Download Life cycles of insects

Life cycles of insects Insects are an important part of agricultural systems as pollinators, scavengers, and beneficial enemies of pests. Insects can also be pests that cause severe damage to crops and act as vectors for diseases. Knowing how to identify insects throughout their life cycle and understanding their feeding behavior and habitat preferences will help you in making management decisions on the farm. Insect Development Metamorphosis is the change in form that insects undergo as they grow from eggs to adults. The two main types of metamorphosis are simple (or gradual) and complete. Simple metamorphosis (Figure 1.) is the most basic form. The egg hatches and the nymph emerges. The nymph will feed, grow and molt, shedding its outer skin, several times before reaching the adult stage. The immature stages of these insects look very similar to the adult stage but are lacking wings and can be a different color. Examples of insects that undergo simple metamorphosis include true bugs, grasshoppers, leafhoppers and aphids. Aphids are a special case and the eggs develop inside the female adult and when ready she gives live birth to nymphs. Figure 1. Simple (Gradual) Metamorphisis : Life cycle of the insidious flower bug Complete metamorphosis (Figure 2) is more complex and the insects undergo drastic physical changes. Larvae emerge from eggs, feed and develop into pupae. Fully formed adults emerge from the pupal case, a little pale and soft but within a few hours are ready to take flight and meet their mate. Figure 2. Complete Metamorphisis : Life Cycle of the convergent lady bug Intermediate metamorphosis Not all insects have a type of metamorphosis that can be classified as simple or complete; there is a third type called intermediate. Thrips, whiteflies, and male scale insects undergo intermediate metamorphosis. Insects can have one, two, three or in the case of aphids many generations per year. Some insects can take multiple years to complete one generation. The number of generations per year can be influenced by climate and as long as conditions are favorable they will continue reproducing. Insects can overwinter as eggs, nypmhs, larvae, pupae or adult. Many insects will spend the winter in a state of dormancy waiting for an environmental or genetic trigger to become active. Diapause is a period in which growth, differentiation, and metamorphosis stop. This resting state allows the insect to survive unfavorable conditions. Diapause is often broken by light, temperature, humidity and mechanical stimuli. Eggs Insect eggs vary in size, shape and color depending on the species, Figure 3. Most eggs are spherical, oval or elongate. It can take a few days to a few months for an egg to fully develop and hatch. Figure 3. Various shapes of insect eggs (from http://www.cals.ncsu.edu/course/ent425/tutorial/eggstructure.html) Nymphs Insect nymphs look very similar to the adult form and feed and live in similar habitats. Each stage of the nymph is called an instar and the number of instars required before reaching sexual maturity varies depending on insect species. Larvae Larvae come in several forms and this impacts where they feed and how mobile they are. The larvae of ladybirds and other beetles are active predators and can move relatively quickly. Caterpillars are more sluggish and feed on vegetation and fruit, not traveling very fast or far. Other larvae do not have any traces of legs and stay in one place or inside one thing during their development; these include larvae of weevils and most of the Hymenoptera. Pupae During the pupal stage the insect is transformed into the adult form. It may appear that not much is happening in the pupal case, but complex biochemical changes are taking place. The insect is enclosed in a pupal case and many have an additional outer case or cocoon protecting them. Adults The primary purpose of the adult insect is to reproduce. The adult from is sexually mature and many are able to fly allowing them to travel to find a mate. Once successful mating occurs the female will deposit eggs with her ovipositor either singly or in clusters on or in plant tissue, soil or other insects. Eggs exposed to the elements often have a protective coating. Thus the cycle begins again. Reproduction In most cases insects reproduce sexually where male and female adults are present at the same time and mate. Eggs are fertilized in the female’s body producing an embryo. The female deposits the eggs on, in or near the feeding source, so when the eggs hatch the larvae or nymphs can begin eating. There are also examples of asexual reproduction, viviparity (aphids giving birth to live young), and polyembryony (parasitic wasps having multiple embryos develop from a single egg). Damage to Plant Material The type of damage to plants caused by insects is often directly related to their type of mouthparts. Being familiar with damage will help you identify pests and which stage is active. Different life stages of insects may feed on different plant parts. Insects with chewing mouthparts feed on leaves, twigs, stems, fruits, and roots. Damage includes holes or notches on leaves and other plant parts, root feeding, leaf skeletonizing, defoliation, boring in wood, stems or fruit. Leaf mining insects feed on tissue between the upper and lower surfaces of leaves. Grasshoppers, earwigs, most beetles and caterpillars are in this group. Caterpillars and larvae usually have chewing mouthparts and at this stage in their life cycle do the most feeding and damage. Insects with sucking mouthparts are able to pierce through the cell wall and suck sap through the plant tissue. Aphids, scales, true bugs and leafhoppers are in this group. Thrips have rasping mouthparts that slash at the plant tissue to make a wound and then suck up sap oozing from the wound. Insects can also cause damage to by injecting eggs directly into fruit or stems causing deformation or dieback of stems and branches. Insects can also be vectors for transmitting plant diseases. Beneficial Insects Not all insects are pests to plants and livestock; some are pests to other insects and weeds. Insect predators are usually larger than their prey, generalist feeders where males and females will kill or consume many of their prey. Insect parasitoids are specialist feeders, smaller than their hosts, the immatures remain on or in the hosts, and the adult stage is mobile and may also be predaceous. Weed seed predators are specific to one plant species and can thrive in all habitats where the pest weed occurs. Resources: http://www.biocontrol.entomology.cornell.edu/bio.html#growth Orchard Pest Management A Resource for the Pacific Northwest edited by
Elizabeth H. Beers, Jay F. Brunner, Michael J. Willet, and Geraldine M.
Warner
http://www.extension.org/pages/19194/ecological-understanding-of-insectsin-organic-farming-systems:-insect-life-cycles
Borrer, D.J., D.M., De Long, and C.A., Triplehorn 1981 An Introduction
to the Study of Insects. Philadelphia: Saunders College, pp 94-105.