Helpers are already at work in your garden and woods, though it’s unlikely you’ve noticed them. They are wasps, and they are busy killing insect pests.
Most readers will remember their first encounter with wasps. We learned that they are big, they sting, and (in the worst case) they live in large groups. Although these familiar wasps do have a colony structure somewhat like that of honeybees, with a hive of sterile females raising the young of a single queen, such a social lifestyle is in fact very rare among wasps. Solitary wasps far, far outnumber the social wasps, but they usually go unnoticed because they are mostly small and they rarely sting.
Solitary wasp females are single parents. They must feed and protect their own young, a challenge they meet by hiding them away in nests or, more gruesomely, inside living insects.
Some of the nest-makers dig tunnels in the ground in which to lay their eggs. Others use small, ready-made holes like those in hollow twigs or in your outdoor electrical outlet (check your woodshed). A nest may hold just one egg, or may contain a row of individual chambers, each with a single egg. The female stings and paralyzes a spider or insect, carries it to a nest, lays an egg on or near it, then seals the nest. After the egg hatches, the larva eats the paralyzed victim and pupates inside the chamber. The wasps in this group are often large, since they have to overpower and carry their prey.
Another route has been taken by solitary wasps whose young develop inside another living insect. How the larvae get inside varies. Eggs may be laid on or in the host, or an egg laid in the open hatches into a larva that must crawl about until it finds a host. In either case, the larva ends up inside the host, which it consumes and eventually kills. Wasps with this lifestyle must sneak up on their victims, so being inconspicuous is an advantage. These wasps can be extraordinarily tiny. Some emerge full-grown from insect eggs. In fact, the smallest known insect is a wasp no bigger than a large amoeba.
When its larvae develop at the expense of just one host animal, and that host is killed in the process, a species is said to be a parasitoid. This distinguishes it from a predator, which kills more than one organism during its development, and a parasite, which does not usually kill its host. Solitary wasps have perfected this parasitoid lifestyle, and some have taken it even a step farther. As the ecology ditty goes, “Little fleas have littler fleas upon their backs to bite ‘em; and littler fleas have littler fleas, so on ad infinitum.” Thus some parasitoids develop only in or on other parasitoids.
Although most parasitoid wasps are small, they loom large in ecological and economic importance. Perhaps one-fifth of all insect species are parasitoids, and most insect groups are attacked by them. The wasps are highly specific in their choice of hosts or prey, a trait that has made them attractive pest controls. This specificity has recently been used in a new way in Maine. There, a colonial digging wasp of the northeast, Cerceris fumipennis, is being used to monitor for the presence of the emerald ash borer, a buprestid (wood-boring) beetle that is a serious threat to native ashes. The wasp supplies its young exclusively with buprestids, and is a champion at finding the ash borer. By looking at the insects that Cerceris brings back to its nest, researchers can determine whether or not the borer is nearby. So far, so good. The borer is not yet in Maine.
Certain parasitoids are widely used commercially. For example, species of Trichogramma (weighing in with a wingspan of 1/50th of an inch) parasitize moth and butterfly eggs and are used to control at least 28 pest species, including those that attack cotton, corn, tomatoes, avocadoes, walnuts, pecans, apples, and alfalfa.
You don’t have to buy these garden helpers, though. They are already here. But you can increase their numbers by providing food and overwintering sites for them. The adults are nectar-feeders, and the number of eggs they lay is directly related to the available food. A poorly-fed wasp may lay 30 eggs; a well-fed one, 300. Attract them with plants that have many small, dish-like flowers. Carrot family members are especially good: fennel, dill, and Queen Anne’s lace. Spread plantings throughout your fields or garden, rather than in one area, and plan to have something blooming throughout the growing season. If you see an ailing caterpillar, let it be. The chances are good that there will soon be more free garden help on the wing.