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A beeline to biocontrol.

As if the honey bee didn't have enough to do already.

Perhaps nature's hardest working insects, bees are usually busy making honey and pollinating billions of dollars worth of plants each year as they go from flower to flower in search of nectar.

Now Agricultural Research Service scientists are taking advantage of the honey bee's work habits, using it as the ultimate biological control courier. While picking up nectar and pollen, the bees (Apis mellifera) are dropping off biological control organisms along the way. The targets: fire blight, a bacterial disease of pears and apples, and corn earworms, one of the worst pests of corn and cotton.

In these studies, unsuspecting bees are delivering biocontrol agents fight where they're needed on the plant. If future experiments succeed, some growers might one day rely on bees to help them protect as well as pollinate their crops.

Bees Fight the Blight

Fire blight is caused by Envinia amvlovora, a bacterium that first colonizes a flower's stigma--the part that receives pollen grains. Although disease outbreaks are quite erratic, warm, wet weather provides prime conditions for E. armylovora to grow. The patbogen can spread quickly, forming cankers on twigs and branches, leaving trees with a bumt appearance; hence, the name fire blight. In some cases, infested trees may die.

At the ARS Bee Biology and Systematics Laboratory in Logan, Utah, bee expert John D. Vandenberg is collaborating with Sherman V. Thomson, a plant pathologist working on fire blight biocontrol at Utah State University.

In the 1980's, scientists discovered that spraying trees with high populations of beneficial bacteria could help prevent the disease. These bacteria outcompete E. amylovora bacteria on the nutrient-rich, moist stigma, so the pathogen can't gain a foothold and invade the tree.

The idea for enlisting bees to spread fire blight-fighting bacteria goes back to a century-old finding, says Thomson. In the USDA's 1895 Yearbook of Agriculture, Merton B. Waite published a study showing that bees-- along with ants, beetles, and several other insects--disperse E. amylovora among flower blossoms.

"I thought, if bees can disseminate the pathogen, why not use them to spread the biocontrol bacteria?" says Thomson.

Since orchardists use bees for pollinating, some of the experimental design was already in place. Each spring, many growers rent hives from beekeepers. These hives are large wooden boxes containing bee colonies, abuzz with anywhere from 10,000 to 100,000 bees.

"On a typical spring day when apple trees are in bloom, bees may be active from midmorning to midafternoon, when it's nice and warm outside," says Vandenberg. During that time, a bee may make several foraging trips from the hive, visiting as many as 100 blossoms each hour.

To automatically dust the bees with the biocontrol bacteria before they head for the flowers, the scientists used a pollen insert, a device already employed to enhance cross-pollination by some orchardists. Cross-pollination, the transfer of pollen from one flower to a genetically different one, fertilizes the flower so the fruit will develop.

The insert attaches to the beehive's entrance, forcing bees to exit through a flat, shallow tray as they leave the hive. Pollen spread in the bottom of the tray sticks to hairs on the bees' bodies. "For our experiments, we mixed bacteria with either apple blossom or cattail pollen," says Vandenberg. The latter, collected from nearby marshes by simply shaking cattails in a plastic bag, had the advantage of being free and abundant.

The bacteria are called Pseudomonasfluorescens A506 and Erwinia herbicola 318. Neither appears to harm the bees, says Vandenberg, noting that the commonly used bacterial pesticide Bacillus thuringiensis is considered harmless to bees.

Last year's experiments proved the bees to be an effective method for spreading the bacteria. "After just 48 hours," Thomson says, "we found the E. herbicola bacteria in 92 percent of 50 blossoms taken randomly throughout one 5-acre apple orchard." The weather on those 2 days was excellent for bee activity, he adds.

In another test on pears, they found that 72 percent of the blossoms had sizable populations of the good bacteria, in an area 20 feet from the hive.

A cold snap that froze the blossoms that year prevented a fair trial of the fire-blight control. But this spring, Thomson and Vandenberg tested the bee method again, on a total of 25 acres of pears and apples in Utah.

"We're working with four cooperating growers in two local counties," says Thomson. "If we're fortunate, the weather conditions will be right so we can see how well this treatment actually works."

Meanwhile, in Corvallis, Oregon State University and ARS scientists are also using bees to study fire blight. But they're dousing the bees with the fire blight bacteria itself, instead of the bacteria meant to fight it.

"We're using the bees to mimic a natural infection of fire blight," explains plant pathologist Joyce E. Loper, of the ARS Horticultural Crops Research Unit. Because fire blight infection is unpredictable, the scientists wanted to find a way to test whether or not the biocontrol tactics were working, rather than wait for a natural outbreak.

She and fellow plant pathologists Virginia O. Stockwell (ARS) and Ken Johnson (OSU) are also working with P. fluorescens strain A506 and a different strain of E. herbicola, C9-1. Their treatment plans include spray applications of both bacteria, alone and in combination, and sometimes with different antibiotics.

Currently, growers continue to spray antibiotics to suppress the fire blight bacteria, but resistant strains of the patbogen have made that practice less effective, says Loper.

Johnson set up a huge screen cage that completely enclosed 40 pear trees. When trees first began to bloom, he placed a bee hive in the cage, complete with a pollen insert that contained not pollen, but freeze-dried (yet still infectious) fire blight bacteria mixed with powdered milk and xantham gum.

Twice during the blooming period, researchers sprayed the trees with either a mixture of the biocontrol bacteria, an antibiotic (streptomycin), or plain water. Just before full bloom, the scientists found the fire blight bacteria in 43 percent of the watertreated blossoms, confirming that the bees were indeed carrying the pathogen to the trees.

What's more, they also showed that the biocontrol-treated blossoms were able to reduce the growth of fire blight bacteria on flowers. "With this disease," says Stockwell, "there has to be a certain amount of E. amylovora present--about 1 million cells per flower--to cause fire blight. We found that only about 3 percent of the blossoms sprayed with the beneficial bacteria had patbogen populations that high, compared to 20 percent of the water-treated controls."

Although the approaches of the two teams differ, they share the same goal. "The exciting thing about these studies," says Loper, "is their interdisciplinary approach--getting entomologists and pathologists working together on biological control. It's really helped advance the field." While working with biological control agents against the corn earworm in the mid-1980's, Harry R. Gross, an entomologist with ARS in Tifton, Georgia, was watching honey bees busily foraging for nectar in a field of crimson clover.

Then it dawned on him: What better way to carry insect pathogens into a field than via a honey bee? "Here's an ideal vehicle that is going right where we want the biological control agent to go," he said. So Gross and fellow Tifton entomologists John J. Hamm and James E. Carpenter began to work on a way to enlist bees in their battle against the corn earworm. The large, green-to-brown caterpillars cause an estimated $500 million damage each year to corn and cotton in the southeastern United States alone.

Bees Battle the Earworm

They chose a natural virus that infects only the corn earworm and tobacco budworm, but doesn't harm honey bees. Called a Heliothis nuclear polyhedrosis virus, it attacks the larvae and dissolves them, turning them to liquid, Gross says.

Instead of using a pollen insert to get the virus into the field, Gross developed a special device that works slightly differently. The structure allows the bees to enter and exit the hive through separate pathways.

Bees enter the hive through the bottom of the device and pass through a wire mesh to reach the colony. As they exit, they respond to light entering the hive through clear plastic panels in the top part of the device. The panel forces the bees to move downward into a metal tray that contains the virus in a fine powder. The bees' legs and underbodies become covered with the patbogen powder as they go back to the field, carrying the virus with them.

The scientists tried the technique with two hives rented from a local beekeeper in two 1-acre crimson clover fields near Tifton.

After letting the bees forage in the clover for a day, scientists collected the clover heads and fed them to corn earworm larvae in the laboratory.

The result: Between 70 and 90 percent of the larvae died, compared to between 10 and 15 percent mortality of larvae put on clover heads that had not been visited by vims-treated bees.

Gross says the technique isn't limited to using only one virus against the corn earworm. "What you choose to put in the hive is limited only by the user's imagination and by how it'll affect the bees," he says.

The technique could likely be used against any insect that feeds on flowering plants that bees visit, Gross adds, although he's not planning to continue the studies himself. "We used clover simply as a model to test the feasibility of the approach," he says.

However, controlling earworms in clover might indirectly benefit other crops. The caterpillar pests feed on clover and other early season plants before they move on to cotton, corn, and other crops.

Bees aren't normally used in corn or cotton fields; corn is wind-pollinated, and most cotton varieties don't require cross-pollination. But bees do like corn pollen and cotton flower nectar--so they might still work as transporters of biological control.--By Julie Corliss and Sean Adams, ARS.

John D. Vandenberg is at the USDA-ARS Bee Biology and Systematics Laboratory, Utah State University, Logan, UT 84322-5310. Phone (801) 750-2524. Joyce E. Loper and Virginia O. Stockwell are in the USDA-ARS Horticultural Crops Research Unit, 3420 NW Orchard Ave., Cotrvallis, OR 97330. Phone (503) 750-8771. Harry R. Gross, James E. Carpenter, and John J. Hamm are in the USDA-ARS Insect Biology Management Systems Research Unit, Georgia Coastal Plains Experiment Station, P.O. Box 748, Tifton, GA 31 793. Phone (912) 3872343.
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Author:Corliss, Julie; Adams, Sean
Publication:Agricultural Research
Date:Jul 1, 1992
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