The first signs of munching caterpillars occurred on the tomato plants a few weeks ago. They quietly increased in number, skeletonizing just a leaflet or two, so I didn’t catch their activity at first. Then whole leaves were eaten and a quick look showed the young larval stage of tiny armyworms. I immediately went to store to purchase the proven bacterial control, Bacillus thuringiensis, or Bt, as it is commonly called, and applied it through a spray application. It took two applications but it did its job. The bacteria attack the caterpillar gut. Feeding activity is reduced greatly in a short amount of time as the caterpillar can no longer feed. But no control is ever 100%. Some caterpillars always manage to escape. And now, a new generation is back and they are on the move. It only takes 28 days from egg laying to adult and therefore, several generations can occur each year. At times generations can even overlap and may appear to be continually occurring. One female can lay an egg mass containing 100-200 eggs. She lays them in neatly layered rows on light colored surfaces such as light-colored leaf undersides or metal gutters and light-colored furniture. Egg hatching occurs within two to four days in warm temperatures. Larvae start to eat immediately, generally in the early morning, late afternoon or early evening, resting wherever they can find protection from the hottest parts of the day. Armyworms attack the foliage of a number of agricultural plants including corn, beans, cucumber, cow peas, cabbage, broccoli, spinach, and kale and turnip. But they also love southern turf grasses.
Lawn care companies are always on the lookout for major caterpillar attacks that usually occur later in the summer and early fall – unless the winter months are very mild. As soon as I mentioned I’d seen fall armyworms (yes, in July) in the garden I maintain, the lawn care specialist immediately came to check on the lawn. St. Augustine lawn will look great, green, lush and healthy … and then patches of the lawn suddenly begin to look ragged with uneven and notched blades, seemingly chewed apart overnight. The larvae begin their feast with tiny bites that simply graze the blade surface, leaving a slight grayish appearance often unnoticed to the untrained eye. But as the larvae reach full size, the bites get much bigger, accounting for the easily seen notches. One armyworm can eat two square inches of turf in its fifth and final larval stage of growth; it will measure 1-½ inches in length. It is green brown or black with a dark head notched with a distinct inverted “Y.” Along the length of the body is a black stripe. Along the middle of the back is a wider yellowish gray stripe with four black dots. An “army” of them can devour a lawn at a rate of 1’ to 3’, usually in a relatively straight line, any time of day or night but mostly at night. Fat, happy larvae eventually burrow to form the pupal stage. Adults emerge from the pupae in about 10-14 days, look for a fresh patch of turf grass, mate and the cycle begins again. Adult moths have front wings that are dark gray with light and dark spots and a white spot near the wing tip. The hind wings are white. During the night hours, the adult female mates and lays clusters of 200 or more tiny light grey eggs on the underside of a leaf blade. The eggs hatch in 2-5 days.
There are two ways to find out if your lawn has these caterpillars. Inspect it for signs of feeding and green pellets (excrement). Active feeding of the caterpillars is usually at night. To see the caterpillars during the day, mix one to two ounces of dish detergent in two gallons of water and pour it on a four square-foot area. This will result in increased activity of the caterpillars within two to five minutes. If your caterpillars don’t look like the description above, you may have another problem, cutworm infestation. These caterpillars also do damage to your lawn. Mature larvae are 1-½” to 2” in length, mottled, dull brown, gray or nearly black and appear plump and greasy. They will curl into a C-shaped ball when disturbed. With three to seven generations a year, they are active year-round. Blades appear roughly chewed off and damage is seen in circular spots of dead grass or sunken spots in the turf.
As mentioned at the beginning of the article, caterpillars will succumb rapidly to the biological control of Bacillus thuringiensis (Bt), a bacterium that attacks the gut of the caterpillar. A second choice of control is the application of an entomopathogenic nematode, Steinernema carsocapsae, which causes a massive infection in the host caterpillar. Follow the directions for application and amounts for best results.
Several turf insecticides also work well, if applied when the caterpillars are still very small and in their most vulnerable stage. Applications late in the day work best, when the caterpillars actively begin to feed. Water in lightly and don’t mow for two days after application. Time the application within one to two weeks after peak moth activity to allow the eggs to hatch into the small caterpillars, keeping in mind that egg development varies with temperature. Read the label before application for best results.