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.
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
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.
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.
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 firstname.lastname@example.org. This article was first published in Rural
Delivery, Volume 29 #2, July/August 2004