Cottonwoods & Caterpillars

Cottonwoods often grow around the wet soils of riverbanks, lakes and irrigation ditches.

Cottonwoods (Populus fremontii) are 40- to 80-foot trees with broad, widely spreading crowns. They grow only in wet soil and have thick, gray, rough and deeply furrowed bark. The cottonwood's triangular leaves turn bright yellow or gold in autumn, and its seeds are contained within cottony masses -- which are, in turn, encapsulated within brown, egg-shaped structures that split open when mature. Several caterpillars affect cottonwoods, including the stinging rose caterpillar, tent-making caterpillars and the redhumped caterpillar.

What Are Caterpillars?

A caterpillar is the larval stage of certain insects in the order Lepidoptera, which includes butterflies, skippers and moths. Lepidoptera is the second-largest insect order, and there are more than 11,000 caterpillar species in North America alone. Most caterpillars feed on plants, and some can cause extreme defoliation. A typical caterpillar has 13 segments and a distinctive head. The first three segments are the thorax, while the last 10 comprise the abdomen.

Stinging Rose Caterpillar

Stinging caterpillars can inflict extreme irritation or allergic reactions, but they sting differently than bees, hornets and wasps. The latter have venom glands and stingers that penetrate skin to inject the venom. Stinging caterpillars, however, have specialized hollow spines attached to poison-containing glands. These stingers are primarily used as self-defense; when people become victims, it is usually due to accidental contact. The stinging rose caterpillar (Parasa indetermina) is known to inhabit cottonwoods. Mature stinging rose caterpillars are between 3/4 inch and 7/8 inch long and have a crayon-yellow body. A wide, purple band runs down the midline of its back and red, white and purple lines down its sides. The body is studded with pairs of horn-like, bristly spines and clumps of smaller spines.

Tent-Making Caterpillars

Tent-making caterpillars include several species that construct silken mats or tents on trees or shrubs. The sonoran tent caterpillar (Malacosoma tigris) and the fall webworm (Hyphantria cunea) inhabit cottonwoods. The fall webworm is more common; in fact, it is the most common tent caterpillar during midsummer, and cottonwoods are among its preferred hosts. Young caterpillars feed en masse on the leaves and can cover a tree in loose tents by August. As they mature, however, these caterpillars tend to confine their feeding to the leaves within the tent. Therefore, the extent of the damage is directly correlated to the number of tents. Fall webworms can produce two or three generations in one season.

Redhumped Caterpillar

The redhumped caterpillar (Schizura concinna) inhabits cottonwoods as an alternative to preferred hosts sweet gum, walnut and plum. It is 1 to 1.5 inches long at maturity, with a yellow body and white, reddish-brown or black stripes running its length. Its head and fourth body segment are orange or brick red; the body segment is humped, with two prominent black spines. Other segments also have these spines, though they are less conspicuous. In the spring, the caterpillars feed in groups, skeletonizing leaves. Later in the season, they consume entire leaves. Damage is usually minor -- often confined to a few branches -- and trees recover quickly. Heavy infestations, however, can completely defoliate a tree.

References

• Colorado State University Extension; Cottonwood Tree (Populus Fremontii); January 2010
• Auburn University; Stinging Caterpillars -- A Guide to Recognition of Species Found on Alabama Trees; L.L. Hyche; February 1998
• Colorado State University Extension; Tent-Making Caterpillars; W.S. Cranshaw; May 2010
• University of California; Redhumped Caterpillar; June 2010