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Mealybugs bug off: clove and spearmint to the rescue

Today, I returned to volunteer at the University of Minnesota Botanical Conservatory for another shift. It was once again a pleasure to roll up my sleeves and continue the never-ending task of cleaning insect pests off the plants. My assignment was limited to a small area: a bench holding about two dozen plants. Each one had to be inspected for pests, cleaned with soapy water, and sprayed with a solution of essential oils to deter further infestation.

While working, I couldn't help but wonder about the efficacy of spraying the plants with clove and spearmint essential oils. Do they really work as well as synthetic pesticides, which are specifically designed to kill insects? How much credible literature supports the use of plant oils as insecticides and repellents? And do they work against all insect pests, or only certain species?

A brief search in Google Scholar produced thousands of articles spanning many years, addressing the use of essential oils in agriculture. It's definitely a big business. Below, I've listed three articles (and there are many more) specifically addressing the use of clove and spearmint oils.

The articles seem to support the use of essential oils as pesticides and repellents. The primary mechanism of action appears to be the inhibition of neurotransmitters in insects. However, the specific molecular mechanisms are not yet fully understood and likely involve multiple pathways.

It also appears that essential oils are effective, though perhaps not as effective as synthetic pesticides. However, they have the advantage of being non-toxic to humans and animals (with rare exceptions, of course).

Clove and other aromatic spices (photo courtesy of Tom Wilson)

To clean the plants, I used a ladder to reach the upper branches of the taller specimens. I sprayed a dilute soapy water solution (a few drops of Dawn per quart of water) onto the plant and then used a soft brush to scrub the pests off the leaves and stems. After rinsing the plant with a hose of tap water, I applied the essential oil solution with a spray bottle. The essential oil solution is available as a concentrate from commercial suppliers.

A ladder is helpful for reaching up into the plant canopy



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