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Entomology
HORT 1003
MASTER GARDENER CORE COURSE
Horticulture for the Home & Garden
Item # 08520
2009 Edition
Item #08520 HORT 1003 Master Gardener Core Course Textbook series
Copyright © 2008, Regents of the University of Minnesota. All rights reserved. Send copyright permission
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Credits
Editor / graphic design: Julie Weisenhorn
Production coordinator: Linda Wasem
Master Gardener Volunteer Manual: David Moen, Julie Weisenhorn, Mary Meyer
Botany: Gary Johnson, Cindy Tong
Soils: Carl Rosen, Peter Bierman, Harlan Finney
Herbaceous Plants: David Zlesak, Mary Meyer
Woody Plants: Nancy Rose, Kathy Zuzek
Turf: Robert Mugaas, Karyn Vidmar
Weeds: Robert Mugaas, Karyn Vidmar, Randy Nelson
Entomology: Jeffrey Hahn
Plant Pathology: Michelle Grabowski
Diagnostics: Jeffrey Hahn, Michelle Grabowski
Fruits: Emily Hoover, Terry Nennich, Doug Foulk, Karl Foord
Vegetables: Vince Fritz, Beth Jarvis
Indoor Plants: Carl Hoffman, Deborah Brown
Living with Wildlife: James Kitts, John Loegering
Thanks to Washington and Virginia Master Gardener Programs for allowing use of information and illustrations from their Master Gardener Core Course outlines.
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ENTOMOLOGY
by Jeffrey Hahn, Professor
and Assistant Extension Entomologist
INTRODUCTION
This chapter will introduce you to insects. We will discuss insects’ role in the
environment, insect anatomy, insect life cycle, and how to identify the common insect orders.
LEARNING OBJECTIVES
Learning objectives of this section for Master Gardeners are to:
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
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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, in particular, are so prolific that even harmless species would become pests if they reproduced without natural enemies to
limit their numbers. Insects are particularly considered beneficial when they attack other organisms,
especially insects, thought by humans to be pests (e.g. ladybird beetles eatinaphids). 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 an individual 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, including 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 at the University of Minnesota in experiments 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. They may be
used in a variety of urban issues, e.g. to help detect abuse of children or elderly people. Insects may
also be involved in stored product cases dealing with contamination. Most people are familiar with
forensic entomology when insects are used to help solve murder cases or accidental deaths. This is
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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 trying solve 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 skeletons (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 in 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 mouthparts
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.
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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.
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, includ-ing 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 important to identify insects to order.
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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.
Orthoptera are the crickets, grasshoppers, and katydids (older classifications also include mantids, cockroaches, and walkingsticks in this order). 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.
Immature Orthoptera
Adult Orthoptera
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.
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Immature Hemiptera
Adult Hemiptera
Immature Hemiptera
Adult Hemiptera
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.
Immature Coleoptera
Adult Coleoptera
Diptera are the true flies. Diptera have complete development. Fly larvae are maggots or wormlike 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.
Immature Diptera
Adult Diptera
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.
Immature Lepidoptera
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Adult Lepidoptera
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chapter
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 my 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.
Immature Hymenoptera
Adult Hymenoptera
Synopsis of 6 Most Common Insect Orders
Order
Metamorphosis Mouthparts
Wings
Other
Orthoptera
Incomplete
Chewing (A & N)
4, 1st pr. Leathery,
2nd pr. Membranous;
a few lack wings
Jumping back legs;
mostly plant feeders
Hemiptera
Incomplete
Piercing – sucking
(A & N)
4, 1st pr. Hemelytrous or leathery
or membranous; 2nd
pr. Membranous; a
few lack wings
Many plant feeders
Coleoptera
Complete
Chewing (A & L)
4, 1st pr hardened
Habits varied incl.
many plant feeders
Diptera
Complete
Sucking (A)
Chewing-rasping (L)
2, membranous
Adults feed on liquids; some larvae
plant feeders, most
decaying organic
matter
Lepidoptera
Complete
Sucking-siphoning
(A), chewing (L)
4, wings covered
with scales
Adults mostly feed
on nectar; larvae
plant feeders
Hymenoptera
Complete
Chewing (A & L)
4, membranous
Adults are pollen,
nectar feeders, also
predators; some larvae plant feeders
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