to borders. Moths congregate on borders and move inward, but pheromone concentration is less on the edges, which sets up the orchard for mating and damage on border edges. The outside four rows need sprays or hand applied dispensers.
5. Be aware of high wind speed in new, high-density plantings. High winds can blow pheromone out. In windy regions, it may be better to use hand-applied dispensers to prevent movement of aerosol pheromone out of the orchard.
6. Aim to manage codling moth populations to keep fruit damage to zero.