Questionable Companions
Mainstay of Organic Pest Control Won't Deter Colorado Potato Beetles

by Tara Moreau

I just completed my Master's degree at the Nova Scotia Agricultural College, where I devoted two years of research to one troublesome agricultural pest: the Colorado potato beetle. Working with the Organic Agriculture Centre of Canada (OACC), I evaluated organic controls for this insect on potatoes. The results, which came as a shock to me, will perhaps be a disappointment to gardeners who place a lot of faith in companion planting.

I realized early on that the 10-striped, leaf-eating Colorado potato beetle (CPB) is no ordinary insect. Its ability to develop resistance to nearly all pesticides used against it, and its global distribution, make it one of the most notorious agriculture pests in the world - and therefore one of the most popular pest species in scientific literature.

My particular interest was in the commonly prescribed technique of companion planting. Many publications devoted to home gardening and organic crop production recommend planting non-host plants or aromatic herbs as a means of reducing insect attack. I was intrigued with the concept. Could the presence of a non-host plant actually work to repel the beetle from potato plants nearby? Is there an unseen level of communication between plants and insects that can be used by growers for a more natural means of pest control?

One of the greatest challenges I encountered in testing companion plants was the lack of information about which companion plant varieties worked best, how many companion plants were required to have an effect, and how to arrange the companion plants within the potatoes. These were the questions I sought to answer.

Delving into the available literature, I realized that companion planting was a controversial subject, with various scientific data supporting and refuting its effectiveness. One study with the CPB reported that adult beetles were less attracted to the odors of potato plants in intercropped systems than in monocropped systems. The researchers speculated that potato plant odors were masked by the odors of non-host plants, and that the CPB's host-plant searching behavior was reduced in the presence of non-host plants. Other researchers demonstrated that CPB populations were lower in high- diversity plots than in lower-diversity plots.

In order to determine which companion plants to evaluate, I reviewed magazines, books, and internet sources, choosing plants that were most commonly recommended and that could be grown in Atlantic Canada. In the end I selected five: Bush beans (Phaseolus vulgaris cv. Provider), flax (Linum usitatissimum cv. Natasja), horseradish (Armoracia rusticana), marigold (Tagetes patula cv. Bolero) and tansy (Tanacetum vulgare), shown at the right. I started the companion plants in a greenhouse in the early spring, in order to grow large plants to transplant into the potato plots.

The next stage of the project was to learn how to grow potatoes. I had no previous experience, and was unaware of the work required, but I managed to get research plots set up. I compared beetle densities between companion planted potato plots and potato plots with no companion plants.

The results from the 2-year study were not what I expected. Analysis of the CPB populations revealed that there were more beetles in plots with flax, marigold, and horseradish than in plots with no companion plants. I couldn't believe it. Not only did these companion plants not decrease CPB densities - their presence near potatoes actually increased the number of beetles. (The plots with Bush beans and tansy showed no difference from the control plots.)

I was surprised to learn that my results were not unprecedented in this field of study. In trials evaluating companion planting for roses, researchers demonstrated that the companion plants increased the incidence of Japanese beetle attack on roses.

While doing this research I had the opportunity to speak with many people who had success with companion planting and believed strongly in its effectiveness. However, my overall conclusion was that successful companion planting depends largely on the insect pest and the companion plants selected. Using Bush beans, flax, horseradish, marigold, and tansy as organic pest controls for the Colorado potato beetle would not be recommended, and I think this study does raise concerns about using companion plants without first verifying their effect on target pests.

Tara Moreau would welcome comments or questions on this subject by email at t@lenouveaupauvre.net.

This article was first published in Rural Delivery, Volume 29 #2, July/August 2004