Pest Versus Pollinator: How Do You Decide?

Black swallowtail caterpillar (left) and adult butterfly (right) on the same dill plant, just weeks apart. Photo by Alissa Freeman.

Q: How do you draw the line between pest and pollinator, in regards to moths and butterflies? — Anthony S., Ruidoso, N.M.

A: That is an excellent question, and the answer really depends on the situation, your point of view, and several other factors. Adult moths and butterflies are harmless to plants and use their siphoning proboscis (long mouthpart that is used like a drinking straw) to feed on nectar and pollinate the flowers they visit. However, the caterpillars (the larval, immature stage of moths and butterflies) use their chewing mouthparts to consume plant matter and can defoliate whole plants. So how do you decide whether the caterpillar is a pest or a pollinator?

Monarch caterpillars, for example, consume a large amount of plant material, yet gardeners rejoice when they discover monarch caterpillars in the garden. Those same gardeners recoil at the sight of a tomato hornworm, which is largely considered a pest. The tomato hornworm will eventually metamorphose into a hawkmoth (also called a sphinx moth), which provides valuable pollination services to native flowers such as sacred datura (Datura wrightii) and is a joy to watch at dusk when they are most active. Another great example of a pollinator that might be considered a pest until it has reached adulthood is the black swallowtail (Papilio polyxenes). These are voracious eaters when in the caterpillar stage and can mow down plants in the Apiaceae family (dill, parsley, fennel, carrots, etc.), while the adult butterfly is a fan favorite. Drawing a line between pest and pollinator is difficult, and the answer is never black and white.

This is where you, as a gardener, need to decide what amount of damage you are willing to allow for these pollinator lifecycles to be completed. It all really depends on your perspective and preferences. To create a garden that’s a true pollinator habitat, we suggest planting more than you need so you can spare some plants for our pollinators, even the not-so-popular ones. Plant extra dill and parsley so that the swallowtail caterpillars can share these herbs with you. If you love watching hawkmoths saunter around in the moonlight, consider covering most of your tomatoes with a row cover to exclude caterpillars, while leaving a few plants exposed just for the caterpillars to feed on.

Keep in mind that broad-spectrum pesticides are extremely harmful to pollinators and beneficial insects and should be used as a last resort. Even certified organic pesticides can still harm beneficial insects.

Alissa Freeman is the Senior Program Specialist and Director of the Learning Garden at the New Mexico State University Agricultural Science Center at Los Lunas.

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