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By Dr. James H. Hunt
Paper wasps Polistes species are important members of natural communities. The adult wasps visit flowers where they feed on nectar, especially in the early spring and in the fall. They may provide some pollination, although they are not efficient pollinators.
In the summer, when there are immature wasps (larvae) in nests, the adult female wasps gather caterpillars to feed to the wasp larvae. Caterpillars are captured using the wasp’s mandibles (mouthparts) without using its sting. The caterpillar is then chewed into a ball of tissue, and the adult wasp drinks the caterpillar’s “hemolymph” (the liquid inside it) during the process. The ball of caterpillar tissue then is fed to larger wasp larvae, and the provisioning wasp then regurgitates hemolymph to feed smaller larvae. The provisioning wasp retains a bit of the hemolymph for her own nourishment.
When paper wasps prey on caterpillars that are pests, they are seen by us to be beneficial members of the ecological community. In fact, paper wasps can be effective biological control agents that can help reduce caterpillar densities in gardens without the use of pesticides. If the caterpillars preyed upon are those of attractive butterflies, however, we might think of paper wasps as undesirable. The paper wasps and caterpillars – whether pests or desirable – interact as part of the balance of nature.
Paper wasps build nests of wood fibers. To do this, a female wasp drinks water, flies to dead wood (a dead tree snag or an unpainted fence plank, for example), regurgitates water to moisten and soften the wood, and then scrapes up a mouthful of fibers. She then flies to her nest, where she mixes the fibers with saliva to form the paper with which she builds the nest. Each trip provides enough paper to lengthen a single cell 1 to 2 millimeters, thus it takes many trips to build a complete nest.
Nests are started in the springtime by the “foundress” wasp. She builds the early nest, lays all the eggs, and rears the first larvae to adulthood – all by herself. The first of her offspring to develop are all females, and they emerge in the early summer. These females become workers, and they take over nest building and care of the larvae in the nest. The foundress then becomes the queen, laying eggs but no longer foraging or building. Through the efforts of the workers the nest reaches its final size, which by the end of the summer can be more than 100 nest cells. The workers do not lay eggs, so the larvae that they are raising come from eggs laid by the queen.
Nests are divided into cells, and each cell receives an egg. The egg ecloses (hatches) into a larva, which an adult female feeds mouth to mouth. Small larvae are fed on nectar or caterpillar hemolymph; larger larvae are fed caterpillar tissue.
A nest is established in the spring by a single “foundress” wasp, which is a female that was reared in the previous summer, mated in the fall, and over-wintered in a protected cavity such as a hollow tree. When the first offspring of the year mature, they remain on the nest where they were reared and behave as “workers.” That is, they forage for caterpillars, feed larvae, make paper and enlarge the nest – but they don’t lay eggs. The combination of an egg-laying queen and her daughter workers constitutes social behavior. The origin of social behavior (i.e. what causes the workers to work rather than building their own nests and reproducing) is a challenging question in evolutionary biology. Paper wasps are one of the best model systems for learning the behaviors, developmental variations, and demographic patterns that combine to yield the evolution of social behavior.
Beginning in mid-summer and continuing into early fall, paper wasp offspring include males plus females called “gynes.” The males do not work – they don’t forage, build nests, or defend the colony (they can’t sting). Their only role in life is to inseminate the gynes. The gynes do not work, either. Instead, they will mate, over-winter in a sheltered place, and then become the foundresses at the start of the next season.
Paper wasp nests are naturally placed on vegetation – the branches of low shrubs or under bunch grasses. Such nests suffer extremely high rates of loss to predators such as birds and deer mice. Perhaps because of this, paper wasps prefer to build their nests on human structures (such as barns or abandoned automobiles) if they are available. Researchers can take advantage of this trait by providing “nest boxes” that are attractive to foundress wasps in the spring. A foundress wasp will select a nest box and start her nest on the underside of its lid with no manipulation by the researchers. If the foundress wasp and her wasps then survive the vagaries of life, the nest will grow to 100 or more cells in size by late summer. Such a nest will have produced more than 100 offspring – workers, males, and gynes. At the end of the summer or in the fall, the queen dies (she has lived for a year or more!), and the workers die. The males and gynes feed on nectar (and possibly pollen) of fall flowers (goldenrod is a favorite), and then the males die. Only the gynes, which by then have mated with a male, survive the winter. They then become foundresses and start the next generation.
A nest box is open on the bottom to let wasps fly in and out. They attach their nest to the underside of the lid. The lid can be removed by researchers to examine the nest, record data, or perform experimental treatments. A screen mesh large enough for the wasps to pass but strong enough to exclude blue jays and raccoons helps protect the nests from predators. The fine-mesh screen on the display nest box is also intended to keep wasps out. The wasps can sting (and it hurts!), but they are aggressive only in defense of the nest in the summer.
Forty nest boxes have been placed at Prairie Ridge in an area away from visitors. These are part of a research project being led by Dr. James H. Hunt of North Carolina State University. The study will be to test a specific hypothesis: that the presence of males and gynes on nests in the late summer leads to the end of colony growth. It is believed that the males and gynes, which do not work, intercept most of the caterpillars foraged by the workers, leaving little or none for the growing larvae. When the larvae stop growing, the colony comes to an end. To test this, a manipulation will be performed in which the males and gynes will be removed from some colonies but left on other colonies. The prediction is that colonies from which the males and gynes are removed should continue to grow or, at least, to persist for longer than the colonies on which the males and gynes are present.
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