How do you feel about the ever-increasing influx of exotic and invasive pests we find in our landscapes and gardens? Just thinking about the misery caused by Japanese beetles, gypsy moths, and azalea lace bugs makes Bug of the Week a bit peevish. Is this a one-way street with the USA the recipient of nasty bugs and the rest of the world getting off scot-free? Absolutely not! One of our pesky "gifts" to the rest of the world is the fall webworm. In the 1940's this insect was accidentally introduced into Hungary from North America. It has now spread throughout large parts of Europe and Asia. In China fall webworm is known as the North American White Moth and it has become one of the most devastating of all forest pests damaging thousands of acres of trees each year.
The fall webworm is a relative of some other moths were met in previous episodes of Bug of the Week including tent caterpillars, gypsy moths, and cankerworms. Although fall webworm is often confused with the Eastern Tent Caterpillar, they are easy to tell apart. Eastern tent caterpillars appear in March and early April in our area and build tents around branches and where branches join the trunk.
The common hosts of eastern tent caterpillars are trees in the rose family such as cherry and apple. The first generation of fall webworms appears in May and June and usually goes unnoticed. The second generation is a lollapalooza and rocks our trees in July and August. Fall webworms build nests on the terminals of branches. They are one of the least finicky of all caterpillars and eat several common trees in our area including cherry, crabapple, maple, sycamore, and walnut. Fall webworms are known to feed on more than 100 species of plants. On a recent outing to southeastern Pennsylvania near Kutztown, I saw an eerie forest where an entire hillside of walnut trees was cloaked in fall webworm tents.
A closer look at these tents revealed writhing masses of hairy, yellow caterpillars with rows of dots lining their backs. As summer wanes, the finale of webworm activity arrives. Webworm caterpillars vacate nests and move to leaf litter and duff beneath trees where they will spend the autumn, winter, and early spring as pupae. Next spring and early summer, adult fall webworm moths emerge from the soil, find mates, and lay hundreds of eggs on the undersides of leaves of susceptible hosts. Tiny caterpillars hatch and spin small webs first around their natal leaf and later around small clusters of leaves on the same branch. Inside the relative safety of these webs, larvae munch leaves. As they grow, larvae enlarge the web to include ever-greater expanses of leaves and branches. When fall webworms are abundant, entire trees may be enclosed in these webs. In most years and most locations fall webworms do not cause serious damage to trees. However, their nests create a bad case of the uglies on trees that persists well into the winter.
Of course, one easy way to be done with these messy guys is to simply remove them by hand or with pruners when you first see the nests. If nests are far above the ground, a pole pruner may be handy. Please don’t just toss the nests aside or you may learn that the caterpillars can march right back up the tree. Place the nest in a bag and discard it. Fortunately, fall webworm cycles wax and wane in a given area with years of great numbers are followed by years of few. This is likely due to the fact that fall webworms are attacked by a great number of predators and parasitoids. For reasons known only to the webworms and Mother Nature, this looks like a great year for fall webworm, so be prepared to observe and contend with this curious caterpillars.