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Sandy's Garden

Potato Beetles

Watch your tomatoes, eggplant, potatoes, and peppers

By: Sandy Feather 2008
Penn State Extension


Q. Our Nicotiana is being chewed-up by some insect, and there is nothing left of the leaves except for the middle vein. When I looked under the remaining part of the leaves, I noticed these small, slimy yellow insects. Do you know what these are and how we can get rid of them?
  

 

A. The writer enclosed a sample of the insect in question. We have had the same insects eating the foliage of angel's trumpet (Datura metel 'Cornucopia') in our demonstration garden at North Park. They are the larvae of three-lined potato beetles (Lema trilinea). The grayish-yellow larvae feed as a group, lined up side by side. They move backward from the edge of the leaf toward the center as they feed. They are "slimy" because they are coated with their own excrement, a disgusting but very effective survival technique.


Feeding Habits

They usually feed on plants in the nightshade or Solanaceae family, which includes Datura, Nicotiana, tomatoes, eggplant, potatoes, peppers and those species considered weeds, such as bittersweet nightshade (Solanum dulcamara). These plants are high in alkaloids that make them toxic to animals, including people.
   

Leaves chewed off by Potato beetles
Leaves are stripped-off to mid-vein of the leaf

One of Penn State's doctoral candidates, Tim Morton, is doing research on these insects. He has found that the alkaloid content of their excrement is key to their survival. When he removed the excrement from the larvae or fed them an innocuous diet of lettuce, predators made short work of them. Those consuming their preferred diet of nightshade were unharmed.

Identification

The adult beetles are reddish-yellow with three black stripes running lengthwise down their wing covers. They become active when tomato and potato plants (or other preferred crops) are planted out in late spring or early summer. They lay clusters of small yellow eggs on the underside of leaves. When the larvae hatch, they begin feeding on the foliage. They are able to coat themselves with excrement because their anus is in the middle of their backs. They feed for about two weeks and then drop to the ground to pupate.

Control Options

Control options include removing severely infested leaves and crushing the insects or sealing them tightly in a plastic bag and sending them out with the trash. You can also make insecticide applications to control them. Rotenone, pyrethrins and Sevin (carbaryl) are labeled to control three-lined potato beetle.
  

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