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Growing More, Spraying Less
Research is helping growers of Wisconsin's most valuable fruit crop to reduce pesticide use without reducing pest control

In 1997, Wisconsin led the nation in cranberry production for the third straight year, producing more than 2 million barrels of the tart fruit. That has made a lot of folks proud in Warrens, Tomah, Black River Falls, Wisconsin Rapids, Manitowish Waters and other communities where the colorful harvest means income and jobs.

Cranberries are the state's most valuable fruit crop. Wisconsin's 1996 crop was valued at $120 million, which exceeded the value of all the state's other fruit crops combined.

Cranberries were once just part of a minor dish served at Thanksgiving and Christmas dinners. But today, processors turn most of this native North American crop into juice blends that consumers enjoy year round. To meet the growing demand, cranberry acreage in Wisconsin has gradually increased over the years, reaching almost 13,000 acres in 1996.

Critics fear that the chemicals growers apply to protect the crop from insects, weeds and diseases might harm wildlife in nearby wetlands. Yet unless growers control these pests, they can reduce cranberry yields by as much as 80 percent.

Now, a decade-long research effort by a CALS team of Extension horticulturists, entomologists and plant pathologists has led to a new computer-based tool that will help Wisconsin growers protect their crop with far fewer pesticide applications.

"We've found new ways, new tools, to control pests," says Dan Mahr, a CALS entomologist who works with Wisconsin fruit growers on ways to reduce insecticide use while keeping insects from damaging crops.

"I can foresee a day when cranberry growers control virtually all insect pests with 10 percent to 25 percent of the insecticide used historically," Mahr says. "Even this small amount will only be used in certain years when other methods have failed.

"The real story here is that growers were faced with a complex of pests that they were using chemicals to control. Largely because they were motivated, they've adopted new practices that reduced pesticide applications."

No more spraying by calendar

Until recently, there's been relatively little known about cranberry pests except what chemicals would kill them. In the past, applying a pesticide that cost far less than one percent of the crop's value was inexpensive insurance. Growers applied pesticides at set calendar intervals to protect the crop.

In 1986, the Cooperate Extension Service and the College launched a formal integrated pest management (IPM) program for cranberries. The IPM program emphasized checking growers' beds for pests called monitoring or scouting and applying pesticides only when pest populations were likely to damage the crop.

"The IPM program was the first time growers made pest-management decisions based on what was out there, rather than how long it had been since they last sprayed," Mahr says. "Growers who had been making several applications a year to control one insect found that they could skip one or maybe two of those sprays in some years." Today, Mahr estimates that 80 to 85 percent of Wisconsin's cranberry acreage is under IPM.

Cranberry processor Ocean Spray was so impressed by the UW efforts that it modified the Wisconsin IPM model for use in other regions.

"After we completed the IPM program, Ocean Spray asked us what more we could learn," says Mahr, who wanted to develop non-chemical alternatives based on biological information about the pests.

"We decided that the people using IPM needed tools that would tell them when to monitor for pests and what pests to look for. The program also needed better information on how numerous pests must be before they warrant control and how to predict economic consequences of decisions."

In the early 1990s, Mahr and Stephen Cockfield, a postdoctoral scientist, spent five years studying the life history and biology of cranberry insects. They focused primarily on the blackheaded fireworm, the most destructive pest of cranberry in Wisconsin.

Mahr wanted to predict when overwintering fireworm eggs would hatch so growers could time control strategies to the pest's most vulnerable stage. Although fireworm eggs hatch as much as a month apart in different years, Mahr can now predict the hatch within a day or two. That astounding accuracy means growers can precisely time any natural control or chemical strategy so it has maximum effect.

Mahr and Cockfield also discovered that if the larvae hatch before the cranberry buds begin growing, the larvae starve to death without control measures. The researchers found that when the eggs hatch later, growers can sometimes control the pest by flooding the beds briefly in spring. If growers must spray, Mahr says, they can use a natural insecticide from bacteria.

Pest Control Begins on the Desktop

For nearly a decade, Mahr dreamed of developing a computer program that supplied growers with the pest management information they needed. Today that program called Cranberry Crop Manager or CCM for short is at work in Wisconsin. Developed specifically for state growers, refined over three years and tested by growers, CCM became available in 1997. The program is a team effort from Mahr and Cockfield, horticulturist Teryl Roper, plant pathologists Patty McManus and Steve Jeffers, and computer programmer Paul Kaarakka.

With CCM, growers can use their computers to enter and track records on weather data, and pest monitoring and pest control applications for each cranberry bed on a farm. When growers encounter a potential pest, they can turn to CCM's plant pest profiles section. It contains an encyclopedia of information about the crop's pests, with information and photos of different stages, and recommendations for controlling 70 weeds, 12 insects, and five diseases.

However, it's the predictive aspects of CCM that make it more advanced than other IPM tools. CCM tells growers when to look for problems and presents alternative control measures for the crop's most serious pests.

"CCM lets growers anticipate events and plan a program that integrates decisions. It brings together the research and tells growers what to scout for and when to scout for problems," says Roper, the Extension fruit specialist in the horticulture department. "CCM tells them the positive and negative consequences of decisions they are considering."

"The new computer program will help growers integrate decisions about managing the crop by showing them how a decision about one aspect of cranberry management affects their options in other areas," says Brent McCown, the UW-Madison's Gottschalk Distinguished Professor of Cranberry Research.

Good Ideas Get Better

Although CCM just became available, the UW cranberry team continues its work to improve the way growers manage the crop.

Fruit pathologist Patricia McManus is focusing her efforts on cottonball, a fungal disease.

"Cottonball is the most economically important cranberry disease in the state," McManus says. "It doesn't affect all marshes. But if not treated, cottonball can damage up to one-third of the berries. Some growers apply fungicides four times a year to control it."

McManus is trying to develop ways to control the disease with fewer fungicide applications. She's testing different cranberry varieties to see if any might resist the disease naturally and studying how spreading sand over cranberry beds affects the disease. Thomas Johnson, one of McManus's graduate students, has identified bacteria on cranberry flowers that inhibit the cottonball fungus in the laboratory. Such bacteria might one day replace chemicals in controlling cottonball.

In the meantime, McManus is testing which fungicide applications are most critical for cottonball control. She is also evaluating several new fungicides. McManus wants growers to have at least two products so they can rotate them and delay the development of pesticide-resistant fungal strains.

Entomologist Mahr is concentrating on cranberry tipworm, a midge larva whose feeding causes cranberry vines to produce vegetative shoots rather than those yielding fruit. As with blackheaded fireworm, Mahr believes that early season control of the first tipworm generation is critical. Currently there is no way to monitor for the tiny midges.

Mahr and entomologist Kathleen Chapman are trying to identify chemicals that attract the insects so growers could put out baited traps and gauge tipworm populations. In the laboratory, the researchers have found that mated female midges are attracted to chemicals from cranberry leaves. They are testing those chemicals in traps and also a sex pheromone they identified that attracts males. Mahr also would like to see if generalized predators such as green lacewings are effective in protecting cranberries.

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