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Traps help monitor pine root weevil in Christmas trees (Research Brief #5)

Posted October 1992

An easy-to-use trap may allow Wisconsin Christmas tree growers in the future to market healthy trees with less insecticide. Developed by University of Wisconsin-Madison entomologists, the traps currently are enabling researchers to monitor the pine root weevil. The insect can inflict heavy damage on Christmas trees and requires a lot of insecticide to control.

Monitoring the pine root weevil could help determine whether insect populations are posing a problem to Christmas trees. If the traps show low weevil numbers, growers could avoid unnecessary insecticide applications, saving them money and reducing potential harm to the environment.

Currently, Wisconsin ranks second nationally in Christmas tree production, with about 500 producers growing 40,000 acres of trees. The wholesale value of Christmas trees sold by state growers each year amounts to $37.5 million.

Several species of pine root weevils attack Christmas trees. Root collar weevils inflict the most serious damage, feeding inside the roots of the trees. The foliage discolors, and eventually the tree dies.

Because the larvae feed underground and the adults are active at night, pine root weevils are difficult to detect. Growers don’t know the insects are present until the trees start to fade in color, and then there’s no way to protect the trees, says Ken Raffa, a UW-Madison entomologist. “In some years, you may have failures of entire plantings after eight or nine years, or just as they reach a harvestable stage,” he notes.

Christmas tree growers must control pine root weevils before they infest a plantation, or trees cannot be marketed, Raffa says. This forces growers to apply an insecticide on a preventive basis, which increases their costs if they spray areas that don’t need treatment.

Growers currently apply the insecticide lindane to control pine root weevils. Lindane can pose serious problems because it persists so long in the environment. In addition, many Christmas trees are grown on sandy soils, increasing the threat of groundwater contamination.

Although pine weevils always pose a potential threat, their populations fluctuate dramatically between seasons and between farms in a given season. Frequently, there is no need to treat trees, says Raffa.

“If growers had a way of knowing when they had a problem, then they could spray only when and where necessary,” he notes. “Growers then could use pesticides that are less harmful to the environment.”

Several years ago, Raffa and entomology graduate student Lynne Rieske developed an ethanol/turpentine trap to monitor pine root weevil numbers. Made from PVC pipe with holes drilled at one end, the traps are placed vertically into the ground with the holes even with the soil surface. The ethanol and turpentine attract the insects to the trap, where they fall in the holes.

Raffa conducted preliminary studies in 1988 and 1989 to compare traditional preventive sprays and sprays based on trap catches. In some areas where he found low numbers of weevils in the traps the damage to trees was also low. “Growers who sprayed during that time were wasting money,” Raffa notes.

Rieske’s studies on five farms in 1990 also showed a strong relationship between the number of weevils caught in the traps and damage to trees. On 15 farms in 1991, Raffa tried to make use of the traps more practical by setting out fewer traps per acre. However, he notes, “Fewer traps didn’t work nearly as effectively as we would have liked.” In addition, the traps were difficult to deploy.

This summer, Raffa is increasing the number of traps per acre. He is also trying a new style of trap — urine sample cups. The new traps are light, easy to use, and readily available, he notes. The goal of the research is to determine weevil threshold levels under practical conditions where eventually growers themselves would deploy the traps and monitor for weevil numbers, says Raffa.

Several companies are interested in the potential use of nematodes as a biological control against pine root weevils. Nematodes are tiny worms that infect the weevil larvae and adults. Infection can kill or sterilize the weevils. Raffa will begin trials next spring to determine the effectiveness of nematodes in controlling weevils and the best time to apply the nematodes. The traps could work well in conjunction with the release of nematodes into the soil, he says.

“Wisconsin growers are much more aware about the need to be judicious in their pesticide applications than they used to be,” Raffa says.

Raffa adds that growers support testing the potential of nematodes to control root weevils but first want to see if they work.

Raffa’s work is tied with Wisconsin’s Christmas tree integrated pest management program to reduce pesticide use. Disease and pest scouting are part of the holistic approach that includes selecting disease-resistant seedlings in an overall management program.

The study has received support from the Center for Integrated Agricultural Systems, which supplements funding from the Christmas tree growers associations in Wisconsin and several neighboring states, the U.S. Department of Agriculture, and the Wisconsin Department of Agriculture, Trade and Consumer Protection Sustainable Agriculture Program.

For more information about this research, contact:

Ken Raffa
Department of Entomology
University of Wisconsin-Madison
Madison, WI 53706
(608) 262-1125

Published as Research Brief #5
October, 1992