Alfalfa and Cabbage Loopers

Alfalfa and cabbage loopers are quite similar in appearance. The greenish larvae crawl by arching their bodies and are 1 to 1.5 inches long when mature. Looper eggs are similar to those of the bollworm in that they are spherical with vertical ridges from top to bottom. However, looper eggs are more flattened and have finer ridges. Alfalfa looper is usually found in May and early June while cabbage looper appears in late June through September.

Loopers feed on leaves, but the resulting ragged appearance of leaves is not important unless it is very extensive (more than 50%) and occurs during squaring. June defoliation is seldom sufficiently extensive to cause economic damage to cotton.

Loopers are seldom numerous enough to cause significant damage and are usually kept under control by natural enemies. Moderate populations of loopers may be more beneficial than harmful, as they support populations of natural enemies that also attack the more destructive bollworms, budworms, and beet armyworms. In most cases, control is needed only where insecticides applied for other pests have reduced natural enemy populations; in the San Joaquin Valley, cabbage loopers often increase in numbers following treatment for lygus bugs. Occasionally, however, widespread outbreaks occur, apparently as a result of unusually favorable weather.

Many predators and parasites combine to substantially maintain looper populations at low levels. The eggs and small larvae are attacked by bigeyed bugs, minute pirate bugs, and other predators. Trichogramma parasites kill the eggs, and several other parasites, especially Hyposoter exiguae and Copidosoma truncatellum, attack the larvae. Loopers are also subject to a nuclear polyhedrosis virus that can reduce populations rapidly.

The use of Bt cotton will help to reduce damage by loopers. A recently developed transgenic cotton, Bollguard II, offers suppression of a broader range of caterpillars than earlier Bt varieties and will also control caterpillars such as beet armyworms, cotton bollworm, pink bollworm, and tobacco budworm, which were not controlled by the earlier Bt varieties.

Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for use on organically grown cotton.

Damage by loopers is not important unless it is very extensive and occurs when it will affect the supply of energy to fruit. During the most vulnerable period, from first square to first open boll, plants can lose 20 to 25% of their leaf area without a reduction in yield. Before and after this period, plants can tolerate a loss of about half of the leaf area.

When loopers are numerous, ragging of leaves is usually obvious, and you will probably find the larvae while sweeping for lygus bugs or monitoring for other pests. Smaller loopers feed on older leaves where they are easily sampled using a beating sheet. Spread a 40-inch square piece of cloth between two rows of cotton and vigorously shake an arm's length of plants from one row onto the sheet. Lay the plants back and look for worms that are looping on the cloth. There are no specific treatment thresholds for loopers. If you find large numbers, check for signs of virus disease before applying an insecticide. Insecticides used for control, including Bacillus thuringiensis, are effective mainly against young larvae. Spot treatments are usually adequate for infestations on seedlings.