By Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Cookies are OK, but I really like ice cream. It turns out that insects have food preferences, too. Extension specialists are working to understand those preferences, and put them to work in a new tool for managing vegetable insect pests, called Perimeter Trap Cropping, or PTC.  For the past few years, Jude Boucher of the University of Connecticut Cooperative Extension and Ruth Hazzard of University of Massachusetts Extension have studied PTC by conducting trials on research farms and in grower’s fields.

Trap cropping is not a new idea. Many growers and gardeners have planted traps crops to protect a main cash crop from a pest. To be effective, a trap crop has to be more attractive to the pest that the cash crop. Depending on the situation, a trap crop can be a different plant species, variety, or just at a different stage of growth than the cash crop. Trap crops work best on pests that are abundant and destructive just about every year, rather than for pests that only appear once in a while in your location.

What’s new about PTC is how the trap crop is managed. In the past, growers tended to plant a single row of a trap crop next to a cash crop. With  PTC, the trap crop is planted to completely encircle the cash crop, like a fortress wall.  Pests are intercepted along the border, regardless of the direction of attack. This simple redesign of the crop production system works equally well on large and small farms and is easy to use because the same machinery, crop spacing, and plant-culture system can be used for both the cash crop and trap crop.

Because PTC concentrates the pest in the border area, the effectiveness of this technique can often be improved by spraying the border as soon as the pest arrives. This prevents pests from moving into the main crop. Border sprays require significantly less time and pesticide compared to full field sprays. By avoiding cover sprays in the main crop, natural enemies are protected, too. PTC with border sprays may not eliminate the pest completely, but it can substantially reduce their populations on the main crop.

Blue hubbard squash has been used in many of the PTC studies so far. It was selected as a trap crop for other cucurbit crops because it is highly attractive to striped cucumber beetles, has vigorous seedlings, and is not as susceptible to bacterial wilt as other attractive varieties.

Blue Hubbard around yellow summer squash has been studied for several years in Connecticut. In experimental plots, over 94% of all the cucumber beetles were found on plants in the perimeter. Beetle populations on the unsprayed main crop in the center of the plots were reduced by up to 93% where sprays were applied to the Hubbard squash border, compared to plots without perimeter sprays or trap crops. Spraying the perimeter trap crop also reduced squash vine borer infestation on the unsprayed summer squash inside the plots by 88%. Six commercial growers employed this technique in 2002 and 2003, and all of them improved their pest control and reduced crop damage. All but one said the system was simpler to use and saved them time and money. Blue Hubbard works equally well around cucumbers or melons.

In Massachusetts, experimental plots planted in 2003 with Blue Hubbard borders had eight times as many cucumber beetles per plant in the border than those with butternut at the border. The number of beetles per plant in the unsprayed main crop was reduced by 60% where there were sprayed Blue Hubbard borders, compared to the control. The same system was tested in six commercial fields of butternut ranging in size from two to six acres. On average, more than seven times as many beetles per plant were found in the borders of protected fields than on the main crop. At one point over a hundred beetles were counted on one plant in the border. Fortunately, the border had recently been sprayed and most of them were dead. Unfortunately, a combination of heavy rains and cold, wet soils led to poor crop emergence and the incomplete borders did not hold up against the beetles. In several cases growers needed to use a full-field spray because of the combination of high beetle pressure and poor trap crop emergence. It may be necessary to use a double border along woods edges, and possibly a systemic insecticide treatment to help keep the borders intact enough to protect the main crop even in times of extremely high beetle pressure.

PTC can also be implemented using a different variety of the main crop species in the border. Prizewinner, a giant pumpkin variety, is highly attractive to cucumber beetles – more attractive than standard pumpkins. One grower in 2003 planted Prizewinner as a perimeter around a field with a mix of other pumpkin varieties as the main crop. Despite the fact that half the field was planted late due to the wet weather, the beetles were heavily concentrated on the border plants. There were three times the numbers of beetles in the border than in the main crop. The grower sprayed the border three times, did not spray the main crop, and was satisfied with his control in the main field.

Cabbage is another crop that may be protected by PTC. Researchers in Florida were able to keep the diamondback moth from reaching action thresholds in nine commercial cabbage fields by surrounded them with two rows of collards. In Massachusetts, it seems that imported cabbageworms also prefer collards to cabbage. A 25% reduction in the number of cabbageworm larvae was measured in the main crop of the plots with a perimeter of collards vs. the plots without PTC collard borders.

Pepper maggot flies lay their eggs in pepper fruit and the maggots feed inside, ruining the fruit. This pests is worse on some farms or fields than others, and only occurs as far north as southern New England. Pepper maggot flies prefer to lay eggs in cherry peppers, compared to regular bell pepper. Researchers in Connecticut have stopped pepper maggots from infesting bell peppers by using a perimeter trap crop of hot cherry peppers and border sprays timed when flies begin to sting the fruit in the border. In experiments, bell peppers surrounded by the trap crop produced at least 98% pest-free fruit at harvest compared with all-bell plots, which had 15% of the fruit infested. Commercial farmers using PTC harvested 99.99% clean pepper fruit, and had better pest control than farms that had used well-timed full-field sprays. They reduced their insecticide use and reported improvement in crop profitability ranging from $5 to $152 per acre.

Results from experiments in Connecticut also indicate that both Colorado potato beetle (CPB) and eggplant flea beetle (FB) prefer the elongated Italian or Japanese eggplant, ‘Vittoria’ compared to standard eggplant varieties and to tomato. If either of these are a problem on your eggplant or your tomato, using Vittoria as a PTC may help, although results are not yet conclusive. Border plants can be sprayed when FB or CPB arrives, or could be treated with a systemic insecticide before transplanting.

PTC may also work with potatoes around potatoes. Since Colorado potato beetles emerge in the field edges and walk or fly into potato fields, an early-planted or early-emerging crop that is planted along the border can serve as a trap crop. Early emergence can be achieved by green sprouting the potatoes before planting. Another option is to plant an early variety such as Superior two weeks before the rest of the crop.

PTC is in its early stages of development, but shows great promise. If you’d like to try it, here are some tips for success. First, be sure to plant the trap crop on good ground where it will grow well, and sow it to completely encircle the cash crop, without any gaps, even in an irregular shaped field. The trap crop can be planted with your regular planter, or by hand. Drive a planter across the ends of the rows, in order to plant crosswise. If you have to cultivate out some plants in the row middles, don’t worry about it. Multiple trap crop rows may be needed, especially near woods where the pest has over-wintered, to prevent large populations of migrating pests from breaching the trap crop barrier.

Scout the border rows frequently and spray the perimeter as soon as beetles (or other pest) appears and begins to feed on the trap crop. Don’t wait to get to a threshold of pest numbers. Use PTC along with crop rotation and other cultural practices to reduce pest populations. Don’t expect PTC to provide perfect control with extreme pest populations.

There are several articles with more details about PTC located on the web at click on ‘vegetable ipm’