Mary E. Barbercheck, Penn State University
In agricultural systems, it is important to be able to recognize both the adult and immature life stages of insects to be able to make appropriate management decisions. Most insects have three life stages: egg, immature, and adult. Because insects have a rigid body covering, called an exoskeleton, they are not able to increase in size by simply expanding. As the insect grows, the body covering is periodically shed and replaced with a larger one in a process called molting. The stage between molts in immature insects is called an instar. Most insects molt several (4 to 8) times between emergence from the egg stage and when they become adults. Most insects do not molt as adults.
The change in form that insects undergo as they develop is called metamorphosis. The life cycles of most insects fall into one of three general types: simple (or gradual) metamorphosis, intermediate metamorphosis, and complete metamorphosis (Gillot, 2005).
The immature insects, usually called nymphs, are similar in appearance to adult, but are wingless. Wings appear as reduced wing buds on nymphs, and are full-sized only in the adult. The nymphs and adults tend to live in the same habitat and feed on the same type of food plant. Common insect groups in which this type of metamorphosis occurs include the grasshoppers and crickets, mantids, true bugs, leafhoppers, and aphids.
Figure 1: Diagram of gradual metamorphosis. Figure credit: Weeden, C.R., A. M. Shelton, and M. P. Hoffman. Biological Control: A Guide to Natural Enemies in North America. http://www.nysaes.cornell.edu/ent/biocontrol/ Accessed February 9, 2009
Some insects undergo metamorphosis that is intermediate between simple and complete. Common insect groups in which this type of metamorphosis occur include whiteflies, scales and thrips.
The immature and adult forms are usually very different in appearance, and often live in different habitats and feed on different food plants. The early immature stages are often wormlike, and are called larvae. Larvae usually have chewing mouthparts. After the last larval stage, the insect transforms into the pupal stage. Pupae are often covered with a cocoon or some other protective structure, for example a chamber in the soil. The final molt occurs at the end of the pupal stage. The final life stage is the adult. Common insect groups in which complete metamorphosis occurs include the lacewings, beetles, flies, butterflies and moths, ants, wasps, and bees. Larvae of moths and butterflies are commonly called caterpillars. Larvae of beetles are often called grubs, and larvae of flies are often called maggots.
Knowing how many generations per year an insect pest will complete and when damaging stages occur can affect management decisions, for example, waiting to plant until after damaging stages are completed. There is considerable variety among insect species in the amount of time it takes to complete a generation. Most insects in temperate climates complete their full life cycle in one year. However, there are some notable exceptions to this pattern, for example, periodical cicadas, that have life cycles from 13 to 17 years. Many insects complete more than one generation per year, and the number of generations can be stable across geographic range, or vary depending on climate in the geographic range. For example, some species may have multiple generations in the southeastern US, while the same species only completes one or two generations in the northern part of its range. Many insects have a life stage that occurs in the winter. Depending on the species, the overwintering stage can be the egg, an immature stage (nymph, larva, or pupa), or adult. In tropical or arid climates. rather than overwintering, insects may enter a quiescent state or diapause during dry or wet seasons.
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