Entomology

By Jeffrey Hahn, Professor and Assistant Extension Entomologist, University of Minnesota

Edited by Clay Kirby, University of Maine

Introduction

This chapter will introduce you to insects. We will discuss insects’ role in the environment, insect anatomy, insect life cycle, how to identify the common insect orders and specific insect examples when using IPM.

Learning Objectives

1. Build a foundation of insect knowledge.
2. Understand insects’ role in the environment.
3. Recognize insect body parts.
4. Understand insect life cycles.
5. Become familiar with the major groups of insects.
6. Learn pest management tactics for controlling insects
7. Use pest management in practice
8. Appreciate Insects.

Introduction to Insects

“If all mankind were to disappear tomorrow, the world would regenerate back to the rich state of equilibrium that existed 10,000 years ago. If insects were to vanish, the terrestrial environment would collapse into chaos.” — Edward O. Wilson, Entomologist and Pulitzer Prize winning author, quoted in Omni, September, 1990.

Insects are the most numerous and successful animals on earth. Entomologists have identified about 90,000 different species living in North America and about one million different species in the world. Taxonomists estimate that there may be as many as two to five million species in the world (some estimate as many as thirty million species); most of these unidentified species are probably living in the tropics. In a typical backyard, there are as many as 1,000 different species of insects at any given time.

Insects have existed on earth for about 350 million years (compared to about 2 million years for humans). Insects occupy virtually all types of habitats except for oceans. Insects use many resources as food and can be divided into three general groups: herbivores, i.e. feeding on plants, carnivores, i.e. feeding on insects and other animals, and saprivores, i.e. feeding on dead or decaying plant or animal material. Insects’ adaptability plus their relatively small size and high reproductive potential has allowed insects to be very successful organisms on this planet.

Insects’ Relationships To Humans and the Environment

The vast majority of insects are harmless or beneficial to people. Insects that are not directly helpful to people are still beneficial to the environment and help maintain the balance of nature. It has been estimated that the beneficial activities of insects are worth approximately $57 billion in the United States. Pollination is a very important insect activity that benefits humans as well as the environment. About 80% of all flowering plants in temperate zones are pollinated by insects. Bees are the most important group of pollinating insects. Flies, butterflies and moths, and beetles also pollinate plants. Many plants, including many fruits and vegetables, are dependent on insects for pollination.

Insect products

Insects also provide us with many useful products. Honey bees are important for producing honey, beeswax (used in cosmetics, candles, waxes, and polishes), and royal jelly (used in health foods and face creams). Other insect products used by humans include silk, shellac, and dyes.

Natural Control

Insects are important as biological control agents. They feed on other organisms, especially insects, helping to keep general populations from becoming too abundant. Insects are particularly so prolific that even harmless species would become pests if they reproduced without natural enemies to limit their numbers. Insects are especially considered to be beneficial when they attack other insects that are considered to be pests by humans (e.g. ladybird beetles eating aphids). Insects are also important as biological control agents of weeds, such as purple loosestrife.

There are two general types of biological control agents that attack other insects, predators and parasitoids. Predators capture and devour insects (called prey), killing them in a relatively short amount of time. Predators eat all or most of the preys’ body. Predators are usually larger or stronger and eat many individual prey.

Parasitoids (also known as parasites) lay their eggs on or in the bodies of other insects (called the host). All life stages can be parasitized, although a specific parasitoid will always parasitize the same stage of a host. After they hatch, parasitoids feed for a relatively long time, consuming all or most of the host’s tissues, eventually killing the host. The host is usually larger than the parasitoid. A parasitoid attacks only one individual host, although many parasitoids can attack a single host.

Decomposers and Recyclers

Insects perform a valuable service as scavengers. They help break down organic material by eating dead animals, dead and dying plants, and dung. These activities help remove unwanted material from the environment and hasten the breakdown of organic material so it can be returned to the soil.

Aerators

Insects living and tunneling in the ground help improve the physical condition of the soil and enrich it. Insects help aerate the soil which improves water and oxygen movement. Insect droppings and their dead bodies serve as fertilizer.

Food Source

Insects are an important source of nutrition for animals. Many mammals, birds, reptiles, amphibians, and fish feed on insects. Some of these animals, rely exclusively on insects for their diet. Humans also consume insects as an important part of their food source. In many cultures throughout the world insects, such as grasshoppers, termites, beetle grubs, and caterpillars, are routinely eaten.

Medicine and Research

Insects also are useful in medicine. For example, live maggots are used to treat some cases of nonhealing wounds (called maggot therapy). Some scientists use insects in their research. Fruit flies are commonly used in genetic experiments. Manduca caterpillars are used to better understand the insect nervous system. This may lead to a better understanding of the human nervous system, which could improve our understanding of diseases affecting the human nervous system, such as Alzheimer’s disease.

Forensic Entomology

Insects are useful any time they are used to help solve crimes or settle legal disputes. Most people are familiar with forensic entomology when insects are used to help solve murder cases or accidental deaths. This is done by collecting insects found on dead bodies and identifying them. Because different insects are present during different stages of body decomposition, the insect fauna at any given time can help establish how long the body has been dead. This information can be critical when solving the mystery of a death.

Human Culture

Insects are part of our everyday lives. They can be found in books, poetry, cartoons, music, movies, arts and crafts, jewelry, and nicknames. They are in our history, our folklore, our myths, and even in our everyday language.

Beauty and Wonder

Finally, insects are appreciated for their beauty and their interesting lives and behaviors. People also enjoy observing and collecting them.

Insects as Pests

Only about 2% – 5% of insect species are considered to be pests. Losses due to these insects have been estimated in the United States to be about $5 billion. Insect pests are especially important because they destroy or spread disease to crops and other valued plants, destroy wood products, clothing and fabrics, and stored foods. They also attack humans and animals by biting, stinging, or spreading diseases. Insects can also be nuisances, especially when large numbers are present.

Metamorphosis: Insect Development

Insects hatch from eggs. Most insects are oviparous, i.e. they lay eggs. Eggs are generally oval and elongate, although there can be much variation. Eggs can be laid singly or in clusters. Many insects surround their eggs in some sort of protective material. A few insects, e.g. aphids, are viviparous, i.e. eggs develop inside the mother and she gives birth to live young.

Immature insects must shed their exoskeletons (outer shells) in order to grow because the exoskeleton does not expand. This process is called molting. The stage of the insect between molts is called an instar (e.g. an insect that has molted once would be a second instar). Insects often have four or five instars, although this varies with different insect species.

A distinctive feature of insects is the phenomenon called metamorphosis. Metamorphosis is defined as a change in form during development. Sometimes this change is gradual but many times it is very dramatic. Insect development is generally divided into two different types, simple or gradual metamorphosis and complete metamorphosis.

The life stages of simple metamorphosis are egg, nymph, and adult. The immature nymphs look similar to the adults except nymphs are smaller, lack wings and are sexually immature. Their wings develop externally (on the outside of their bodies). The last instar nymph molts into an adult. Nymphs and adults often live in the same habitat, eating the same food.

The life stages for complete metamorphosis are egg, larva, pupa, and adult. Larvae look very different from adults, usually feeding on different types of food, and living in different habitats. Last instar larvae molt into a pupa or resting stage. In the pupal stage, the insect does not feed and usually does not move. Inside the pupa, the insect changes form through tissue breakdown, tissue reorganization, and the development of new structures, such as wings, legs, antennae and mouth-parts from masses of specialized cells. Wings develop internally (inside the body). The insect eventually molts one last time with the adult insect emerging from the pupa.

Many insects take one year to complete their life cycle. They are said to have one generation a year. A generation is the time it takes an insect to develop from a particular stage to a descendant in the same stage (e.g. from an adult to the next adult stage). Other insects may have more than one generation in a year. Others may require more than a year to finish one generation.

Anatomy: Insect Form and Structure

Insects (and their relatives, including spiders, mites, centipedes, millipedes, sowbugs etc.) possess a hard external integument known as an exoskeleton. This differs from the internal skeleton, or endoskeleton, that humans and other vertebrate animals possess. The insect exoskeleton is important for protection from the outside environment, prevention of water loss (i.e. drying out), for attachment of muscles, and support of internal organs.

Insects possess three major body parts: head, thorax, and abdomen.

Several important features are found on the head. The antennae are prominent and distinctive. One pair is nearly always found on the adult’s head (there are only a few exceptions where insects lack antennae, e.g. some scale insects). Antennae are usually found between or in front of the eyes. Antennae are segmented, vary in form and complexity, and are sometimes referred to as feelers or horns. Antennae are used by insects for many types of sensing, including smelling, hearing, tasting, and feeling. Antennae can be an important feature to identify some insects, although not for insect orders.

Compound eyes are made up of varying numbers of facets. Each facet sees a small part of what the insect is viewing. Together, they comprise what the insect sees. The size of the compound eye (i.e. how many facets it has) varies. It can be a few as several facets as in some subterranean ants and as many as 50,000 facets in the large eyes of dragonflies.

Chewing mouthparts

Sucking mouthparts

Insect mouthparts are described as being mandibulate, i.e. possessing mandibles, although there is much variation. Mouthparts can be generally divided up into two types: chewing mouthparts, i.e. mandibles (jaws) are prominent, or sucking mouthparts, i.e. the mouthparts are modified into beak-like or tube-like mouthparts. While they differ considerably in appearance, the same basic parts are found in both types. Mouthparts are often very important in identifying insects to order (i.e. beetles, true bugs, flies etc.). Immature insects may have the same mouthparts as adults or they may have very different mouthparts.

The second major body part is the thorax. It is divided into three sections, prothorax, mesothorax, and metathorax. There is a pair of legs on each thoracic segment, a total of six (a few adult insects possess no legs). Legs, like so many other insects parts, vary much in form and function. Legs are used for running and walking, jumping, grasping, swimming, and digging. The type of leg and the number of segments the legs are composed of can be important in identifying insects.

The wings are also attached to the thorax. Most insects have four wings, attached to the second and third thoracic segments (mesothorax and metathorax). Flies have only two wings (attached to the mesothorax). A few insects lack wings altogether. There are a variety of modifications of insect wings, including hardened wings (e.g. beetles), leathery wings (e.g. grasshoppers, crickets, and katydids), hemelytrous wings which are half leathery and half membranous (e.g. true bugs), and thin membranous wings (e.g. flies). Wings are very important to identify insect orders.

The abdomen generally possesses eleven segments and usually lacks appendages. The abdomen is relatively simple in structure compared to the head and thorax. Some insects possess a pair of appendages on the last segment of the abdomen known as cerci. Cerci vary in form and usually function as sensory organs. Appendages for mating and laying eggs, including claspers and ovipositors, are also found on the abdomen.

Some insects, such as bees and wasps, possess a stinger on the abdomen which is a modified ovipositor. Aphids possess a pair of appendages on the abdomen known as cornicles. Cornicles secrete a fluid used by aphids for defensive purposes. Butterfly and moth caterpillars and sawfly larvae possess fleshy false legs on the bottom of the abdomen known as prolegs. When they turn into adult insects, these prolegs are lost.

Insect Classification

Living organisms are classified by scientists into broad groups that share similar characters which are in turn classified into smaller and more specific groups, each sharing a certain set of traits. A commonly used classification for insects is orders.

There are 31 orders of insects (this number may vary depending on which authority you are reading). They are usually separated by comparing the type of metamorphosis, the type of mouthparts, and the number and form of wings. Most of the insects you are likely to see belong to one of six common orders. You can recognize these orders from the following descriptions. (Also see Synopsis of 6 Most Common Insect Orders.)

Orthoptera are the crickets, grasshoppers, and katydids. The Orthoptera have an incomplete life cycle with nymphs resembling adults. Both adults and nymphs have chewing mouthparts. Orthoptera usually have four wings which are folded behind their back. Their first pair is somewhat thickened or leathery. Orthoptera are typically plant feeders.
Hemiptera are the true bugs, cicadas, hoppers, aphids, scales etc. (these two groups were formerly in separate orders). They have an incomplete life cycle. Nymphs resemble adults. The Hemipterans have piercing sucking mouthparts, some of which are slender and needle-like while others are more stout and beak-like. The wings of the Hemiptera are variable. Nearly all possess four wings.The forewing on true bugs is called a hemelytron (plural = hemelytra), i.e. the first part of the wing (closest to the head) is thickened or leathery while the second section is membranous. The hind wings are membranous. Other Hemiptera possess either four membranous wings or the first pair is leathery and the second is membranous. A few lack wings altogether. Many Hemipterans are plant feeders, although a few are predaceous.
Coleoptera are the beetles, the most abundant group of insects. They develop through complete metamorphosis. Larvae usually possess legs and have a definite head with chewing mouthparts. Their general appearance is grub-like or worm-like. Adults also have chewing mouthparts. Beetle adults have four wings, the first pair is hardened while the second pair is membranous (fly-like). Habits for both larvae and adults vary, including herbivorous, predaceous, and saprophagous.
Diptera are the true flies. Diptera have complete development. Fly larvae are maggots or worm-like in form. They are usually legless (although a few possess legs). They generally possess chewing mouthparts but lack a conspicuous head. Their habits are variable, although only a few are herbivorous. Adult flies have two clear, membranous wings and generally have sucking mouthparts, ranging from sponging-sucking (e.g. house fly) to siphoning-sucking (e.g. bee fly) to piercing-sucking (e.g. mosquito). Adult flies feed on liquids or liquified foods from many different sources but are rarely plant feeders.
Lepidoptera are the butterflies and moths. They develop by complete metamorphosis. The larvae are caterpillars. In addition to 3 pairs of true legs on the thorax, there are also 2 – 5 pairs of prolegs (false legs) found on the ventral, posterior end of the abdomen. Lepidoptera larvae have chewing mouthparts and are plant feeders. Adults have 4 membranous wings, usually covered with scales. They use siphoning mouthparts to feed on plant nectar and other liquids.
Hymenoptera are the sawflies, wasps, bees and ants. They develop by complete metamorphosis. Larvae are caterpillar-like (sawflies) or grub or worm-like (wasps, bees, ants). Hymenoptera larvae may have legs and 6 – 10 pairs of prolegs (sawflies) or are legless (wasps, bees, ants). Hymenoptera larvae may have a definite head and chewing mouthparts (sawflies) or the head and mouthparts may be inconspicuous (wasps, bees, ands). They can be herbivorous (sawflies), predaceous (wasps and ants) or pollen feeders (bees). Nearly all adults have 4 membranous wings, although a few are wingless. They have chewing mouth-parts and feed on a variety of food sources, including nectar and pollen.

Integrated Pest Management

A general discussion of Integrated Pest Management (IPM), along with the steps that are used, are covered in the IPM links on the Master Gardener Manual website. We will discuss some of these steps here giving specific insect examples.

Monitor for Pests. It is important to check for insects regularly. Check for all stages, adults, immatures, eggs and pupae, as well as any signs of their presence, such as droppings, honeydew, and webbing. Also watch for symptoms, anything that isn’t normal on the plant, like chewed or discolored leaves.

There are a variety of tools that can help you detect when insects are present, such as traps, e.g. pheromone traps for corn earworm moths. You can also use calendar dates, i.e. the average time during the year a pest first appears, e.g. forest tent caterpillars emerge early to mid May. Their actual first appearance will vary according to where you are in the state and what kind of spring we are experiencing but it does it give you a ball park estimate as to when you can expect to first see them.

Plant phenology is the time an insect is expected to appear corresponding with the first appearance of a specific plant event. For example, pine needle scale crawlers hatch about when common lilacs are first blooming. Plant phenology takes into account the earliness or lateness of a spring. Also, be aware of conditions that could be conducive to pests, e.g. damp conditions for slugs or hot and dry conditions for spider mites.

Use evidence of feeding, calendar dates, plant phenology as guides and rely on sightings of pests to determine their actual presence. However, there are times if you wait to detect the presence of the insect, e.g. iris borers, then it will be too late to manage the insects. In these cases, consider how often you have detected the pests in previous years as a guide. The more frequently you have noticed them, the more likely you will treat them.

Identify Pests. Identifying insects helps you understand their biology, life cycle, and habits and helps distinguish potential pests from those that are harmless. Knowing what insects you have found also helps us understand when the pests are most vulnerable to management tactics and how severe their damage can potentially become. Two different insects that appear similar may actually require different management tactics. Always know what insect is present before implementing IPM. There are numerous fact sheets, reference books, and web sites to help you identify insects. The University of Maine Cooperative Extension’s Homeowner IPM website is particularly useful.

Establish action thresholds. Before selecting your management tactics, determine how many insects can be tolerated. Consider the actual amount of damage that has occurred and what the potential for injury is. You can determine the threshold by the number of insects present or the amount of damage that is present. Sometimes there may be an established threshold when dealing with a particular insect pest in a specific situation. Other times the threshold may be more subjective. Keep in mind that the action threshold for a particular pest problem may have already been reached.

Select Management Strategies. Determine which combination of methods will be most effective. There are several from which to choose.

• Do nothing Not every situation requires action. You may decide to do nothing when you discover the insect is not a pest or the damage or the insects present are tolerable. You may also take no action if the insect is already (or essentially) done feeding. Or management may not be practical and you may choose do nothing.
• Genetic is a method where you choose plants that are resistant or tolerant to pest problems. For example, if you are trying to minimize problems with bronze birch borer, plant a river birch or Whitespire birch which are resistant to bronze birch borer (i.e. they are rarely attacked). If are concerned with iris borer, you may plant Siberian iris. Although Siberian iris is still attacked by this pest, it is more tolerant of the damage.
• Sanitation are methods that you use to alter or modify the environment to minimize pests. Weed management, for example the removal of solanaceous weeds, reduces alternative food sources for Colorado potato beetles which helps reduce their numbers. Some pests, like slugs, can be reduced through moisture management, i.e. water deeply but less often. Sanitation can be helpful. If you clean up the debris in your garden you can reduce the number of squash bugs that can use it for shelter when they overwinter. Sanitation is also useful for household insects. Removing or thinning mulch around homes helps to minimize home-invading insects, like crickets and earwigs. Many household pests, like fruit flies and Indianmeal moths, can be managed by removing their food sources.
• Physical are methods where you use an object or force to manage pests. Probably the most common type of tactic is physical removal, e.g. handpicking Japanese beetles (and thrown into a container of soapy water), using a hard spray of water to knock aphids off a plant, or pruning a heavily infested branch off of a houseplant. You might even try vacuuming to remove insects, like Colorado potato beetles. Another tactic is the use of barriers, like floating row covers to protect crops from cabbage maggots or a cheesecloth fabric to protect plants from Japanese beetles. Traps can be useful, e.g. sticky red sphere traps for apple maggots. When discussing management of indoor insects, physical removal can be vacuuming insects, like boxelder bugs or lady beetles, or using sticky traps for crickets and spiders. Finally, exclusion, i.e. sealing cracks and spaces, like installing a door sweep to help prevent insects from entering buildings.
• Biological methods involve the use of other organisms to reduce insect pests. First, use biorational insecticides when possible, e.g. using parasitic nematodes against white grubs. Preserve natural enemies, predators and parasitoids (parasites), by not making unnecessary pesticide applications. Recognize natural enemies that are in your garden so you do not unnecessarily spray them. There are cases where lady beetle larvae have been sprayed because they were “ugly and therefore must be bad.” You can encourage natural enemies by planting a variety of flowering plants in your yard and garden, especially ones that will flower at different times of the year. Predators and parasitoids feed on nectar and pollen as alternative food sources attracting them to your garden and yard which helps establish a population of natural enemies to help deal with insect pests.
• Chemical Insecticides are an option but should be used thoughtfully and judiciously. Use low impact products when they are practical and effective. Some common examples of low impact insecticides are:
  • Horticultural oils – Petroleum or vegetable oil products that are generally effective against soft-bodied insects. They work by suffocating the insects.
  • Insecticidal soap – This product is composed of potassium salt of fatty acids and is generally effective against small, soft-bodied insects. They are believed to kill insects by disrupting their cell membranes.
  • Bacillus thuringiensis (B.t.) – This is a naturally occurring bacterial disease of insects. Different strains are effective against different insects, e.g. caterpillars, leaf beetle larvae, or fly larvae. It’s mode of action is as a stomach poison.
  • Spinosad – This is the fermentation product of naturally occurring bacteria. It is effective against caterpillars, sawflies, thrips and other insects. It is primarily effective by attacking the insects’ nervous system.
• Legal methods would not be a strategy most gardeners would use. A good example of a legal method would be quarantines, i.e. federal laws to restrict the movement of invasive species, like emerald ash borer. Home gardeners can use legal methods by observing any quarantines that are in place in counties you live in or visit.