Insect Development and Life Cycles

THE EGG STAGE

Most insects begin their lives as eggs although there are some exceptions such as aphids, which are born alive. Eggs can be laid singularly as with native budworm or in rafts like the spined predatory shield bug. They may be laid on, in or near the host plant or animal. The death rate of eggs may be very high. Wind and heavy rain can dislodge eggs, or predators and parasites may destroy them.

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 unfavourable seasonal periods. Some eggs of the redlegged earth mite and lucerne flea survive over hot dry summers.

GROWING STAGES

Young insects and mites grow by undergoing a series of moults. From time to time they need to cast off their entire exoskeleton to make room for the next stage. Under the old, hard exoskeleton a new flexible one forms and, after moulting occurs, this is able to expand until the new outer layer hardens. These growth stages are called instars and the number of these may be over twenty in some species but is generally between four and eight. Food source, temperature, humidity and other environmental factors govern the duration of the growth stages. It is often the young growing stages that cause the most damage.

ADULTS

Young insects and mites grow by undergoing a series of moults. From time to time they need to cast off their entire exoskeleton to make room for the next stage. Under the old, hard exoskeleton a new flexible one forms and, after moulting occurs, this is able to expand until the new outer layer hardens. These growth stages are called instars and the number of these may be over twenty in some species but is generally between four and eight. Food source, temperature, humidity and other environmental factors govern the duration of the growth stages. It is often the young growing stages that cause the most damage.

METAMORPHOSIS

There are two main ways that insects develop before reaching their adult stage. These can be described as incomplete or complete metamorphosis; a Greek word meaning change.

Some groups of insects have an incomplete metamorphosis in which the immature growth stages (nymphs) look like scaled down versions of the final adult, but with some lack of development evident, especially the wings. Incomplete metamorphosis is seen in the aphids, true bugs, grasshoppers and locusts.

Other groups of insects have complete metamorphosis with an immature growth stage or larva (plural larvae) that is visually unlike the adult form. "Caterpillars", "grubs" or "maggots" bear no resemblance to the adult moths, beetles and flies that they become. These groups also have another stage, the pupa, between the larva and adult. The pupa (plural pupae) is a transition stage, where larval characters are lost and the adult features develop. This type of development is seen in moths, flies, beetles and wasps.

LIFE CYCLES

Only one cycle or generation occurs per year in some species such as the brown pasture looper and bronzed field beetle. The life cycle may be longer in some insects, such as redheaded cockchafers which is every two years. Two or more generations can occur in one year in other species such as the oat aphid and green mirid. Where several generations occur in one year, all stages of a species may be present at the same time.

Knowledge of insect life cycles and generations is very helpful in understanding the likely insect growth stages that can be expected and periods of risk to crops. For example, the native budworm adult (moth) and eggs may be recognised and provide early warning of the damaging larval stages. With the pasture webworm, there is only one generation per year and the timing of moth emergence and laying of eggs (oviposition) is very predictable.

Arthropoda (plural arthropods): the animal group (phyla) that includes insects, springtails such as lucerne flea (Collembola), mites, spiders and ticks (Arachnids) and other creatures with hard outer plate coverings (exoskeleton) joined by softer tissue.

Beetles: a group of insects (Coleoptera) that have complete metamorphosis and chewing mouthparts. Adults have strongly hardened bodies and the forewings are modified as hardened elytra (wing covers).

Bugs: a group of insects (Hemiptera) that has piercing and sucking mouthparts. These are further divided into "true bugs" which have a hard forewing covering the membranous hind wing (eg. Rutherglen bugs) and other bugs without the hard wing coverings (eg. aphids and leafhoppers).

Earwigs: a group of insects (Dermaptera) that have incomplete metamorphosis and elongated flattened bodies terminating in characteristic forceps (pincers).

Frass: Insect excreta.

Insect: true insects are identified by their three body segments that are clearly visible in the adult form: head, thorax and abdomen. They also have three pairs of legs arising from the thorax. Adult insects have two pairs of wings attached to the body at the thorax. Wings may be modified or absent.

Instar: a stage of growth between each moult of an insect's life between the egg and adult stages.

Invertebrates: animals without a backbone. A host of animals including some that may resemble, or are commonly referred to as, insects eg. spiders, mites, millipedes, slaters etc.

Larva (plural larvae): the immature stage of insects that have a complete metamorphosis such as beetles, moths and flies. They are commonly referred to as a grub, caterpillar or maggot.

Locust and grasshopper: a group of insects (Orthoptera). Locusts and grasshoppers sometimes group together when in large numbers to form large congregations called bands (nymphal stages) and swarms (adult stages).

Mites: arthropods belonging to the order Acarina that have two body divisions and four pairs of legs (larval stages have three pairs). Mites do not have antennae and wings and are minute in size, generally less than 2.0 mm in length.

Molluscs: invertebrates that include snails and slugs. These have a muscular foot, mouthparts with a set of hooked teeth (radula) which cause rasp like damage on plants, and may have shells.

Moult: to shed the outer exoskeleton or "skin" during the process of growth. This occurs between each instar or nymphal stage.

Nymph: an immature stage occurring between the egg and adult stage of insects that have an incomplete metamorphosis e.g. true bugs, aphids, earwigs, locusts and mites. They are usually smaller wingless versions of the adult.

Parasites: organisms that feed on or in the body of another, the host. Some eventually kill their host and are free-living as an adult (parasitoids) e.g. aphid parasites.

Polyphagous: eating many different kinds of food.

Predators: mainly free-living species that consume a large number of prey during their lifetime e.g. ladybird beetles.

Pupa (plural pupae): the resting immobile stage between the larva and the adult in the lifecycle of insects with complete metamorphosis.

INSECT BODY PARTS

Antenna (plural antennae): paired and segmented sensory "feelers"which are attached to the front of the head. In aphids, the last antennal segment is divided into a basal part and a terminal process.

Abdomen: the third and rear section of an insects three main body divisions, and contains the ovipositor in the adult female. This part is more segmented than the head and thorax. In larval forms prolegs are present for some groups (eg. moths) and spiracles (breathing holes) are usually visible.

Cervical shield: in moth larvae the hardened plate on top (dorsal) directly behind the head.

Forewings: the front or top pair of insect wings. These are modified to hardened solid coverings in beetles and some bugs.

Head: the first and front section of an insects three main body divisions bearing the eyes, mouthparts, and antennae.

Hindwings: the second pair of wings, usually membranous, that are often covered or hidden by the forewings when the insect is at rest.

Mandible: jaw-like mouthparts that are used for chewing on plant material or crushing prey in predatory species.

Ovipositor: an egg-laying tube on the abdomen of a female insect. Not present in all insects, but well developed in groups such as parasitic wasps, thrips and some grasshoppers and locusts.

Pronotum: the upper surface of the first segment (prothorax) making up the thorax body part.

Proleg: a non-segmented appendage used for grasping. Abdominal prolegs can be found on moth and butterfly larvae and some sawfly larvae. Anal prolegs can also be present on the abdomen.

Rostrum: The snout of weevils that bears bent-like antennae. Also applies to the protruding piercing and sucking mouthparts of all bugs.

Siphuncle (cornicles): a tubular wax-secreting structure (projection) on either side at the rear of the abdomen on aphids.

Tubercle: in aphids a small protrusion between the antennal segments.

Thorax: the middle section of an insect's three main body divisions. Legs and wings are appended to the thorax.

Umbilicus: in snails this is the hole that the inner surface of the shell is coiled around.