Trench trapping, a technique pioneered in Canada, is the most recently-developed alternative control strategy for CPB adults. In 1992, plastic-lined trench traps were established at five commercial farm sites on Long Island. The traps provided excellent control of the overwintered adult CPB. At one location, over 100,000 CPB were trapped in 300 linear feet of trench in the spring. Over 50,000 migrating beetles were collected in the fall as well. The trench traps eliminated border sprays, saving growers up to $80 per treated acre.

Before constructing traps, you must know where the beetles are hibernating. Most beetles typically hibernate in the sunny, south or west facing sides over overwintering sites such as hedgerows or in the same field where they fed and developed. These sites can be identified during emergence (which may be too late for installing trenches) or, better yet, by recalling where serious border damage occurred in previous years.

Trench traps should be constructed either between the overwintering sites (woods, hedgerows, ditches) and the recently planted potato fields or between adjacent fields in an alternate rotation.

Trenches should be installed as close to overwintering sites as possible, at least one week before adults are expected to emerge. The closer the trench is to the hibernation site, the less chance of beetles flying before they are trapped.

The trenches are made with a single-bottom plow, a V-shaped blade or a commercial trenching machine and lined with 4-to-6-feet-wide, 1- to 1 1/4-mil black ploy mulching plastic. There appear to be no differences in control between smooth and embossed-type plastics. The trenches should be at least one foot deep and 69 to 24 inches wide at the top, depending on its shape. Trench traps can be either U- or V- shaped with side walls sloping at angles between 45 and 90 degrees. The mulching plastic can be laid by hand or by a modified mulch layer. Researchers at Agriculture Canada in New Brunswick are currently developing plans for a machine which will construct the trench and line it with plastic in one operation.

Relatively soft, fallow ground will make the construction of these traps much easier. When digging a trench, hills of soil are placed at the top of the trench. Before lining the trench with plastic, this crown of soil should be leveled since it will interfere with the movement of adults into the trench.

The plastic should be kept taut when laid in the trench to eliminate wrinkles, which provide a potential escape route for CPB. Installation of the plastic on a warm, sunny day will aid in minimizing this problem. For rain water drainage, small perforations spaced ten feet apart should be made in the bottom of the trench. The trapped beetles will try to escape from the trench by crawling through the drain holes. However, in the moist conditions under the plastic, these CPB are attacked by a fungus parasite, Beauveria bassiana, and die.

Ends of trenches need to be constructed very carefully. A mound of soil will suffice. However, heavy rains may was the soil away. A 3/4-inch (by the width of the trap) board placed into the trench and secured firmly in the soil has provided the most effective ends. The plastic will have to be pulled tightly over the board and secured outside the trench.

Usually during the installation of the plastic or within a day after the trench is completed, a fine coating of dust will cover the plastic. The dust is the key to preventing the CPB from escaping the trench. It accumulates on the beetles' tarsi (feet) causing them to slide into the trench when attempting to crawl out of the trap. Although rainfall will remove some of the dust, a sufficient amount will remain to prevent the beetles from escaping. Within a week, the trapped beetles die of dehydration.

Very few adults escape from the trap by flight. A few try to fly but usually crash into the walls. Other potential escape routes are wrinkles on the walls and at the ends of the trenches.

To maintain the trench for use in the fall, when the adults migrate out of the field, the traps must be kept relatively free of weeds. An application of herbicides along the top edges of the trenches during the middle of the summer should keep the traps clear of weeds. Studies will be conducted in 1993 to determine if the trenches can be used for a second season without replacing the plastic.

In the spring, 59 to 75 percent of the overwintered adults disperse by walking. These will be caught in the lined trenches. The number of CPB trapped will vary, however, depending on location of traps with respect to the overwintering site and the air temperature (which affects the number of beetles walking). The traps should catch a higher percentage of CPB migrating out of the fields in the fall since the summer adults emerging from pupation after mid-August will not develop flight muscles until the following spring.

Note: For more information about trench traps, a six-minute video, Trench Trapping the Colorado Potato Beetle, is available for $20 from Cornell Cooperative Extension, Suffolk County, 246 Griffing Avenue, Riverhead, NY 11901-3086.

Colorado Potato Beetle Pictures and Links

Vern Grubinger, Cooperative Extension System, University of Vermont

Published in Grower, Vegetable and Small Fruit Newsletter. April 1993. Vol. 93-4. p 1-2.

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