Trap crops: pesticide-free ways to keep pests away from you vegetables

What is trap cropping?

Growing trap crops is a way of luring insect pests away from vegetable plants that they would normally attack. Effective trap crops are those preferred by the pest. The home gardener monitors the trap crop and destroys the pest by other means.

Does it work?

Apply a healthy dose of scepticism about trap cropping or companion planting information you find on the internet. Many of these ideas are not scientifically tested and evaluated and do little to rid you of problem pests. Some of these cures may make you feel like you’re doing something, when in fact, you may attract more pests to your garden than you deter.

That said, trap cropping is studied by horticulture scientists because of insecticide resistance and because many growers do not want insecticides on vegetable produce. Trap crops may not completely eliminate pests but can reduce the severity of attack if properly timed and managed. Read on for specific trap crop strategies.

 Factors that affect the success of trap cropping:

• Know your insect enemy. What is its life cycle? When does it emerge? Where does it lay its eggs? When is it most destructive? The timing of growing trap crops is essential.
• Are they mobile? Small, light, winged insects like aphids fly or are moved by wind over relatively large distances, while crawling insects tend to stay in one place.
• What do they eat? Many pests feed only on specific plants. For example, Colorado potato beetle eats only potato plants and cabbage moths feed on all kinds of brassicas.
• Weather plays a role. A severe winter, late spring and extreme weather events affect the timing of insect life cycles. Your garden is not a lab where factors like temperature and moisture can be controlled. 

Trap cropping is more effective when used with other best practices for growing vegetables:

• Mulch your soil. We can’t say it enough. Mulch protects soil from erosion, improves soil structure and supports biodiversity which creates habitat for beneficial insects.
• Rotate your crops. Growing the same thing in the same spot every year allows pests to build up to damaging levels. Rotating what you grow starves pests when they cannot find their desired host plant.
• Refrain from using insecticides which kill all insects, including beneficials.
• Choose insect and disease-resistant varieties of plants.
• Grow a variety of vegetable plants. Include flowering plants like borage, sunflower or yarrow (to name a few) in your vegetable garden to feed insect pollinators like bees.
• Stop rototilling. Rototilling damages soil structure and can move diseases and insects to other parts of your garden.
• Pick your weeds. Weeds attract certain insect pests. For example, several types of wild mustard weeds are found on the prairies. These are brassicas which are food sources for pests that feed on cole crops.
• Scout for destructive insects and hand pick when you find them.
• Avoid overcrowding your plants. Thickly planted vegetables are more prone to disease and insect infestations.
• Go easy on the fertilizer. While vegetables do benefit from modest applications of fertilizer, too much nitrogen results in lush, tender leaf growth – fodder for insects and makes plants more vulnerable to attack. Follow the directions on the package: more is not better. 

Spring trap cropping for Colorado Potato Beetle

The strategy for trapping Colorado potato beetle is to do two plantings: a small early spring trap crop followed by a second main crop planting in early to mid-June. Overwintering adults emerge in late May and early June. They mate and the females lay eggs in June to late July.

1. Select a new site for your potato crop, preferably one where no potatoes have been grown for at least three years.
2. Plant an early trap crop of potatoes in the plot as early as the ground can be worked in spring, perhaps one row or about 15 -20% of the total you wish to grow. Use only certified new potato seed. Space these potatoes about 6” wider apart than you would normally. This makes it easier to scout for beetles.
3. Once the trap crop emerges, look at the plants daily to scout for potato beetles. Be sure to look under the leaves. Pick off the potato beetles. No need to use insecticides – just crush them, place them in a bag or jar and dispose of in the garbage.
4. Plant your main crop in early to mid-June.
5. Continue to check all of your plants for potato beetles and for egg clusters in late June to late July.  The eggs are in yellow clusters on the underside of potato leaves. Remove and destroy.

Be diligent: finding and destroy most of the potato beetles in the spring trap crop will interrupt their life cycle.

Perimeter trap crop of collards around cole plants to deter diamondback moth and flea beetles

The diamondback moth is a problem for canola growers and can also be a problem for home gardeners who love growing cole crops like broccoli, cabbage, brussels sprouts and kale. While flea beetles do less in the way of damage to cole crops, they are known to infest other crops like tomato and potato.

Researchers at the University of Connecticut have found that collards – an edible cole crop popular in Southern US cuisine – are an excellent trap crop for both of these pests. They recommend planting collards like 'Vates', 'Georgia' or 'Champion' on all four sides of your main cole crops. Collards tend to grow a bit slower than other cole crops, so get them planted as early as you can. 

It appears that scouting and destroying diamondback moth and flea beetles is not necessary. The pests are happy to munch on collards and tend not to move on to the main crop. Collards also attract beneficial insects like parasitic wasps that control moth and beetle populations provided no insecticides are used in the garden.

Perimeter trap crop of Hubbard squash to control squash vine borer

Squash vine borers are occasional problems in cucurbit crops like cucumber, squash, pumpkin and zucchini.  These piercing, sucking insects damage plant tissues but also invade the inside of vines by larvae that hatch from eggs laid at the base of plants. Result – a once healthy plant is quickly reduced to a rotting mess. As such, they are hard to find and by the time they are at their peak, the damage is irreparable.

These insects have a strong preference for one cultivar and given a choice, will always select ‘Blue Hubbard’ over all others.  Use this to your advantage if you are growing zucchini or summer squash: these are much less appetizing to squash vine borers.

Growing a perimeter of ‘Blue Hubbard’ around your preferred crop is a way to manage infestations. When the borer infests the lure crop, remove and destroy the plant to prevent the borer from moving on to more desirable cultivars. Here again, insecticides are not required. Do not put infested plants in the compost: bag and dispose of in the garbage.

Sources:

https://academic.oup.com/jipm/article/5/4/A1/2193920?searchresult=1

https://extension.tennessee.edu/publications/Documents/W235-F.pdf

http://ipm.uconn.edu/documents/raw2/Perimeter%20Trap%20Cropping%20Works/Perimeter%20Trap%20Cropping%20Works.php

https://ipm.missouri.edu/MEG/2017/3/Trap_cropping/

https://academic.oup.com/jipm/article/7/1/1/2658001?searchresult=1