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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.

ovipositor complete metamorphosis complete metamorphosis

Figure 5. The imported cabbageworm is an example of an insect with complete metamorphosis. After hatching from the egg, the larva grows, occasionally shedding its skin, until it is fully grown.The larva then molts one more time and transforms into the pupa.The pupa in turn molts and transforms into the adult winged and reproductive stage, after which it no longer grows.

Figure 5. The imported cabbageworm is an example of an insect with complete metamorphosis. After hatching from the egg, the larva grows, occasionally shedding its skin, until it is fully grown.The larva then molts one more time and transforms into the pupa.The pupa in turn molts and transforms into the adult winged and reproductive stage, after which it no longer grows.

Simple metamorphosis

(immature stage—nymph)

bigeyed bugs damsel bugs minute pirate bugs predatory mites stink bugs

As both nymphs and larvae grow, they periodically have to shed their skins (the exoskeleton), through a process called molting.Most species of insects molt a set number of times before they become adults.The distinct immature stages between successive molts are called instars.For example, greenhouse whiteflies have four instars while vegetable leafminers have three instars.The first instar is that which hatches from the egg, the second instar is after the first molt, and so on. Many natural enemies, especially parasitic wasps, attack only certain instars of the target pest.This can be an important factor for effective biological control, especially when timing releases of natural enemies.

Adult insects are characterized by the presence of wings and by reproductive maturity. (With insects it seems that there are exceptions to every rule; most adult aphids and all fleas are wingless.) Once an insect reaches the adult stage, it doesn't grow any further and never molts again.Therefore, small beetles do not grow into large beetles; small flies do not grow into large flies; and so forth (figure 6).

Figure 6. The upper life cycle shows the common misconception that little insects are smaller versions of adult insects, while the lower life cycle illustrates the correct growth pattern of a beetle.When two similar adult insects are substantially different in size, they are likely different species with different habits.

The rate of insect growth and development depends largely on environmental factors (e.g., temperature, humidity, and availability of food) and the genetic traits of the species.Within limits, the warmer the temperature, the more rapid the development and the shorter the generation time.The length of time required for an insect to complete one generation varies considerably with the type of insect, the availability of food, and, to some degree, the location and climate. Greenhouse environmental conditions usually promote more rapid growth of pest populations than would occur outdoors, although population development often slows down under cooler winter conditions.

Table 1. Common natural enemies with simple and complete metamorphosis.

Complete metamorphosis

(immature stage—larva)

ground beetles hover flies (syrphid flies)

lacewings lady beetles parasitic wasps (such as ichneumonids, braconids, and chalcids)

tachinid flies

In temperate climates with cold winters, insects either die or go into an overwintering protective state of arrested development called diapause.A given species diapauses in a specific stage of development. Outdoors, aphids overwinter in the egg stage, caterpillar pests such as the cabbage looper overwinter as pupae, and vine weevils or root weevils diapause as adults. In the greenhouse, many pests do not enter diapause; they develop year round. But certain natural enemies do diapause in the greenhouse.The predatory aphid midge Aphidoletes aphidimyza,for example, diapauses as pupae, and the predatory mite Neoseiulus cucumeris as adults, although there are nondiapaus-ing strains of the mite.

egg small beetle medium beetle

large = MA

beetle egg large = MA

beetle

larva

pupa adult heetle

Insect feeding

Insects with simple metamorphosis often feed as both nymphs and adults in the same location and on the same food.This is true of aphids, mites, mealybugs, scales, and thrips.The larvae of insects with complete metamorphosis often feed in a different location and on a different food than the adults. For example, black vine weevil larvae feed on roots, but the adults feed on foliage. Some insects, especially those with complete metamorphosis, feed primarily in the immature stages,and adult feeding may be insignificant. For example, leafminer maggots feed by chewing on plant tissue in their mines, while the adult flies feed by sucking plant juices from holes made in the leaves.

Insects may feed in two basic ways: by sucking fluids or by chewing. Mosquitoes, for example, suck blood, butterflies suck nectar from a flower, and aphids suck plant sap from leaves. On the other hand, grubs chew on roots of plants, and caterpillars or grasshoppers chew holes in leaves. Nymphs and adults of insects with simple metamorphosis usually have the same type of mouthparts. In all stages of development, aphids have sucking mouthparts and grasshoppers have chewing mouth-parts.The adults of insects with complete metamorphosis do not necessarily have the same type of mouthparts as the larvae. Caterpillars have chewing mouthparts, whereas adult butterflies have sucking mouthparts.

Many beneficial natural enemies, such as lady beetles, praying mantids,and parasitic wasps, have chewing mouthparts. Other natural enemies, such as the larvae of lacewings and the nymphs and adults of the true bugs, have sucking mouthparts.

Insect classification

Classification is the process of categorizing organisms into related groups, based on a standard hierarchy of categories (see table 2).The most basic category in classification is the species, a grouping of animals that is functionally capable of reproducing and routinely does so, thereby perpetuating the species.The scientific name of a species consists of the genus name (capitalized) plus the species name (not capitalized), both italicized or underlined. For example, the scientific name of the species we call greenhouse whitefly is Trialeurodes vaporariorum.Using scientific names is preferable to using common or colloquial names because in different places the same common names sometimes refer to different species. Also, only very common, very showy, or very pestiferous species have common names.The majority of insects, including most beneficial natural enemies,are uncommon, small, or nondescript, and have never received common names.The Entomological Society of America publishes a list of approved common names that can be used for individual species. In this publication, we will use approved common names in conjunction with the scientific names.

We also describe insects using their family name.Thus, mirid bugs, Deraeocoris brevis and Macrolophus costalis,are both members of the family Miridae.

A note about mites

Mites are not insects but are classified with the insects and several other groups in the phylum Arthropoda. Arthropods are categorized by their hard exoskeleton, or skin, and by their jointed appendages. Mites are in the class Arachnida, which also includes spiders and scorpions, and the order Acari (sometimes called Acarida), which includes all mites. Mites have two body regions instead of three and usually have four pairs of legs. No species of mite has wings.They develop by simple metamorphosis, although in some cases the immatures are markedly different from the adults in structure and biology. Most mites are very small.Those important to agriculture are about Vso inch (V2 mm) in size or less when fully grown.There are many mite pests of greenhouse crops, as well as several predatory mites, especially in the family Phytoseiidae,that are important natural enemies of small insect pests such as thrips and spider mites.

Table 2. Example of insect classification, based on the greenhouse whitefly, Trialeurodes vaporariorum.

Category

Classification of greenhouse whitefly

Kingdom

Animalia

The animal kingdom

Phylum

Arthropoda

The arthropods

Class

Hexapoda

Insects and their relatives

Order

Homoptera

Aphids, cicadas, leafhoppers, mealybugs, planthoppers, scale insects, treehoppers, and whiteflies

Family

Aleyrodidae

Whiteflies

Genus

Trialeurodes

A group of several closely related whitefly species

Species

vaporariorum

Greenhouse whitefly

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