Biocontrols: An Alternative To Pesticides?

A plan to screen pesticides, fungicides, and herbicides for endocrine disruptor activity could be bad news for the manufacturers of those synthetics, but good news for research in biocontrols, a natural alternative to those chemicals. Recently, the Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC) recommended that about 15,000 commercial chemicals be tested for their interaction with reproduction and development (P. Smaglik, The Scientist, 12[18]:1, Sept. 14, 1998). Although those screenings have not officially begun, the completion of the project could mean the discontinuation of many products that have already been anecdotally implicated in birth defects in animals. Jo Handelsman "If we lose some of the key fungicides on which agriculture is currently dependent, then we need to supply some alternatives," remarks Jo Handelsman, a professor of plant pathology at the University of Wisconsin at Madison. Alternatives include boosting existing soil bacteria to fight fungus, transgenically transforming plants to fend off predators, unleashing native species such as spiders to eat other pests, breeding resistant species, and understanding plant biology enough to lessen existing chemical applications. Handelsman and colleagues have been working for years to develop one such alternative. Their approach relies on boosting the amount of Bacillus cereus--a fungus-fighting bacteria--in the soil surrounding soy bean plants to fight the fungus Cercospora. "We're not talking about importing species that might go wild because there's little competition. We're usually talking about augmenting the population of something that is there naturally." In one long-term study, her group found that applying the bacteria to soybean seeds resulted in protection similar to that of an equivalent chemical (R.M. Osburn et al., Plant Disease, 79:551-56, 1995). In addition, the amount of the bacteria in soil drops over time--so the additional bacteria doesn't appear to disrupt the ecosystem long-term. "It's a short-term disruption of the microbial flora," remarks Handelsman. "But it's long enough of a disruption to suppress disease and increase plant growth. The appeal is [that] you are taking a naturally occurring very, very common organism and increasing its population for a short period of time." Photo: Agricultural Research Service, USDA BUG SLINGER: A lightweight, biodegradable Aerodynamic Transport Body, loaded with insect predators or parasites, can quickly distribute beneficial insects throughout a field of cotton. However, the Environmental Protection Agency has been hesitant to approve the strategy, since the bacteria acts as a food contaminant when food is improperly stored or handled. Handelsman admits to some frustration over this roadblock, because Bacillus thuringiensis, which is almost identical in genetic composition' has been used as an organic control against insects for about 30 years. Handelsman hopes to cope with the problem by creating a transgenic strain that produces the antibiotics, but not the compounds that make people sick. Otherwise, the researcher may seek a new bacteria, but scientists have not yet discovered any viable alternatives. Plant pathologists Greg Upchurch, with U.S. Department of Agriculture's Agricultural Research Service (ARS), and Jon Duvick, with Pioneer Hi-Bred International Inc., based in Des Moines, Iowa, have adapted a different strategy to fight the same fungus in corn. Cercospora secretes the toxin cercosporin, which allows the fungus to consume plant tissues--decreasing crop yields by up to 50 percent. Ironically, no-till cultivation, which is intended to be environmentally friendly by reducing soil erosion, provides a better home for the fungus and, therefore, increases the need for more fungicide. The two researchers' approach basically turns the fungus' toxin on itself. Upchurch and his research team at the ARS Soybean and Nitrogen Fixation Unit in Raleigh, N.C., first isolated and cloned a gene that protects the fungi from its own toxin. They found the gene's function by inactivating it through mutation. When exposed to the toxin, a mutant strain with the nonfunctioning gene had 60 percent less growth than a strain with the intact protective gene. The researchers hypothesize that the mutated gene manufactures a protein that prevents the toxin from building up in plants. They named the protein cercospora facilitator protein ( cfp ). The scientists have taken the mutated cfp gene and expressed it in tobacco, since that plant transforms with relative ease . The transformed tobacco performed well against the fungus. "Our initial results look promising in terms of less disease in tobacco plants containing the cfp gene," Upchurch remarks. However, expressing the mutated cfp in other crops, such as corn, may be more difficult. "Corn requires a gene gun and some other technologies we don't have here." Pioneer scientists have made those transformations and will next test altered cfp corn in a greenhouse and in test fields. Rather than placing his trust in transgenics, ARS entomologist Matthew H. Greenstone suspects spiders may suffice. Spiders, like the bacterium favored by Handelsman, occur naturally. So finding ways to boost their numbers could limit pests more harmful to crops. Greenstone admits that before spiders can be used as agents of biological control, researchers need to understand more about what insects they eat and in what quantities. To aid that endeavor, Greenstone uses monoclonal antibody-based assays he helped develop to discern the contents of spiders' guts. Without those means, such analysis is impossible, since spiders suck the innards out of their prey without chewing. That tactic makes it hard for researchers to find out more about their prey. "It's just gunk." Photo: Agricultural Research Service, USDA SPREADING MITES: The Mite Meter, attached to a tractor, can dispense 500 to 20,000 biocontrol mites per acre. Last year, Greenstone and University of Georgia entomologist John R. Ruberson used the assays to determine which predators are attacking budworms and bollworms in cotton fields. Ruberson collected more than 3,000 predatory insects and spiders from one field and determined that about 6.5 percent were positive for budworm and bollworm egg remains. In other countries, such as Indonesia, rice farmers count the number of spiders and other insects in one area of the crop. "They know which are the good guys and which are the bad." They use that information to determine whether or not to use pesticides. "When there are (enough) spiders around they don't spray," Greenstone notes. "This has tremendously reduced the use of chemical insecticides." Using the assays in the U.S. could provide more sophisticated indicators. Greenstone notes that experiments by Susan E. Riechert and Leslie Bishop, both then at the University of Tennessee, Knoxville, showed that spiders can provide effective biocontrol in some situations (S.E. Riechert, L. Bishop, Ecology, 71:1441-50, 1990). They encouraged spider populations in vegetable gardens by putting down mulch between the rows. Normally, the exposed dirt--unprotected from the beating sun--provided a hostile habitat for spiders. Their experiment yielded fewer pests and more healthy vegetable plants. Still, Greenstone admits the approach may be difficult to implement in large-scale agribusiness--especially in corn fields, because the approach could be time-consuming and labor-intensive. And much remains unknown about the concentration of spiders in a given area necessary to control a particular kind of pest. "We don't know how many there should be for effective control." Finding those "threshold levels" will be a key avenue of research, Greenstone predicts. Photo: Agricultural Research Service, USDA ADJUSTABLE LEVEL: USDA agricultural engineer Joe Chesson (left), machinist Vic Penner (center), and agriculatural engineer Lyle Carter adjust the application level of the Mite Meter used to apply predaccous mites in a newly planted cotton field. ARS scientists Bruce W. Wood and Charles C. Reilly are also trying to reduce chemical use on pecan crops. However, they know of no natural predator that attacks the fungus that causes pecan scab. And "transgenic strategies--at least for the moment--don't seem viable. "For some reason, this species is very difficult to propagate and regenerate from cells," Reilly comments. "There are several genes involved in resistance--probably dozens, at least." So, instead, they have closely studied how that disease progresses in the environment. Both had assumed that recommended application levels were based on good information--a sentiment that echoes the EDSTAC report. "What you find in a lab may or may not be true with about what you find in the environment," Reilly comments. In this case, they noted a significant difference. "We discovered that the persistence of the fungi was less than originally assumed." Their experiments determined that dry weather makes the fungus less likely to appear, and also that applications early in the growing season were also unnecessary. Those findings could save farmers money, while also reducing the use of suspected endocrine disruptor triphenyl 10 hydroxide. An economic argument like that--rather than a purely environmental one--will help convince more farmers to reduce fungicide use. "As long as we can show that we can maintain profitability, it will be an easy sell."

Biocontrols: An Alternative To Pesticides?

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