In the battle of the bug, the University of Florida is allying its high-tech resources with mother nature to control unwanted pests.
After half a century of relying almost exclusively on chemicals to combat agricultural and household pests, scientists now try to exploit natural enemies to do the job.
The 1940s’ philosophy that chemicals are the solution to pests has been supplanted by a more environmentally benign, and fiscally prudent, approach in the 1990s.
“Biological control can make an impact on agriculture, in Florida and nationwide, by improving crop yields and quality,” says UF entomology Professor Marjorie Hoy, Davies Fisher Eckes Eminent Scholar in Biocontrol. “Biological control also helps environmentally. It reduces pesticide applications, which cuts production costs, reduces impacts on the soil and groundwater, reduces food residues, reduces our impacts on other species and makes things safer for our agricultural workers.”
John Capinera, chairman of the Department of Entomology and Nematology in UF’s Institute of Food and Agricultural Sciences (IFAS), adds: “We are gaining a greater appreciation for the limitations associated with near-total dependency on chemicals and we recognize the need for options.”
Some of those options take advantage of a greater understanding of the thousands of predatory and parasitic species available in nature, while others employ new knowledge about genetics that allows scientists to manipulate nature.
Biocontrol seeks to establish a natural balance in which the pest, while never totally eliminated, is reduced to a level where it causes no serious damage.
With year-round tropical weather, Florida is particularly vulnerable to a wide variety of pests, so IFAS researchers have devoted considerable effort to developing new ways of eliminating them.
Biological control research at UF is conducted primarily in the departments of plant pathology and entomology and nematology. Plant pathology seeks ways to make the plants themselves more resistant to attack, while entomology and nematology seeks to identify natural enemies.
South Florida citrus farmers had just replanted thousands of acres of groves after the devastation of Hurricane Andrew when they identified a new kind of natural disaster in May 1993.
Phyllocnistis citrella, the citrus leafminer, had arrived in Florida. And all those young trees were “nothing short of leafminer paradise” for the tiny moth, according to one IFAS publication.
The silvery and brown moths lay their eggs on the underside of young leaves. The hatching larva bores directly into the leaf and begins “mining” (eating), leaving trails that zigzag around the leaf. Ultimately, the larva pupates at the edge of the leaf, emerging from its cocoon as an adult. For the tree, the result is lost leaves, retarded growth and reduced yield.
By the time growers detected the leafminer, more than 90 percent of Dade County’s newly replanted lime groves were already infested and it soon spread to all of the state’s citrus-growing counties.
Hoy and her colleagues on a task force of state and federal entomologists and growers quickly determined that chemical pesticides were not going to be particularly effective against the leafminer, so they began to concentrate on alternate ways to control the bug.
Although the leafminer is native to Southeast Asia, logistically it was easier to go to Australia, where the leafminer has been around since the early 1940s.
After consulting with Australian scientists and growers, Hoy returned to Gainesville with several thousand adult specimens of a tiny wasp, Ageniaspis citricola, which acts as a parasite on the leafminer.
The wasps lay their eggs on the leafminer larvae. The immature leafminers continue to eat the citrus plant, but what emerges from their pupal chamber is not an adult leafminer, but a new generation of wasps, breaking the reproductive cycle.
“Instead of a moth, you get anywhere between one and 10 wasps,” Hoy says. “As efficient as the leafminer is, these parasites might be better.”
The wasps do not attack any other species and they seem to be successfully adapting to Florida.
“In some groves, there is a 99-percent parasitism rate,” Hoy says. “For every 100 leafminer pupae you open, 99 of them have the parasite. They’ve also spread rapidly, with groves 10 to 12 miles from the nearest release site showing signs of them.”
Hoy is continuing to research biological control of the leafminer, working with other natural enemies she has found in Thailand and Taiwan. This work is being done in collaboration with Ru Nguyen of the state Division of Plant Industry and IFAS scientists Jorge Pena, Robert Bullock, Phil Stansly, Joe Knapp and Harold Browning.
For most persons, the most common use of pesticides occurs right in their own home — when they spray for insects in their kitchen or garden.
“Think of what you do when you see a roach in your kitchen,” says UF entomology Professor Jerry Stimac. “You spray it so much you wet it down. That’s much more pesticide than you need in your environment.”
To combat excessive exposure to humans, Stimac is developing a line of nontoxic home pest control products that use a natural fungus, Beauveria bassiana, instead of chemicals. Stimac found the fungus while searching for a control for fire ants.
Native to Brazil, red imported fire ants were first seen in Mobile, Ala., in the 1930s and are now firmly established in the southeastern United States, ranging as far west as Texas and as far north as North Carolina. Experts believe the ants were introduced by ships transporting goods from Latin America to the United States.
Fire ant control is a serious problem in Florida. Over the past 10 years, state and federal agencies have appropriated more than $200 million toward wiping out the pest.
In addition to the danger of multiple stings to children or animals who accidentally disturb their mounds, fire ants damage crops by feeding directly on the plants or by protecting other harmful insects. They chew the bark and growing tips of citrus trees. They have even caused sections of roads to collapse by undermining the asphalt.
Powerful pesticides may knock out the tenacious little insects temporarily, but in the long run chemicals actually help them.
“When you use a broad-spectrum insecticide against them, it kills everything, including the beneficial insects,” Stimac says. “And the fire ants are the hardiest, so they are the first to come back. And when they come back, there aren’t any natural enemies to hold them in check, so the fire ants become even more established.”
Like Hoy, Stimac decided to go to the ants’ native habitat in search of a natural enemy. In this case that habitat was the Brazilian Patanal, a region similar to the Everglades.
“The levels of fire ants in the United States are 10 times higher than in their native habitat,” he says. “It was obvious that something was controlling them.”
But identifying that limiting factor was not easy because the swamps where the ants originated are some of the most complex ecosystems on Earth.
It wasn’t until Sergio Alves, a colleague at the University of São Paolo, noted that fungus seemed to affect fire ant populations that Stimac redirected his search.
“We were looking for some sort of predator or parasite,” he says, “but it turned out that the answer was in the dirt under our feet.”
The fungus in the soil of the Pantanal adheres to insects it contacts. The spores then penetrate through the outer shell and begin reproducing within the insect’s body. The insect eventually dies and a fuzzy coating of the fungus grows on the carcass.
But identifying the fungus was only the first step in a long process toward a commercial product.
It wasn’t until after Stimac had isolated the fungus and brought samples back to his lab in Gainesville that he and his colleagues realized that it also worked on other common household insects, including a dozen species of ants, cockroaches and termites.
Stimac and his colleagues are now developing baits that will lure different types of insects, which are naturally wary of the fungus, to come into contact with it.
“Our ultimate goal is to replace most toxic chemical pesticides used in and around the house with a nontoxic biological pesticide,” Stimac says.
Stimac envisions fungal traps similar to the ant and roach traps now on the market. The fungus can live for about three or four months, but since it cannot reproduce efficiently in Florida’s climate there is no danger of any adverse effects from it.
If You Can’t Find It, Make It
While Hoy and Stimac found natural enemies for their pests, UF plant pathology Professor Ernest Hiebert is using genetic engineering to create a natural enemy for the tomato mottle geminivirus, which can cause millions of dollars in damage annually to Florida’s $650 million tomato industry.
Until now, the only way to control the virus, which stunts growth of the tomato plant and reduces the size and number of fruit, has been to use pesticides to control the whitefly, which transmits the virus.
“We decided that the approach to take was to concentrate on the virus itself and not worry about the whitefly,” Hiebert says.
Typically, researchers use a tumor-causing bacterium, Agrobacterium tumefaciences, to introduce new genetic material into tobacco plants, which accept genetic changes faster and easier than other plants. Researchers replace the tumor-causing gene with whatever gene they are studying.
Hiebert’s research team members were doing some preliminary genetic work on tobacco plants when they discovered a nondestructive mutation of the geminivirus gene that can be used to “inoculate” tomato plants against the disease.
Over the next three years, Hiebert’s team will genetically engineer a significant number of tomato plants, then evaluate them for virus resistance and horticultural traits. Plant breeders will use selected lines with desirable traits to develop seed stock for farmers.
Hiebert’s research is funded by UF, the U.S. Department of Agriculture and the Florida Tomato Committee. He has applied for a patent for the new gene.
Hoy also engages in genetic engineering, with a tiny mite that may someday help to control spider mites, a well-known pest of strawberries and ornamentals in Florida.
Hoy has a request pending with state and federal regulatory reviewers to release a transgenic arthropod — a genetically altered predatory mite that feeds on the disease.
While scientists around the world have genetically transformed plants, animals and insects, this is the first time a researcher has requested field-testing of a genetically engineered arthropod.
Hoy has inserted a bacterial marker gene into the small, flightless mite called Metaseiulus occidentalis, which is no larger than the period at the end of this sentence.
To get new genes into the mites, Hoy and her colleagues trap the mites between layers of tape and use tiny needles to inject DNA into their abdomens. The genes find their way into the mites’ eggs and some offspring then carry the gene. These successful offspring are themselves bred and individual lines are developed which all carry the gene.
Although at this point Hoy is using only a non-beneficial, but easily identified, marker gene to test the viability of the process, ultimately she hopes to endow the mite with useful genes.
Western farmers have used the mite for years to biologically control spider mites in fruit and almond orchards. One report says they have saved $20 million annually in pesticide costs in almonds. But repeated attempts to establish the mite in Florida have failed because of the climate.
As the first scientist to seek permission to release a genetically engineered arthropod into an outdoor test plot, Hoy has taken great pains to alleviate fears about mites escaping the test site and mutating into something harmful.
She will transport the transgenic mites, which have already been studied through 150 generations in the laboratory, to the test plot in sealed plastic boxes surrounded by water moats. In addition, the researchers will soak their lab coats in alcohol before returning to the lab, scatter sticky insect traps throughout the test area, spray pesticides around the perimeter every two weeks and destroy mite-covered leaves in sterilization chambers after they have been examined.
While Hoy acknowledges there is a small chance some of the mites could escape, she says in her application to the U.S. Department of Agriculture: “In the worst-case scenario, a desirable natural enemy valued for its ability to control pest spider mites would be added to the fauna of Florida.”
Hoy’s field trials will be monitored by the USDA’s Animal and Plant Health Inspection Service, the state Division of Plant Industry and UF. She plans to collect data on survivability, stability of the transgene and fitness of the new strain as a biological-control agent.
By: Michael Podolsky