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Giant silk moth caterpillars

Jeff Hahn

Cecropia caterpillar

Photo: Jeff Hahn

Cecropia caterpillar

Polyphemus caterpillar

Photo: Jeff Hahn

Polyphemus caterpillar

It is common during late summer and early fall to find caterpillars on trees or in nearby areas of your yard as they search for a place to pupate. Two of the largest and most striking moth caterpillars in Minnesota are the cecropia, Hyalophora cecropia, and the polyphemus, Antheraea polyphemus.

Both belong to the family of giant silk moths (Saturniidae) and are active as caterpillars from summer into the fall, before they pupate and spend the winter as cocoons. They emerge as adult moths the following spring. Adults are short lived, surviving just one to two weeks, and do not feed. The basically have enough time to mate and lay eggs. These moths have one generation per year.

The cecropia caterpillar grows as large as 4 inches long. It is light green with a double row of reddish orange knobs (turbercles) on the thorax behind the head. There are also series of smaller yellow and blue knobs (turbercles) on the abdomen. These knobs are presumed to be harmless to people. A cecropia feeds on the leaves of many different hardwood trees, including cherry, linden, maple, boxelder, elm, oak, birch, willow, hawthorn, and poplar. The insect constructs a cocoon, often inside a group of leaves, that typically is attached to a branch or to the trunk of the host plant.

The adult cecropia, sometimes referred to as a robin moth, is a spectacular insect. It is reddish brown with a wingspan of five to six inches. Cecropia moths are common in states east of the Rocky Mountains.

The polyphemus moth caterpillar grows to be about 3 1/2 inches long when fully grown. It is a pale green with sparse long hairs along the top and sides of the abdomen. These hairs are non-stinging (urticating). A polyphemus feeds on the leaves of many deciduous trees and shrubs, including ash, birch, maple, oak, and willow. It has also been known to eat grape leaves. It forms its cocoon inside leaves on the ground.

The polyphemus moth is a striking brown moth with a large blue and yellow eyespot on each hind wing. A polyphemus has a wingspan of four to five inches and is found throughout most of the United States and Canada.

If you find one of these caterpillars in a tree, do not worry about it. Despite their size, these caterpillars cause little, if any, actual damage to trees and shrubs when they feed. No control is necessary.

If you find a caterpillar or cocoon and are interested to try to rear it to an adult, follow these guidelines. Place it in a wide-mouthed container or a terrarium. Remember that the adult moths can have a wingspan up to six inches and it is important that they have a large enough space so they can expand their wings after they emerge. If you are trying to rear a caterpillar (especially a cecropia), be sure to place a twig or small branch so they can hang their cocoon on it. The adult moths also like to be able to climb up twigs or similar objects after they emerge. Sprinkle the inside occasionally with water to keep it moist and be sure that the container is well ventilated to avoid mold growth. It is best to keep the container in an unheated site. It will have an easier time completing its development and will also emerge in the spring when it can find a mate and complete its life cycle.

Published in Yard and Garden Line News, August 15, 2005

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