Sharon Evans, age 12, of Winston Salem, No. Carolina, for her question:

How do bagworms feet from tree to tree?

The female bagworm has no wings and the grabby young caterpillars can crawl only to the next twig. Yet somehow these insect pests manage to spread from tree to tree. Now maybe, we know how they do it. At least we know how certain African bagworms spread. Of all things, their eggs are transported by birds.

Bagworms are unusual moths and various species live in different parts of the world, mostly in warm regions where evergreens grow. Our native North America species plagues parts of the eastern United States, where its pesky caterpillars devour the foliage of evergreens and sometimes other trees.

There is no mystery about how other moths spread from place to place. The adult females fly far and wide, seeking suitable plants where they lay eggs to establish the next generation in a .new location. The crawly caterpillars cannot travel very far, but sometimes they too manage to spread from one plant to the next.

But for the bagworms, these methods of transportation are impossible. The adult female is a legless, wingless creature who cannot possibly travel to the next tree to lay her eggs. The newly hatched caterpillars may crawl perhaps a few feet. Then each one spins a silken bag around his body   and stays inside it. His hungry head pokes through a hole and sometimes he totes his portable home to the next green twig. But certainly he cannot travel from tree to tree.

Scientists studied the bagworms of Africa, where these insects are serious pests. The bagworm bag is a sort of cocoon, woven with bits of bark and other natural debris. Usually the feeding caterpillar is not noticed. But during his pupa stage, he dangles his sack from a twig. Later the female also hangs up her sack. Inside it are her eggs and her dead body. A hungry bird may notice and grab a dangling bagworm bag. He may get a little nourishment from the pupa. If he eats the egg case he may get some nourishment from the departed female – but those tiny tough eggs are downright indigestible. Meantime, the bird flies far and wide    as the bagworm eggs pass safely through his body. Eventually they are dropped with other waste bird material    often far from home. The eggs hatch and the next generation of bagworms spreads to a new location.

The adult bagworm that hatches from the pupa may be male or female    naturally. The male is a drab little moth with wings that shed their flakey powder and turn pale. His fragile wings enable him to visit the helpless female, who cannot leave home. Possibly American birds also devour the egg cases and help to spread our destructive bagworms from tree to tree.