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Abundant rainfall attracts fall pests early

Posted: August 19, 2012 - 12:16am

It has been a long time since I have seen an August with so much rain. Usually, plants in the area are suffering from a lack of water.

With all the lush plant growth, insects such as the fall armyworm, red-headed azalea caterpillar and the orange-stripped oakworm can become a problem.

Fall armyworms (Spodoptera frugiperda) are from the Gulf coast and south Florida. They tend to show up in our area around the last week of August or first week of September and can last until frost. Fall armyworms are pests of Bermuda grass, zoysia, millets and fall vegetables.

Armyworms get their name from the large number of caterpillars that can be in an area. The adult stage is a moth. The female moth lays about 400 eggs in a mass on anything that is close to a food source.

When the caterpillars are small, they will chew on the bottom side of the leaf and usually don’t consume the whole leaf. As the caterpillar gets larger, they consume the entire leaf or plant. When the food source becomes scarce, the entire population of caterpillars will move en masse to a new food source. I have seen a lawn that measures 2,000 square feet with tens of thousands of armyworms. They can eat a lawn this size overnight.

Armyworms can be treated with Bacillus thuringiensis (Bt, Dipel), carbaryl (Sevin) or various pyrethroids found in garden centers. We usually have up to two generations of armyworms in the fall and this year there could be up to four.

There have been reports of fall armyworm in pastures and hay fields, so keep an eye on Bermuda and zoysia lawns.

Another common caterpillar found this time of year is the red-headed azalea caterpillar (Datana major). This caterpillar is a major pest in the Southeast and azaleas are the preferred host. Datana major have also been reported on apple, blueberry and red oak. In our area most of the damage occurs in late August and September with mid-September being the time when they are most often seen.

When these caterpillars reach ⅜-inch long they will have a yellow body, seven red longitudinal stripes and a black head. Mature caterpillars reach 2 inches in length and have a black body, red head and legs and broken yellow (occasionally white) lengthwise stripes. When disturbed, they raise their heads and tails into a u-shape.

Even though the caterpillars are hairy, they are harmless to humans and can be picked off by hand. It is best to remove the caterpillars as soon as an infestation is detected. Bacillus thuringiensis (B.t.) is a low-toxicity biological control that is very effective if the caterpillars are found when small (less than ¾-inch long). Larger caterpillars will require a more toxic pesticide for control, such as carbaryl (Sevin 50WP) or cyfluthrin (Bayer Advanced Garden Multi-Insect Killer). As with all pesticides, read and follow all label instructions and precautions.

The orange-striped oakworm caterpillar usually appears in our area in August, September and sometimes as late as October. They can defoliate sections of oak trees and will completely defoliate smaller trees. These caterpillars can be a nuisance near patios, driveways and sidewalks because of the large amount of droppings they produce.

The winter is passed in the pupa stage in the ground. Adult moths emerge from June to August, mate and deposit eggs on the underside of foliage. Usually, there is only one generation per year and control measures are not necessary. By the time these caterpillars attack trees, the trees have stored enough energy to survive the winter and come back the next spring. If control measures are necessary, such as to protect a small seedling oak, treat with Dipel or Thuricide (both are Bacillus thuringiensis, a biological control) or use an insecticide such as cyfluthrin or Sevin.

All of these caterpillars are easier to control when they are small.

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. Reach him at cphillipshort@comcast.net or at (706) 836-2152.

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