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What natural plant pest-repellents haven't worked for you?

16 years ago

In planning my spring garden, I've been doing a lot of research on cucumber beetles and potato beetles. I didn't have problems with them this past summer, but I sure did during 2006!

But to be on the safe side, I thought I'd make sure I planted as many repellent plants for beetles as I could next spring. I've faithfully believed those internet lists of plants that are supposed to repel beetles until I ran across this, a study on plant repellents for potato beetles.

From the link:
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?

....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 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.

What natural pest repellents have you tried that you don't think worked, absent a controlled experiement as the woman above conducted?

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