Insect Life Cycles 

It is useful to understand life-cycles when you want to get control of pest insects.

Insects have external skeletons of chitin. The limited flexibility of this chitin cuticle stops insects being able to grow without shedding the cuticle. So, insects grow in a series of periodic moults; exiting their exoskeleton, expanding and hardening a new cuticle.

Insects also develop in a process called metamorphosis. Metamorphosis is the change in form from egg to adulthood. There are three main types of metamorphosis; primitive, incomplete and complete.

• Primitive – Springtails, are an example of insects that grow through a primitive metamorphosis where a nymph hatches from an egg and grows, by moulting and expansion, through several stages which look very much like each other but a little larger each time.
• Incomplete – Aphids, shield bugs and termites are examples of insects that grow through incomplete metamorphosis, by moulting and expansion, through several stages that have the same basic structure but which have significant differences such as the development of wings and reproductive organisms.
• Complete – Flies, butterflies/caterpillars, grass grub (and other beetles), ants, whitefly and wasps are examples of insects that grow through complete metamorphosis, by hatching from eggs as larvae which do not look at all like the adult. The larvae are ‘eating machines’ which when fully fed change into a pupa/cocoon. In the pupal stage, the insect transforms into the mature adult form which is very different from the larva. It breaks out of the pupal cocoon and enters the reproductive adult phase.

Knowledge of the life-cycle of the insect pest you want to control can help you in approaching the problem. For example, in control of grass grub, it is useful to know that the larvae are barely mobile grubs that eat the roots of grass and other plants, the pupae are immobile and protected in their cocoon, and the adults are much more mobile beetles with chewing mouthparts and feed on the foliage of plants.

Note: In many cases for insects that go through complete metamorphosis it is the larval stage that is a pest; Porina moths, grass grub. Whereas it is the adult that is the pest for ants and wasps. Flies are pests both as maggots and as the adult fly.

Note Also: Creatures such as spiders, mites, slaters, centipedes and millipedes (i.e. not insects) do not go through metamorphosis but their life cycle is similar to that of primitive metamorphosis.

David Brittain
Kiwicare