The biology of insects

Biological control is most successful when the pest manager has a fundamental understanding of the biology of the pests and their natural enemies.This section explains some of the principles of insect biology and defines terms used throughout this publication.

Insect structure

The insect body is divided into three regions: the head, thorax,and abdomen (figure 1).The head contains the brain, the mouth and mouthparts,and important sensory organs such as the eyes and antennae. Behind the head is the thorax, to which the legs and wings are attached. Insects have three pairs of true legs, although some larval insects, especially caterpillars, possess fleshy auxiliary legs, called prolegs,that are attached to the abdomen (figure 2).There are usually two pairs of wings, except in true flies, which have only one pair. In some insects such as grasshoppers and beetles, the front pair of wings has become thickened to serve as a protective covering over the abdomen while the insect is not flying. The abdomen is located behind the thorax. It is usually distinctly segmented and as long as, or longer than, the head and thorax combined. Internally, the abdomen contains most of the digestive system, the reproductive system, and other important organs. It is usually more flexible than the head and thorax because it needs to stretch to accommodate food, water, air, fat reserves, and eggs.

Insect growth and development

Most insects start life in an egg stage. The act of egg laying is called oviposi-tion.The reproductive adult females of many species lay their eggs specifically in the area where the offspring will feed. For example, greenhouse whiteflies lay their eggs on the foliage where the immatures will feed, and fungus gnats oviposit on the surface of the soil or growing medium where the larvae will develop. Insects have specialized organs called ovipositors for depositing the eggs in the appropriate location (figure 3).Some ovipositors are internal except during oviposition (as with most flies); others are external and very obvious (as with crickets and ichneu-monid wasps). A few insects, such as aphids, give birth to live young.

Figure 3.A cricket (left) and an insect-parasitic wasp (right), each with an obvious ovipositor.The cricket inserts its eggs in soil and the parasitic wasp "stings" its host insect, laying its egg inside the host.

ovipositor

The two major categories of growth and development are simple and complete metamorphosis. Simple metamorphosis

(sometimes called incomplete metamorphosis) occurs in those insect species in which the young usually look very similar to the adults, except that wings are absent and they are not reproductively mature. In the immature stage, these insects are called nymphs (figure 4). Common insects that undergo simple metamorphosis include dragonflies, mayflies, grasshoppers, crickets, cockroaches, true bugs, and leafhoppers. Greenhouse pests with simple metamorphosis include aphids, whiteflies, thrips, mealybugs, and scales. (Whiteflies and thrips have an inactive nymphal stage that is commonly, but incorrectly, referred to as a pupa, even though these insects do not have a true pupal stage like insects which undergo complete metamorphosis—see below.) Mites, which are not insects, also develop by simple metamorphosis.

Most insects undergo a more complex form of juvenile development called complete metamorphosis.These insects are worm-like, maggot-like, or grub-like in the immature stage.These types of immature insects are called larvae (singular—larva).The major change in body shape between the larval and adult stages requires an intermediate stage of development, the pupal stage (figure 5). Pupae (singular— pupa) are nonfeeding and inactive.They usually change shape (metamorphose) in a protected location—such as within a cocoon, under tree bark, or in the soil—because they cannot fly, walk, or otherwise avoid natural enemies and environmental extremes. Insects with complete metamorphosis include all beetles, butterflies, moths, bees, wasps, ants, flies, and lacewings. Greenhouse pests with complete metamorphosis include leafminers, fungus gnats, shore flies, caterpillars, and weevils.

Figure 3.A cricket (left) and an insect-parasitic wasp (right), each with an obvious ovipositor.The cricket inserts its eggs in soil and the parasitic wasp "stings" its host insect, laying its egg inside the host.

Figure 4. A plant bug is an example of an insect with simple metamorphosis. After hatching from the egg, the nymph grows, occasionally shedding its skin, until it reaches the adult winged and reproductive stage, after which it no longer grows.

simple metamorphosis

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