Chapter 22

Greenhouse Insect and Mite Pest Management

Biology of Insects

Knowledge of the biology of insect and mite pests and their natural enemies is a prerequisite for pest management methods compatible with Integrated Pest Management and organic pest management, which rather than eliminate insect pests aim to manage them. A successful management plan requires information about a species biology including its diet and lifecycle, how it interacts with the environment and with other species as well as species behavior and how the behavior of both pest and beneficial insects can be manipulated to prevent or reduce yield losses.

Insect Growth and Development

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.

The Egg Stage

Most insects begin their lives as eggs although there are some exceptions such as aphids, which are born alive. They may be laid on, in or near the host plant or animal. Eggs may hatch soon after they are laid or they may have a long incubation period. In other cases, they have mechanisms that allow them to survive between seasons or during unfavorable seasonal periods.

Molting

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. The first instar hatches from the egg, the second instar is after the first molt, and so on.

Metamorphosis

There are two main ways that insects develop before reaching their adult stage. These can be described as incomplete or complete metamorphosis. Insects that undergo incomplete metamorphosis have three distinct life stages (egg, nymph, and adult). The nymph is the immature stage, which often looks like a smaller, wingless version of the adult. As the nymph grows, it slowly matures into an adult with fully formed wings and reproductive organs. Nymphs usually eat the same food and live in the same habitat as the adults.

Insect Seasonal Cycles

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.

Insect Feeding

Insect mouthparts are of two main types: chewing and piercing-sucking. Some insects have modifications of these two basic types. Mouthparts determine how an insect feeds and therefore play a role in the type of insect control that is most effective. Insects feed on leaves, buds, stems, roots, fruits and seeds, as well as on plant tissue at various stages of decay.

Injury by Chewing Insects

Chewing insect pests cause damage by consuming plant parts such as leaves and stems or burrowing in plant tissues, which damages the host plant. Symptoms of chewing insect pests include holes in leaves, silvering of leaf tissue, complete removal of leaf tissues and burrowing in or around plant stems, branches or trunks.

Injury by Piercing-Sucking Insects

Another important method which insects use to feed on plants is piercing the epidermis (skin) and sucking sap from cells. In this case, only internal and liquid portions of the plant are swallowed, while the insect feeds externally on the plant. These insects have a slender and sharp pointed part of the mouthpart which is thrust into the plant and through which sap is sucked.

Feeding Habits

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.

Mites

Mites are not actually insects, but belong to the related class Arachnida, which also includes spiders, scorpions, and ticks. The major morphological differences between mites and insects are found in the number of major body parts and the number of legs.

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