Other Controls

Insects, Enemies of Insects

Barnard D. Burks.

A common sight in the country is the large gray paper nest built by the bald-faced hornet. Generally it hangs from the stout limb of a tree at the edge of a forest.

Within the nest the hornets live a social life much like that of the honey bee. Each nest has a single queen, the mother of the other members of the colony. The workers, the sterile females, care for the developing brood and perform other housekeeping duties in the nest or serve as foragers, ranging the countryside to gather food and materials for building and maintaining the nest.

The adult hornets ordinarily feed on the nectar of flowers, but they will take almost any available fluid foodstuffs. They take only liquids because their mouth openings are so small they cannot swallow solids. The food of the adults is mainly carbohydrate, but the growing brood back in the nest require proteins. To get that the hornets capture caterpillars.

The hornet recognizes her prey by sight. When she is out hunting caterpillars, she flies along near the ground or threads her way among leaves, intently looking for a suitable victim. When she sees one, she pounces on it.

She does not sting her victim to death but butchers it alive. She quickly dismembers the struggling caterpillar. Sometimes she carries it into the nest to cut it up, but usually she does so outside. If the caterpillar is small, the hornet suspends herself head down, hanging by one foot from a convenient twig, and proceeds to chew up the victim. If the caterpillar is large, she drops to the ground and cuts it up there.

Her treatment of the victim is cold-blooded and methodical. First she kneads it all over with her mandibles to soften the muscles and other tissues. Then she cuts it up with her teeth. As she proceeds with her carving, she swallows the liquids and forms the solid parts into pellets, which she carries back to the nest for the growing brood. If the caterpillar is too large to be disposed of at once, the hornet cuts up part of it, takes that to the nest, and then returns for the rest. When she must leave a part of her victim behind, she carefully reconnoiters the surrounding territory and notes the landmarks, for which she has a remarkable memory, so that she will be able to find again the spot where she left her prey. When she has finished slaughtering the caterpillar she discards the inedible parts.

Back in the nest, the nurse hornets take over the pellets of solid caterpillar flesh. They break them up into morsels and feed them to the brood. The inedible bits are carried out of the nest and thrown away.

THE HORNETS are not always the best of housekeepers, especially toward the end of the season, when the vigor of the colony declines. Some remnants of food may accumulate in the bottom of the nest. Various scavenger flies then move in to lay their eggs and rear a crop of maggots in the refuse. The nest may even become infested with cockroaches. And, although the hornets are quick to defend their nest, a parasitic wasp, Sphecophaga burra, sometimes is able to invade and lay its eggs in the bodies of some of the developing hornet larvae. The grubs that hatch from the eggs feed at first internally and later externally on the bodies of the immature hornets, eventually killing them. This parasite is itself sometimes parasitized and killed by a minute chalcid wasp, Dimmockia incongrua.

Nest of bald-faced hornet.

The bald-faced hornet is one of thousands of insects that live at the expense of other insects. Its life and activities exemplify a mechanism whereby nature keeps a species in check. We have all heard about the enormous reproductive capacity of insects how, for example, flies would cover the entire earth to a depth of 47 feet within 5 months if all the progeny of only one pair of house flies lived to maturity and reproduced. Many insects often do become disastrously plentiful, but no species has yet even remotely approached the number of individuals it theoretically could, simply because no insect ever gets the chance to continue multiplying as fast as it can. Like most living things, insects are susceptible to bacterial and fungus diseases. Another ever-present control is their enemies.

Often their worst enemies are their own relatives.

Throughout their lives from egg to death, most insects are surrounded by others that are trying to eat them or to lay their eggs in them or on them or to seize and carry them off to become food for their own developing brood. Many insects pillage the food their more industrious relatives have accumulated. While the interlopers grow fat, the helpless young that should have had the food are killed or die of starvation.

No insects are completely safe from these depredations no matter if they live in apparent safety inside trees, in nests of clay, beneath the surface of ponds or streams, or deep within the soil.

CONSIDER ALSO the horse guard, which is well known in rural districts of the South. It is a large, aggressive, loud-buzzing, black-and-yellow wasp. Farm animals, which are distressed by the relatively modest humming of the horse flies and bot flies, will stand quietly while the horse guards drone loudly all around them they learn quickly that the wasps are catching the flies that are tormenting them. It is a mutually beneficial relationship. The animals attract the flies, so that hunting for flies is easy around them, and the wasps, in catching the flies, help rid the livestock of the pests.

The horse guard, which is equally watchful for the comfort of cattle and mules, does not catch the flies for food for herself. Like most of the adult wasps, she feeds on the nectar of flowers or the sweet honeydew that is given off by aphids and scale insects. The horse guard catches flies to feed to her young.

She rears her progeny in nests she digs in sandy, loose soil. Ordinarily she spends 2 days in excavating and finishing a nest. She tunnels into the earth for about 18 inches and at the end of the passageway she makes a small chamber where she lays one egg. When she has finished excavating the nest, she has heaped up a large pile of sand outside the entrance. This she is careful not to leave as a tell-tale marker to the portal of the nest. The horse guard carefully closes the entrance with sand and then scatters the heap of diggings, thoroughly smoothing the surface of the soil around the nest. When she has finished, there is no sign of any kind to show where the entrance to the nest lies, although she does not herself have any trouble finding it again.

The egg, deep within the nest, hatches within 2 days. Then the horse guard begins bringing in flies, one at a time, to feed the grub. She cruises over the surrounding territory, looking for flies of a suitable size. As the flies are most easily found , around livestock, that is where she hunts. She catches the flies in the air, on the wing, and stings them to death. (Many other wasps that stock their nests with flies do not kill them, but only paralyze them by deftly stinging a vital nerve.)

The horse guard grasps the fly between her legs and carries it to the entrance of her nest. She scrapes aside the sand before the entrance and carries the fly in to the young larva, which begins eating the freshly killed fly at once. Then, having settled her offspring to its meal, the horse guard leaves the nest and again closes the entrance. She will return with another fly when her offspring has consumed the first one. She will continue bringing in one fly at a time to feed her larva until it is fully grown and refuses to eat more.

The growing larva eats all the soft parts of the flies, but rejects the legs, heads, and the wings. These it casts aside. But because the larva is essentially motionless, the refused parts of the flies do not get broken up. Consequently we can open one of the nests, remove the wings, and count them. Dividing the total by two gives a close reckoning of the number of flies the grub has consumed. In midsummer a grub will complete its growth in about 2 weeks. Counts of wings taken from nests of the horse guard show that the average number of flies one grub consumes during this growing period is 5o. As most of the heads of the flies also are preserved intact, the flies often can be identified accurately. Nests which have been studied contained predominantly or exclusively female horse flies.

As the larva of the horse guard nears maturity, it consumes flies at a greater and greater rate. Since each horse guard rears several young simultaneously, she is kept extremely busy, catching flies to feed to her ravenous progeny.