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Stalk borers

Jeff Hahn

stalk borer in stem

Stalk borer, in stem
Photo credit: Dept. of Entomology

Stalk borers are occasionally found in home gardens. They have a very broad host range, attacking over 200 species of plants, including vegetables, such as tomato, pepper, potato, corn, eggplant, asparagus, and bean, and perennials, including lily, anemone, canna, carnation, cosmos, daisy, gladiolus, hollyhock, iris, peony, phlox, and purple coneflower.

They are also known to attack weeds, including ragweed, quackgrass, ground cherry, goldenrod, lanbsquarters, smartweed, and thistle, as well as fruits, such as currant, strawberry, gooseberry, and cantaloupe and crops, including alfalfa, barley, oat, red clover, sugar beet, sweet clover, and wheat. They have even been known to attack the twigs of some trees.

Stalk borers overwinter as eggs on plant stems or on the ground on fallen leaves. They hatch sometime in May and usually attack grass stems first. As they become older and larger, they outgrow the plants they are infesting. They move at night to bore into bigger stemmed plants. They may actually move several times during their larval stage, depending on the availability of food. Stalk borers are aggressive and cannibalistic and you rarely find more than larva per plant.

The larvae are a little over one inch long when fully developed. When fully grown, stalk borers have an orange head and most of its body is chocolate brown. There is a white stripe that runs on the top of the body from the head to the end of the abdomen and another white stripe running along the sides of the body from the middle to end of the abdomen. There are also a couple of short white stripes on the side of the body near the head. Stalk borers have a smooth body with only a few hairs on it.

The larvae feed for about two to three months before moving into the soil to pupate. They remain as pupae for about three weeks before emerging sometime in September as adult moths. They lay eggs primarily in the folds and creases of grasses where they remain until the following spring. There is one generation per year.

stalk borer out of stem

Stalk borer, out of stem.
Photo credit: Dept. of Entomology

The first sign that you have stalk borers infesting your plants is wilting leaves. Sometimes, the plant may break off, especially after strong winds, to let you know your plant has a pest. You may also see the small hole they create when they enter the stalk. When you cut into the stem, you'll find it hollowed and possibly even see the culprit still there. Younger plants are killed when attacked by stalk borer. Plants with larger, more woody stems may survive an infestation. Fortunately, this is a infrequent problem in home gardens and you are likely to only find a few stalk borers at a time there.

By the time you notice a stalk borer problem it's too late to save the plants in most cases. Your best bet is remove infested plants and kill the stalk borer as it can move on and attack other plants. It also reduces the number of stalk borers that survive to lay eggs later. It is possible to cut into infested stems with a very sharp knife to kill the stalk borer and then bind the plant and hope it survives (there isn't much to loose by trying).

Problems are more likely to occur in garden with adjacent grassy, weedy areas. Control weeds to help reduce the risk of stalk borer infestations. If this isn't possible, keep weeds cut short, especially in the spring when stalk borers are looking for feeding sites, and late summer/early fall when adults are laying eggs. Insecticide treatments is not suggested. Stalk borer problems are too sporadic making it difficult, if not impossible, to predict which plants will be attacked and when.

Published in Yard & Garden Line News, July 15, 2003

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