A new approach yields great results

A close-up of a larva emerging from a cherry and exit hole (on left).
 
Tim Smith applying bait to a backyard tree.

Solutions sometimes come from the most unexpected places. That’s what Tim Smith found when he experimented with controlling the cherry fruit fly with GF-120 bait, the substance the USDA developed to replace aerial malathion spraying for the Mediterranean fruit fly (Medfly).

“The USDA wanted some way of controlling the Medfly and other tropical fruit flies,” says Smith, a researcher and educator at Washington State University’s Tree Fruit Extension program. “It wasn’t intended to be used on the cherry fruit fly.”

Growers needed a replacement for the two most common insecticides that were effective against the pest, azinphos-methyl and carbaryl. The organophosphates were receiving special regulatory attention, he says, and it was feared that if their use was restricted, there would be few alternatives.

Other control measures, mainly parasitic wasps and picking all the fruit from the trees, weren’t effective enough. California, Australia and Japan, major cherry importers, have zero tolerance for the cherry fruit fly, Smith says. Even one larva in a shipment of fresh-packed cherries can cause the entire shipment to be rejected.

Another researcher was trying the new bait on the walnut husk fly, a close relative of the cherry fruit fly, so Smith decided to give it a shot.

“The idea of using this seemed ridiculous,” he says. “The bait gets spattered on.” Normally, growers spray carefully to get uniform coverage.

There were virtually no cherry fruit flies for him to experiment on in commercial orchards because growers were using the organophosphates. The only places he could find the flies were in backyard trees.

In 2002, he sprayed some backyard trees that had been infested the previous year and was surprised to find no larvae at all at harvest. He expanded the experiment the following year to about 30 trees that had also been badly infested the year before, and he again had success.

The bait does need two seasons for 100 percent control in heavily infested orchards, and it didn’t completely control infestations in areas near infested trees that weren’t being sprayed.

It was registered for use on conventional cherries in 2001 and on organic cherries in 2004. “Organic growers jumped on it wholeheartedly, as did conventional growers,” he says.

Life cycle

Cherry fruit flies are native to North America. They were first found in commercial cherries in the Pacific Northwest in the early 1900s, and are the primary insect pest for sweet cherries there. They live on sweet, tart and wild bitter cherries.

The flies spend the winter as pupae under cherry trees, 1 to 6 inches below the surface of the soil. Adults emerge over an eight-week period. The single generation begins in May, peaks at harvest time, when the fruit is soft and it is easy for females to deposit their eggs, and tapers off until it ends three or four weeks after the harvest. The adults live for 16 to 35 days.

They fly up the tree and begin searching the leaves and fruit for food: pollen, yeast, honeydew, microorganisms and bird droppings. Sometimes females will probe ripe cherries and feed on the juice. The flies may spend their entire life on the same isolated tree, and are closely associated with only this host, Smith says, which is the reason the bait works so well.

Females become sexually mature about seven to 10 days after they hatch. They lay between 50 and 300 eggs, typically one in each cherry. They also lay a little bit of hormone that other females can detect, which lets them know the cherry is being used. In highly infested trees, there may be more than one larva in a cherry.

When the eggs hatch, five to eight days later, the larvae feed on the cherry and burrow toward the pit. Ten to 21 days after that, they bore their way out and drop to the ground, where they burrow into the soil under the tree and overwinter as pupae. A few remain dormant for two years.

How it works

GF-120 bait contains feeding stimulants and spinosad, a nerve poison that comes from a naturally occurring soil bacterium called saccharopolyspora spinosa.

“Soil is a zoo,” Smith says. “It’s full of organisms we don’t even know about.” Every once in a while, one produces an insecticide or herbicide when fermented. “It turns out that Mother Nature is a better chemist than humans.”

The bait sticks to trees in small gobs. Although the coverage is spotty, it works because the flies go to the toxicant instead of the toxicant going to the flies, he says. They’re so thorough at foraging that when they get within a few inches of the bait, they find it and eat it eagerly.

Although there’s a lag time before the flies find the bait, it kills them before the females can lay their eggs, Smith says. “It only takes a tiny amount of toxicant. Spinosad is very toxic to flies.”

Although spinosad controls many species of fruit flies infesting tree, fruit, nut, vine and vegetable crops, as well as ornamentals and non-crop vegetation, GF-120 bait doesn’t work as well on most of them because they fly from hosts in the general environment into the crop, ready to immediately lay eggs.

Bait being applied in an orchard.

Spray

Spraying should begin when the adults emerge from the ground, and continue once a week for about six weeks, depending on the cultivar, until after the harvest. The maximum number of applications allowed is 10.

It’s sprayed in a pulsating spray from a 10-gallon tank mounted on the back of a four-wheeled ATV, Smith says. The nozzle points in two directions, so trees on both sides of the row are sprayed at the same time, and the ATV can skip alternate rows. It only takes about two minutes to spray an acre.

The stream breaks up into fairly large particles, which are best because they allow the bait to last longer, and since it works as a bait, thorough coverage isn’t necessary.

The bait works best in areas with dry summers, Smith says. “In Washington, it sits there like a dry gooey blob for a week while flies are looking for it.” It washes right off if it rains and can dissolve in humid regions, and even with heavy dew. If it rains, the bait should be reapplied when the trees are dry.

“This product is incredibly safe,” Smith says. It has no impact on the natural enemies of other cherry pests, it’s safe to apply, and it can be used up to the day of harvest.

It’s also economical, in terms of labor, material and application. The bait has saved growers about $1.5 million a year in Washington and reduced the use of other insecticides by 70,000 pounds in 2006 and in 2007, he says.

Using ATVs, which use about 3 gallons of gas per acre, instead of tractors saves the cherry industry in Washington more than 2,000 gallons of gas per year, he says. The bait takes less time to apply than conventional sprays, and only 1 gram of spinosad—the equivalent of a packet of sugar—in a gallon of molasses covers more than 6 acres.

Future

GF-120 bait is now the most commonly used product for controlling the cherry fruit fly in Washington, one of the nation’s largest cherry-producing states. It’s also widely used in British Columbia.

Although it was developed as a conventional spray, organic growers have benefited from it, Smith says.

“This is a conventional product that organic growers have accepted,” he says. “It has absolutely bailed out organic growers. Three years ago, the cherry fruit fly was at the top of their list. They’ve announced that it isn’t a problem [now].”

The author is a freelance writer based in Altadena, Calif.