Aug. 03--I found some caterpillars in my garden and am worried about the damage their feeding will cause. Should I spray them with an insecticide to protect my plants?
-- Emily Lewis, Wheeling
The picture you attached to your email shows the caterpillar of a white-lined sphinx moth (Hyles lineata). These large caterpillars have black markings and can be yellow, green or mostly black, with considerable variation. They resemble the tomato hornworm that you might find on tomato plants, but typically are found in larger numbers and are easier to see in the garden.
The caterpillars feed on the foliage of a variety of perennials, shrubs and trees, but rarely do significant damage. There is no reason to spray for them. Instead, look forward to the beautiful and fascinating insects these larvae will turn into.
The white-lined sphinx moth is one of the species commonly called hummingbird moths because they can be as big as hummingbirds. They often are seen hovering to feed on the nectar of flowers, beating their wings rapidly like hummingbirds. They even emit an audible hum, similar to that of a hummingbird in flight.
The moths usually fly at dusk, during the night and at dawn, but you also may see them during the day. They feed on a variety of flowers including phlox, bee balm, verbena, honeysuckle, columbine, larkspur and lilac. They tend to be most active in summer when bee balms are in flower.
The population of this moth normally varies from year to year, so an increase in sightings is neither a good nor bad thing for your garden.
If a particular plant is sustaining heavy damage, carefully move the caterpillars to another corner in your garden to feed on another plant.
Any time you find an insect in your garden that you think might be a pest, always identify it before taking any action. Then decide whether or not the amount of damage it is causing really warrants treatment.
The Plant Information Service of the Chicago Botanic Garden can help with insect identification and recommendations. Whenever you decide to try and control an insect, choose the method that will have the least amount of impact on the environment.
Tim Johnson is senior director of horticulture for the Chicago Botanic Garden in Glencoe.
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