Woollybear Caterpillars

This article was originally published on August 6, 2012 and expired on August 13, 2012. It is provided here for archival purposes and may contain dated information.

A few species of caterpillar are generally called "woollyworms," correctly termed woollybears, but of those species yellow woollbears, "Spilosoma virginica," are commonly noted in the majority of crop scouting literature. Salt marsh caterpillars, "Estigmene acrea," are very similar in appearance to yellow woollybears and are also noted in many resources.

Yellow woollybears are actually native to most of the continent, reaching almost two inches in length when full grown. They emerge from small spherical eggs, laid in mass. While young, the caterpillars usually feed discretely on the undersides of leaves also in mass. Feeding injury at that time appears resembles somewhat minor "windowpaning." As the hair/setae-covered larvae grow in size, they often shift from lighter colors to darker colors (dark colors are often therefore an indication of maturity rather than a rough winter), and they move away from one another to feed in solitude. There are six instars or growth stages. At that time, feeding injury suddenly shifts to easily observed holes chewed in the leaves. Yellow woollybears feed on many different plants (i.e. they are polyphagous) and often feed toward the top of those plans. Economic injury from the woollybear in field crops does not usually occur unless their feeding accompanies severe injury from several other pests.

After the final molt, larvae move about in a seemingly desperate search for residue cover. The larvae spin a hair-covered cocoon around themselves in which they pupate. In season, the pupae stage lasts for about two to three weeks. The second generation pupates in the fall and remains in that state for the winter months. Adults emerging from the cocoon are white colored moths.

Woollybears usually appear most when conditions are hot and dry for extended periods of time. In such conditions, fungal pathogens that deter wollybear populations naturally are inhibited. Likewise, periods of abundant moisture during either generation can allow that fungal pathogen to thrive affecting the woollybear population for much of the growing season.

Pull date: August 13, 2012