WHAT ARE THE GOALS OF THIS
MODULE?
This module provides a general overview of natural enemies, including predators
and parasitoids, and their impact on our everyday lives. Insects may be natural
enemies and feed on other organisms or they may be the prey (i.e., food) of other
organisms. In other words, insects may terminate or they may be terminated!
Either way, they play an important role in our world.
This module focuses on helping you to understand the basic concept of natural
enemies and provides learning opportunities to help you to meet the expectations of
this module. This module also addresses the many ways that insects protect
themselves from termination. Life sure is complex in the world of insects!
WHAT ARE NATURAL ENEMIES?
Natural Enemies are basically organisms that feed on other organisms. If you have
watched nature shows, you have probably seen the lion chasing and killing the
zebra. In this example, the lion is the natural enemy of the zebra. In order to stay
alive, the zebra must avoid the lion. But, on the other hand, for the lion to survive it
must hunt and kill the zebra. What a catch 22??!! Natural enemies are common
among the insect world., where insects are always eating or being eaten.
Insect natural enemies come in many shapes and sizes, but typically they are
classified as predators or parasitoids (often you may hear the term ‘parasite,’ but it
is not a natural enemy). General definitions of each term (from Webster's
Dictionary) are:
predator - one that preys, destroys, or devours; an animal that lives by
predation (think zebra and lion) (predators kill their prey)
parasite - an organism living in or on another organism (a parasite obtains
benefits from a host which it usually injures) (think tick and human)
(parasites do not generally kill their host) (although parasites may weaken
their host, they are not generally considered to be insect terminators because
their host does not typically die)
parasitoid - an insect that develops on or within the body usually of another
insect and eventually kills it (remember the movie ‘Alien’ where the creature
erupted from the human’s body? Well, welcome to the world of parasitoids.
They can develop internally or externally, as well as solitarily or
gregariously. Parasitoids ultimately kill their host.
WHAT IS BIOLOGICAL CONTROL?
"Biological control" is an important natural phenomenon in our world and is a
major element of the force that keeps living creatures in “balance.” The basic
premise of biological control is that organisms have natural enemies; we then
attempt to increase the use or effectiveness of these natural enemies. Biological
control takes advantage of one organism's ability to reduce the viability of another
organism. In some systems, biological control can play an important role in
regulating and/or reducing pest populations, with minimal adverse environmental
impacts. Biological control can be simply defined as the science of natural enemies
(predator, parasitoid, pathogen, antagonist, other competitors, etc.) in regulating
the numbers of their hosts. Without these natural enemies, our world would be
overrun with some species of insects and other animals.
The United States has a governmental unit called the National Biological Control
Institute (USDA/APHIS/PPQ). This Institute has defined biological control as the
use of live natural enemies of pest species ("biological control agents") to reduce
populations of pest species to levels below which would occur in the absence of the
natural enemies. From a practical standpoint, the use or encouragement of
"beneficial" living organisms for the reduction of pest organism populations has
occurred for many years. For example, the earliest record of biological control
dates back to about 4th century China, where ants were used to suppress pests in
citrus.
Why should we be interested in biological control? It is nature’s free gift to us –
why not take advantage of it, if we can? To paraphrase Johnathan Swift:
"Big fleas have little fleas upon their backs to bite 'em, and little fleas have
lesser fleas, and so ad infinitum."
Biological control offers us several advantages to its use to reduce pest populations.
It is: 1) permanent (self-sustaining, self-perpetuating) (generally), 2) safe (less
disruptive -- environmentally), 3) economical, and 4) compatible with other tactics.
On the other hand, biological control does have some disadvantages or limitations to
its use: 1) may still have occasional pest outbreaks, 2) has been criticized as lowyielding, 3) is not as fast acting as other control measures, and 4) may require
additional input and knowledge.
Insect Orders
Hymenoptera – Bees, Wasps, and Ants
The Hymenoptera exhibit a great diversity of habits and behavior, from the solitary
wasps to the highly structured social classes of ants. Although this order contains
many types of predators (see other part of this module), the most unique and
unusual specialized predators are the parasitoids, often referred to as parasitic
wasps. Adult parasitic wasps feed on nectar and pollen, but the immature are the
specialized predators. Both life stages have chewing mouthparts.
Parasitic wasps are unusual creatures because their immatures use the body of
another organism, usually another insect, as their home. Basically, a parasitic wasp
uses its modified ovipositor (egg-laying structure) – we usually call it a ‘stinger’ – to
lay an egg in, or on, the bodies of other insects, spiders, etc. To lay an egg inside the
host, the stinger actually penetrates the body wall and the egg is inserted inside the
host. This egg hatches and the developing immature feeds on the hemolymph and
eats the internal organs, ultimately killing its host. When the parasitoid larvae are
mature, they bore through the host’s skin and spin a cocoon. The adult develops
inside this cocoon. A new adult wasp emerges and then the cycle starts over again.
In some cases, the parasitic wasp may parasitize the host, allowing the developing
larvae to have available food for a longer time. If you watched the movie “Alien,”
you already have a good grasp of how parasitic wasps function. In the movie, the
queen Alien laid an egg in the body of one of the men. In the climatic scene, the man
becomes ill, his chest rips open, and the young Alien enters the world to begin the
cycle again. Of course, the man who was host for the Alien died. What a way to go!
Although these parasitic wasps are deadly to their hosts, they are harmless to us.
Parasitic wasps may have a stinger, but they do not use it against us. We should be
thankful that these parasitic wasps do not lay eggs in humans – at least they do not
lay eggs that develop into adulthood! Two groups of hymenoptera contain many
parasitoid species – these are the ichneumon wasps (Family Ichnemonidae) and the
braconid wasps (Family Braconidae).
Insect Terminators-How Do Natural Enemies Impact Humans?
Natural enemies – predators and parasitic wasps - are important to us for many reasons and impact humans
in a positive manner. These groups of insects help to keep the natural order of other insect populations ‘in
balance.’ They are extremely important when they feed on or parasitize and kill pest insects – those insects
that destroy crops or fiber products, such as corn and cotton, or invade our homes (such as ants,
cockroaches, and termites).
Biological control is a normal, natural part of everyday life in our environment, but it also can be
implemented as a tactic to suppress or reduce pest populations of certain insects and weeds. Natural enemies
play a tremendous but relatively unnoticed role in our environment every day - they feed on insect pests,
other organisms, and weeds, reducing or preventing problems that these pestiferous organisms may cause in
crop production, natural areas, landscapes, and forests. Ecologically, natural enemies are an important
regulation factor in the population dynamics of many organisms - without these beneficial insects, our lands
and homes would be overrun by pest insects, a fact that is not known or appreciated by the general public.
Environmentally, natural enemies help to protect our environment from overuse, misuse, and abuse of
chemical pesticides, maintain the stability of our ecosystems, and provide long-term, sustainable reductions
in pest numbers. Economically, natural enemies save us billions of dollars annually in reduced pesticide
costs and increased yields.
Some biological control agents can be purchased for release against certain pests. Yes, people make money
by raising ‘good’ bugs to release against ‘bad’ bugs. A few common natural enemies that can be purchased
are preying mantis egg cases, lady beetle adults, lacewing larvae and adults, egg parasitoids (Trichogramma)
of moths, whitefly and aphid parasitoids, other parasitic wasps, and even thistle-feeding weevils.
Natural enemies occasionally may impact humans in a negative manner. A good example is the Asian
multicolored lady beetle, which was introduced from Asia as a predator of aphids and other insects that feed
on trees, such as pecan and fruit trees. This insect is extremely beneficial, as it is one of the few voracious
feeding lady beetles that are arboreal (like to inhabit trees). Unfortunately, this insect has a really bad habit
of overwintering in homes – something that many humans just cannot seem to tolerate! In this case, the
beneficial has become a pest. Ecologically, this introduced lady beetle also seems to directly impact and
displace native lady beetles – something that conservationists abhor!
Fortunately for us, many types of beneficial insects are present in our environment and benefit us “behind
the scenes.” Some of these natural enemies are shown in this module, but many other natural enemies are
found all around us. Look around you and watch nature at work!
WHAT KINDS OF ORGANISMS
TERMINATE INSECTS?
(Natural enemies of insects)
Many different kinds of organisms kill and feed on insects, which serve as
a major protein source for many animals. One of the most common
terminators of insects is other insects – either as a carnivore on other
species (such as a lady beetle feeding on an aphid) or as a cannibal
feeding on members of their own species (such as a female preying mantis
feeding on a male preying mantis).
Other organisms that are natural enemies of insects are:
- spiders
- centipedes
- scorpions
- lizards
- skunks
- birds
- others???
Oops – I have overlooked one other important group of insect terminators.
HUMANS – as Ripley would say, “Believe it or not.” In many parts of the
world, insects serve as an essential ingredient of the diet of human
civilizations. In other parts of the world, certain insect species are
delicacies. In the United States, insect consumption has not really gone
very far. In some larger metropolitan areas, you may find insects on the
menu. But in most parts of the United States, we steer as far away as we
can from restaurants that serve insects. It is interesting how many people
eat close relatives of insects (including crab, shrimp, and lobster) but
would NEVER knowingly place an insect in their mouth. If we did start
eating insects, how quickly could we eliminate some pest populations?
There is one other group of “insect eaters” to mention – insectivorous
plants! Yes, some plants do attract, kill, and digest insects for their own
nutritional gains. Sounds odd, but true. The most famous of these plants
is the Venus fly trap. Did you ever see the play (or movie) titled “Little
Shop of Horrors“ – Audrey II (the human-eating plant) was probably
modeled after this plant.
Insects are an important food for the survival and continuation of many
other species. Without them as a food source, the diversity, abundance,
and composition of our ecosystems would be entirely different. Think
about it the next time you sit down to eat a plate full of mealworms!
HOW DO INSECTS PROTECT
THEMSELVES FROM
TERMINATION?
Put yourself in the place of an insect? You are minding your own business
and doing your own thing – then suddenly from out of nowhere comes
another organism who has you on his/her dinner menu. What are your
options? Do you give up and shake salt and pepper on your body before
you are eaten? Do you offer up a couple of legs hoping that will satisfy
his/her appetite? Do you beg? Do you run? Or do you “turn on” your
protection devices and quickly remove your name from the menu?
Many insects have unique ways to protect themselves from being eaten. If
they did not have these protections, their species would not be able to
exist very long or at least in large numbers. Some of these protections are:
-
-
chemical defenses (“squirters”) (blister beetles, lady beetles, ….)
alarm coloration (adult moths)
warning coloration (velvet ant, lady beetles, ….)
camouflage – blending in plant stem, foliage, etc. (walking sticks,
treehoppers,….)
camouflage – resembles ‘dirt’ or animal ‘droppings’ (toad bugs,
cereal leaf beetle, some lacewing larvae, some moths,….
sound (hissing cockroaches)
Batesian mimicry – a good-tasting species resembles a badtasting or aggressive model (clearwing moths that resemble
wasps; flies that resemble bees; viceroy butterfly that resembles
the monarch butterfly)
Mullerian mimicry – they all taste bad and all look similar
(example?)
These protection types are very important to insects that are potential food
for other organisms. They certainly make our ecosystems more complex.
The next time you are outside, take a few minutes to examine closely the
bark of that tree – you may be surprised to learn what you may find!