Perimeter Trap Cropping for Cole Crops

Trap crops are used to protect the main cash crop from a pest or complex of pests. The trap crop can be a different plant species, variety or just a different growth stage of the same species, as long as it is more attractive to the pests when they are present. Trap cropping is most worthwhile for pests that are abundant and destructive in most years. It works best for insects of intermediate mobility rather than those, like aphids, passively dispersed by air currents or those strong fliers that descend on a crop from high elevations. Trap crops are more economical to use if the system is easily planted and maintained, and the crops have some other beneficial use (e.g., support beneficial insects or are marketable). Trap crops that require a small amount of space relative to the main crop are more economical, as well. The required trap crop planting size depends on the number of pests and the intensity and direction of the attack expected. Also, consider the mobility of the target insect and not the size of the main crop planting. However, because the edge to area ratio shrinks the bigger the field gets, perimeter trap cropping is more economical for larger cole crop plantings (i.e., proportionally less area is dedicated to the trap crop).

Perimeter trap cropping involves planting the attractive plant species so that it completely encircles the main crop like fortress walls. Perimeter trap cropping is useful when it is necessary to protect the crop from an attack that may come from several or unknown directions. Understanding how the insect moves in and uses its environment is crucial to developing or deploying a successful trap crop system. In some cases when perimeter trap cropping has been attempted, but the barrier was not completely closed around the main crop, the pest poured through the open gap like water into a leaky boat. Wider trap plantings may be necessary along field edges that border known sources of infestation such as insect overwintering sites, non-crop breeding sites or alternative food sources. Trap cropping effectiveness may improve when used in conjunction with chemical, biological, mechanical and cultural control tactics; or pest attractants and repellants.

Why use perimeter trap cropping?

1. The pest control benefits of planting in a logical spatial pattern seem obvious. Put the plant the pest likes the best on the outside, and it may not need to travel any farther;

2. Trap cropping often results in dramatic pesticide savings;

3. Less spraying usually translates into lower costs;

4. Less spraying usually translates into fewer environmental problems;

5. Less spraying usually translates into fewer safety concerns;

6. Less spraying delays the development of pesticide resistance;

7. Increasing the species diversity in a monocrop system should help balance the ecosystem and prevent pest population "explosions." For example, limited pesticide applications, targeted at the perimeter rows only, leaves most of the crop unsprayed and preserves pest natural enemies that can help prevent future crop damage. Alternatively, leaving the trap crop perimeter unsprayed may provide a refuge for higher parasite populations and enhance the effectiveness of biological control by natural enemies. If a market exists for the trap crop, applying a spot spray to the perimeter rows with a selective pesticide (i.e., microbial products like Bt or spinosad) may be beneficial. It may preserve the quality of both the trap and main crops, spare beneficials and lower the cost of trap cropping;

8. Trap cropping provides another alternative to chemical pest control. A major tenet of the IPM concept is that, over time, a mix of pest control techniques will outperform any single method, such as pesticides, and help maximize profits.

Diamondback moth parasatized by
Diadegma Insularis Puparium of parasite

Perimeter trap cropping for cole crop pests

Perimeter trap cropping can be used to help protect major crucifer crops (cabbage, broccoli and cauliflower) from damage by the diamondback moth (DBM) and/or flea beetles. Perimeter trap cropping may not eliminate these insects completely, but it substantially reduces the pest population on the main crop and reduces pesticide applications for the target pests by 75 to 100%, while helping to maintain or improve crop quality.

What you need to know about the DBM and flea beetles

Adult DBM and flea beetles may survive the winter under the residue of previous cole crop plantings or cruciferous weeds like wild mustard or radish. Therefore, it is important to rotate your crops and eliminate weeds in the cabbage family from your fields, to prevent these pests from invading the new planting from within the protective perimeter trap planting. Both pests are mobile insects, but not strong fliers.

The DBM is among the most resistant insects in the world. Using perimeter trap cropping against DBM can improve the effectiveness of your current control efforts. In addition, perimeter trap cropping can help preserve the usefulness of the newer selective pesticides registered for this pest by reducing the insect's exposure to these products. Diamondback moth larvae may also arrive on infested southern-grown transplants or may arrive as adult moths on storm fronts later in the season. This pest has three to five generations per season. The DBM is treated as part of a cole crop caterpillar complex, which includes the imported cabbageworm, cabbage looper and sometimes the cross-stripped cabbageworm.

Diamondback moth

Diamondback moth pupa

Flea beetles primarily damage seedlings during establishment or prior to the five-leaf stage. In addition to surviving the winter under cruciferous weed and crop residue, they hide under leaf litter in tree lines and hedgerows. They move into the field from shelter sites daily, especially on sunny days. Leaf feeding is often concentrated on crop rows near the field margins. Several different species make up the flea beetle complex that may damage cole crops. Individual species have one or two generations per season, and the population peak of one species may be closely followed by that of another.

Example of trap cropping in cole crops

The IPM program recommended for cole crop insects involves weekly scouting and using action thresholds (e.g., 3 to 5 flea beetles/plant or 20% caterpillar-infested plants after heading). In addition, the program entails preserving natural enemies, trap cropping and selective pesticide applications with sound resistance management practices.

Perimeter trap cropping recommendations for DBM

For early spring plantings, transplant two rows of collards, or the equivalent width of barrier, on all sides of the main cole crop planting. The collard seedlings can be transplanted by machine on two sides of the field, while seven to nine collard transplants should be set by hand at the end of each row to provide a barrier on the third and fourth sides. Transplant the collards at the same time as the main crop and provide both with similar fertilizer, irrigation and cultivation programs. For late spring or summer plantings, direct seed or transplant the collard barrier 1 to 2 weeks before establishing the main crop, to assure a trap crop of adequate size and attractiveness. Remember that collard seedlings develop a little slower than traditional cole crops. One possible spacing would be 40 inches between rows and 10 to 12 inches between plants within rows (or the same as your main crop spacing). 'Vates', 'Georgia' or 'Champion' collard varieties make good trap crops. Make sure the trap crop barrier remains lush all through the season to attract the pest and the barrier extends all the way around the crop you wish to protect.

Unless you plan to market the collards, researchers in Florida recommend not spraying the trap crop to allow it (1) to serve as a refuge for insecticide-susceptible DBM individuals (for resistance management); and (2) to build up high populations of DBM parasites. Researchers in Florida and Wisconsin have observed 80 to 88% DBM parasitism by the ichneumonid wasp, Diadegma insulare, in unsprayed barriers and plantings (fig. 4 & 5). Fields in the Northeast may have D. insulare parasitism rates as high as 50 to 70%. There are also many other effective, naturally-occurring predators and parasites that can build up in the trap crop area to help provide control of DBM and other cole crop pests. If DBM or other caterpillar populations exceed action thresholds in the main crop and insecticide applications are required, it is recommended to alternate between selective products that spare the beneficials, like spinosad (SpinTor), Bacillus thuringiensis aizawai (XenTari) or neem (Neemix).

Perimeter trap cropping for flea beetles

Perimeter trap cropping can be used for flea beetles by simply replacing the collard barrier with Chinese 'Southern Giant' mustard. Alternatively, you can plant several rows of mustard on the outside of the collards to trap out both pests. It is best to make multiple sowings, several days apart, to provide an ample and continuous supply of attractive, young mustard plants that protect the main crop adequately. Do not let the mustard go to seed, or it will become a weed in future plantings. Unfortunately, selective insecticides are not yet available to aid in flea beetle control.

Intercropping cole crops with tomato plants

Intercropping cole crops with tomato plants has been shown to deter flea beetles and DBM. Although untested, spraying the main crop with a "tomato tea" may also repel these insects and enhance the effectiveness of a perimeter trap crop system.

T. Jude Boucher, Vegetable Crops IPM Coordinator, University of Connecticut

flea beetle image from Extension Entomology, Texas A & M University

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