A bug-eat-bug world
by Mary Aegerter | © Washington State University
Next time you sip a good beer, pause a moment and thank your neighbors. Chances are, the hops that give your beer most of its bitterness and some of its flavor and aroma were grown in Washington state. But what you can't see or taste in your brew is the change that's coming to the growing of hops.
David James, associate professor of entomology and extension specialist, calls himself a "pest manager with a focus on biocontrol." Biocontrol means using living organisms to control pests, whether plant or animal. Biocontrol also means establishing a lasting relationship between the control organisms and the pests, rather than eradicating the latter.
Classical biocontrol stems from the fact that many pests, especially plant pests, are introduced species that arrived here without the predators and pathogens that control them in their native habitat. Classical biocontrol entails a carefully regulated release of natural control agents into the pest's new habitat. It has been used safely and successfully, even spectacularly, on a variety of weed species, including tansy ragwort, purple loosestrife, and diffuse knapweed (see sidebar).
Conservation biocontrol is based on controlling what usually are native pests with the natural enemies or predators that should already be out there doing the work. The reason they're not out there working is that when we spray with broad-spectrum pesticides, we kill them. Fortunately, the use of broad-spectrum pesticides is on the decline. That means the natural control agents may soon be back in the fields doing their appointed tasks.
Biocontrol often is part of an integrated pest management (IPM) program that takes into account the economic, environmental, and sociological effects of any proposed pest management strategy. Washington has been the worldwide leader in IPM for apples since the 1960s (see sidebar), but the concept has been slow to take hold in some of the state's other crops. James came to Washington State University four years ago to help change the latter condition .
James started with hops, converting a conventionally treated hop yard outside his Prosser office to biocontrol. Since the state's growers collectively pay about $2.6 million for a single industry-wide spray for spider mites, the major pest of hops, they were interested in how biocontrol and IPM would work in his yard.
James's hop yard has not been sprayed for the past three years, and it produces a commercial crop. Thanks to the presence of three times as many predators of spider mites, the number of mites is the same as in conventionally sprayed yards. Which hops would you rather have in your favorite brew?
Intensive monitoring of pests and predators in his hop yard has shown James that the key to controlling spider mites is the early season. If predators come early, they can control hot spots of mites and limit their spread for the rest of the season. Those predators that come—not just one, but a complex army of them—seem tuned in to the smell of the mites, says James, and he's exploring the possibility of artificially enhancing the numbers of predators with chemical attractants. This year James will run a second test of methyl salicylate as an attractant. Last year, it worked well.
James also has been working on biocontrol systems for the state's grape growers. Deirdre Prischmann, a doctoral student studying in James's lab, compared spider mite populations in unsprayed vineyards to those in vineyards that had received traditional chemical sprays. She found significantly more mites in the latter. "If you use chemicals, you're going to have more mites," says James. The benefits of such work will be monetary as well as environmental—monetary not just in savings on the cost of pesticide application, but in the opening of new markets for Washington state wines.
At this point, little Washington wine is exported. It hasn't been necessary—which was not the case in Australia, where James first worked. Wine is that country's second-leading export, behind sheep and wool products. While developing its export market, Australia learned that European countries are strict about residues in wine. Australia took most of the chemicals out of its grape growing and keeps the remaining levels low. The country's "clean and green" image serves its wine making industry well.
As Washington produces more wine and looks to export, residue restrictions will be a powerful motive to drive change even without increasing domestic demand for clean and green.
Protecting the generalists
Obviously, the key to success for these biocontrol efforts is to not kill off all the good guys, whether or not they kill just one pest or many—as do the generalist predators such as those Bill Snyder, assistant professor of entomology, studies.
"Generalists often eat anything they bump into and can subdue," says Snyder. That can include good guys like pollinators or even each other. Spiders are generalists, as are praying mantises and ladybugs.
Generalists have special problems when you use broad-spectrum pesticides, for they tend to live longer than other insects and sometimes produce only one generation in a year. A fatal spraying may wipe out the entire year's predation possibilities. The multigenerational pests, however, will still haunt you later in the season.
With a switch from broad-spectrum to softer pesticides that target and kill specific pests, the generalists should survive. And softer pesticides are where we're going, whether you look at upcoming government regulations, a desire or need to export to countries that demand clean and green, the increased demand in this country for organics, or concern about what pesticides are doing to people and to the environment.
In addition to using softer pesticides, we also can enhance the environment so as to encourage the predators to stay, prosper, and reproduce, says Snyder. That may involve breaking up the monoculture and providing places for the beneficial predators to overwinter, such as James is suggesting for the parasitic wasp, or providing places for other stages in their life cycles to feed.
Several of Snyder's graduate students have worked on projects involving generalist predators. Renee Prasad is looking at the effectiveness of beetle refuges, or "beetle banks," for organic growers of cabbage crops. Cory Straub is experimenting with flowering plants that may be added to beetle banks to provide nectar for parasitic wasps. Amanda Koss, who has since received her master's degree, worked on potatoes, one of our most heavily sprayed crops. Aphids cause a disease in potatoes that results in poor storing qualities. In the past, growers have sprayed regularly with broad-spectrum pesticides. Koss looked at whether aphid predator densities increase when the new softer pesticides specific for sucking insects like aphids are used.
It makes wonderful sense to do all this, says Snyder—identifying predators and planting refugia or flowers. But as yet there has been little follow-up on whether any of this helps. It's not known whether you get more beneficials with refugia or beetle banks, or whether the beneficials you do get ever actually move out into the field and feed on the pests.
"You could actually be attracting all the good bugs out of the field and into the refuge or providing a refuge for the pest, not the beneficial," says Snyder.
The tools he and others need to determine what does and doesn't happen may well come from another WSU researcher, Vince Jones, associate professor of entomology, Tree Fruit Research and Extension Center, Wenatchee. Jones works on developing sampling programs to determine pest and natural-enemy densities, models that will determine the best time to sample for a given insect, and techniques that permit the study of how both pests and predators move through the landscape. Though he specializes in tree fruit, his work is applicable to a wide variety of crops.
"Movement and dispersal [are] intimately entwined with biocontrol and the newer management strategies" says Jones. And they aren't activities that have been easy or even possible to document before.
One technique Jones uses to study insect movement is to spray areas with marker substances that are not normally found outdoors—like soy milk or egg substitute. The low-tech spray then can be identified on insects found in traps by means of higher-tech laboratory procedures.
The markers can tell Jones where orchard pests are overwintering, where beneficial insects are coming from, or which insects are moving into or out of refugia and beetle banks such as those Snyder and James are looking at. He also can determine how far into an orchard the beneficials from refugia actually move.
Understanding the generalists, however, requires more. In addition to not yet knowing how they move around the landscape, we don't know what, how often, or how much they eat—things we need to know in order to understand their role in the system. It's not practical or probably even possible to get this kind of information with lab studies or by having students or technicians sit out in the field and watch the bugs. It is practical, however, to use DNA and sophisticated molecular techniques.
Tom Unruh, research entomologist, United States Department of Agriculture-ARS, and adjunct professor of entomology, WSU, is fine-tuning a method that will provide quantitative information on predator diets. He can look at specific DNA sequences to determine what pests or other beneficials are being eaten, even when during the season a predator might switch from one prey species to another. He also can determine how many prey a predator eats in one day by looking at how long the predator has gone without a meal. Large pieces of DNA digest more rapidly than small ones, and Unruh uses the abundance of different-sized DNA pieces in a predator's guts to determine how long it's been since the predator last ate.
The biology of the insect may dictate the ultimate usefulness of this. A lacewing will eat many insects at one time, stuffing itself as if at an all-you-can-eat buffet. Another predator may be more of a gourmet, eating just one, then going off to savor and digest it. The DNA pieces won't tell Unruh how many pests either predator ate at that one sitting. But he'll know about when it had its last meal. That, coupled with behavioral studies of how many prey a given predator eats in one sitting, can be used to estimate how many it eats in a day.
Unruh's technique is a far cry from the old "count the predators and prey" system. When you count, you only get numbers, he says. You get no information on whether the predator is eating that prey or whether it's feeding on something else. Given that there might be 10 or 20 species of predator out there, growers need to know which are the most important for controlling particular pests. They are the species he needs to monitor in order to determine whether to spray.
But knowing which predator is the most important doesn't come easy and may not yet be possible. The techniques that will provide this and much more information are under development and being tested. When they are ready—something both Jones and Snyder suggest might be the case in five years or so—growers may have the information they need to truly implement biocontrol.
Biocontrol is knowledge-based. It often means not doing what your first impulse says is right, says Jones. It's knowing that, because you haven't sprayed and killed all the predators, you can afford to wait a bit. It's knowing that the insects that can control the problem probably are out there, and it's knowing what to look for in order to determine whether they really are.
In some places, the process is well underway and showing promising results. That's the case for pear psylla, the major pest of pear orchards. Because one or two nymphs per leaf can cause economic problems, pear orchards have a low threshold for damage. Until recently, this meant many sprays with broad-spectrum pesticides. Things have changed, thanks to a substance more often found in cosmetics than in an orchard. Kaolin, a clay, originally moved into agriculture as a part of a technique developed to prevent fungal infections, says John Dunley, associate entomologist at the Tree Fruit and Research and Extension Center in Wenatchee. But kaolin also works to prevent pear psylla. When pre-bloom pear trees are sprayed with it, pear psylla don't come into the orchards early, don't feed, and don't build up populations. How it works isn't known.
A couple of years ago, Dunley recommended that Wenatchee River Valley growers use kaolin, and last year 95 percent of them did. If this continues and accomplishes what he hopes—essentially, chasing the bug out of the area—it will open the way for controlling other pests with softer, even organic, pesticides, and for biocontrol.
A group of growers in the Pashastin Creek area, south of the Wenatchee River between Leavenworth and Wenatchee, went that step further last season and used kaolin and organics. Their results were promising. The region is relatively isolated, so that the area-wide tactics weren't confounded by what happened in surrounding orchards. The costs to growers were about the same as they had been before. The growers who already were organic spent an average of $270 an acre for pest control, while those in transition from conventional to organic spent $360. Nearby conventional growers spent $490.
Dunley says the area is well on its way to creating an environment where natural enemies can live, work, and prosper. In time—all biocontrol is long-term—it will take less work and less money to manage pests this way.
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