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Common Stalk Borer

Rhubarb, peony & coneflowers are affected

By: Sandy Feather 2007
Penn State Extension


Q. I have rhubarb, peony and coneflower plants that have been infested with some kind of borer. I noticed that individual stalks on those plants were wilting due to insect damage. Can you tell me what they are?

A. It is unusual that the same insect would have damaged three very different plants, since most have a limited host range. It took some research to learn that this insect is known as the common stalk borer (Papaipema nebris). Common stalk borer is best known as a corn pest in eastern North America, but is also reported to feed on over 175 types of plants, including vegetables, fruits and ornamentals. All of the plants in question here are known to be susceptible.
  


Life cycle of common stalk borers

Common stalk borer adults are moths that hatch in late summer (September). Females lay eggs on unmown dead grass and curled up weed leaves, or in bark crevices on trees. Common stalk borers overwinter as eggs that hatch into small larvae in mid-spring. They immediately tunnel into a host plant for food and shelter. Although they prefer to tunnel into grass stems, young larvae can also tunnel into leaf veins and stems (petioles) on many weeds. As they develop, they outgrow the first host and migrate to the larger stems of vegetable, fruit, and ornamental plants, or larger weeds. This is the stage that causes the most damage to crops and ornamental plants.

Coneflower
Coneflowers

They bore their way into the stem, pushing sawdust-like frass from the hole as they tunnel. They usually move upwards in the stem, boring holes to the outside as needed to clear the stem of frass. They also migrate to adjacent stems, with each larva damaging many plants. (There are species of borers that spend their lives in one plant, causing less overall damage). Infested stems may wilt as their feeding destroys the plants' vascular system, or it may break in the wind since it is weakened from their tunneling activity. Common stalk borer occasionally infests small twigs on woody plants, too.

 


Life Cycle

It takes two or three months for the larvae to mature. They pupate in mid-summer, with adults hatching in three to four weeks. We have one generation a year. Common stalk borer adults are grayish-brown with a wingspan of an inch or so. They are not attracted to lights, so you probably will never see them. Larvae range from three-quarters of an inch to two inches long. They are purplish-brown, with a white stripe down the middle and two white stripes running down either side when young. Mature larvae lose this distinctive coloration and are purplish-gray and rather non-descript.


Control options

Control options include controlling weeds and cutting tall grass, especially in mid-August, to reduce suitable habitat for the adult moths to lay their eggs. Avoid cutting weeds between late May and late July. This can actually force common stalk borer larvae out of those weeds and into your garden. You can also try to find the larvae in infested stems and destroy them. Fortunately, common stalk borer is not considered a major insect pest in our area. The mechanical methods described above should provide sufficient control. The insecticides labeled to control common stalk borer in agronomic crops are not available to home gardeners.

peony
Red Peony

There are other species of insects - primarily beetles - whose larvae cause similar damage. I usually see borer damage on herbaceous plants every year, but it seems to be limited to a few stems or a few plants here and there. Mechanical controls generally provide effective control without using insecticides. Contact insecticides such as insecticidal soap, pyrethrins or Sevin (carbaryl) are not effective once insects are inside plant stems. Systemic insecticides such as Bayer Advanced Rose & Flower Insect Killer (cyfluthrin and imidacloprid) can provide control of borers that are beetle larvae. It is not effective against moth or butterfly larvae.

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