Do Companion Plants Offer Protection from Insect Pests?

This past week, I attended the Pacific Branch Meeting of the Entomological Society of America (PB-ESA). The meeting was held in Portland, so it was a wonderful opportunity to interact with and learn from entomologists from CA, HI, ID, UT, MT, WA, AZ . . . and of course, OR. What I love so much about scientific conferences, such as the PB-ESA, is that we get a chance to hear about hot and happening research results. Scientific conferences are an opportunity present your work to your scientific colleagues for consideration and critique, before you publish the results of your research.

One talk that really stood out to me was the fantastic work of Joyce Parker. Joyce was examining the efficacy of two ecologically-based pest management practices in broccoli: trap cropping and companion plants.

Trap cropping is a practice where farmers plant a 'trap' crop next to the target crop that they want to protect. Pests are attracted to the trap crop, where they can be managed via targeted chemical controls. The pesticides applied are greatly reduced to a few strips of the trap crop, rather than being broadcast across the larger geographic area of the target crop.

Companion planting is a practice where farmers plant aromatic crops next to the target crop that they want to protect. The aromas of the companion plants are said to 'hide' the target crop from insect pests. Many gardeners also use companion plants in their garden, by planting marigolds next to marigolds next to cucumbers, or dill next to cabbage.

Joyce's results suggest that using mustard as a trap crop was an effective, ecological pest management strategy in broccoli fields. However, her companion plants (Yukon Gold potato, marigolds, dill, and bunching green onions) didn't protect broccoli from the crucifer flea beetle. Her results align with several others studies which found that companion plants don't mask target crops with their aroma:

• Finch et al. 2003: cabbage root fly and onion fly were distracted from target crops by the leaf color of some companion plants - but not by the aroma of companion plants.
• Held et al. 2003: companion plantings of geranium actually INCREASED Japanese beetles on roses. So too did fennel seeds, cedar shavings, crushed red pepper, and osage orange.
• Latheef and Erwin 1979: cabbage interplanted with peppermint, nasturtium, marigold and/or thyme had just as many caterpillars as cabbage planted with other cabbage.
• Latheef and Ortiz 1984: hyssop and santolina did not protect cabbage from crucifer flea beetles. Wormwood and tansy reduced crucifer flea beetle numbers by about half (as compared to control cabbage - planted without companion plants). However, damage from crucifer flea beetles was still high on cabbage interplanted with wormword or tansy.

Nonetheless, some gardeners swear by the use of companion plants in the garden. I always tell gardeners that if they feel it's working for them - keep doing what they're doing. Companion plants can be a great way to add color and variety to the vegetable garden. However, if you're planting companion plants purely for the pest control benefits that you've head they might confer . . . you may be disappointed.

Instead, I would suggest planting insectary plants - such as coneflowers, tickseed, bee balm or spirea. These plants have been tested by researchers at Michigan State University and ranked according to their attractiveness to pollinators, parasitoids and predators.



Yarrow, such as this specimen in the Lincoln County Master Gardener Demonstration Garden, attract a wide variety of beneficial insects when in bloom.