[ General Control
Principles: preface |
cultural | mechanical
CONTROL PRINCIPLES [continued]
Ch. 3, pp. 73 - 76
||BIOLOGICAL CONTROL [continued]
|Parasitic Wasp laying
Eggs in Mexican Bean Beetle Pupa
Parasites may be insects (wasps, flies, and beetles), mites, or
nematodes. Parasites are usually free-living adults which lay eggs
on or within a living host which is larger and/or stronger than
themselves. The immature(s) gradually feed on host tissues until
the host is killed. Parasitic immatures can complete development
in one host. Because they are extremely specialized, they often
only attack one or a few closely related species of insect. They
DO NOT harm humans or their pets.
|Parasitic Wasp Laying
Eggs in Aphid
Some parasites have extremely complex and wondrous life
cycles. For example, a eucharitid wasp that is a parasite of ants
lays her eggs on the leaves of trees. The eggs hatch into a mobile
immature that is able to crawl about on the leaf surface. In the
spring, worker ants climb into the trees in search of aphids and
other insects for food. The parasite larva attaches itself to any
worker ant that comes close and, when the worker ant returns to
its nest, it carries along the parasite. Once in the nest, the
parasite drops off and attaches itself to a larval ant. The wasp
larva feeds on the ant larva, eventually killing the ant. After
emergence from the pupa, the adult wasp flies out of the ant nest
to lay her eggs on leaves once more.
Other types of parasitic wasps control aphids. The
female lays an egg inside an aphid. The activity of the immature
wasp within the aphid causes it to form a stiff, immobile form
called a "mummy." Homeowners should leave mummies alone,
and not wash them off the plant, because the new adult wasp will
emerge and attack more aphids.
Gardeners may encounter caterpillars, such as
hornworms, suddenly decorated with white egg-like structures.
These are actually wasp pupae within silken cocoons. The
caterpillar should be left alone, because it will soon die, and
the wasps will emerge and attack more caterpillars.
Sometimes homeowners may find wasps with long
projections or what they may call "stingers" at the end
to their abdomen. These are members of another group of parasitic
wasps called "ichneumonids." The tail is a long tube
used for laying eggs, or "ovipositor." They use the
ovipositor to lay eggs in insect larvae found feeding deep within
plants, or even wood. They are completely harmless to humans.
Bee flies are examples of flies that are parasites of
other insects as larvae. The adult flies mimic bees and may be
mistaken for the predatory flower flies discussed in the previous
section. The larvae attack the eggs or immatures of grasshoppers,
beetles, moths, bees, and wasps.
Nematodes are hair-like worms found naturally in the
soil. Many are microscopic in size and vary in life-style. The
parasitic forms generally feed on insects that are found in the
soil during one or more stages of their life cycle, such as white
grubs, root maggots or weevils. Some forms are available
commercially for insect control.
The advantages of using a parasite to control an insect
is that they tend to be very specific, and attack only one or a
few closely related species. Some have short life spans, and can
build up to high numbers quickly. An example of a successful use
of parasites is to control flies in dairy barns. The parasitic
wasp lays an egg in the pupa of the fly, and thus prevents
emergence of the adult. Dairy farmers must change their management
practices in order to maintain the wasps, however. Because the
wasps emerge from the fly pupae, the farmer must be aware of where
flies pupate and reduce insecticide use in those areas. Fly
predators may be found in those areas as well.
All the different disease organisms, including viruses,
rickettsia, bacteria, protozoa and fungi, attack insects. Some
disease organisms have been grown commercially, and sold
over-the-counter for insect control. A classic example is the
bacteria, Bacillus thuringiensis (also called Bt). This bacteria
produces a toxin which disrupts the gut of the insect that eats
it. Commercial preparations are available from several nurseries
and garden supply companies for the control of various pests with
chewing mouthparts, especially Lepidoptera. A related bacteria,
Bacillus popillae causes milky spore disease of Japanese beetle
larvae and other white grubs. Unfortunately, it seems to be
adversely effected by the high temperatures found in Arizona.
Although they have yet to be used commercially, insect
viruses can control pest insects successfully. They are passed
from insect to insect in much the same way as between humans, but
are specific to insects.
Fungal pathogens are also known to attack insects, but
are often more difficult to grow commercially than bacteria. Fungi
generally require high relative humidity to germinate and to
infect the target insect, but do not need to be consumed to be
Indirect evidence suggests biological control agents may be
extremely important for keeping pest insects in check. When an
insect is introduced into a new area and leaves its natural
enemies behind, it often will become a serious pest. There are
many examples of insects that are of little or no importance in
their native land, that devastate our crops when introduced here.
One such example is the Russian wheat aphid. In Eastern Europe,
where it is native, the Russian wheat aphid is of little concern.
But once it was accidently introduced to North America,
populations exploded and it became a major pest in the mid-west
and western wheat producing areas. In order to control the
outbreak, scientists visited areas where the pest was native in
search of predators and parasites that fed on the aphid. These
insects were gathered and shipped back to the United States where,
after a period of quarantine, they have been released.
There are disadvantages of using solely biological
control, however. First of all, the insect pests do not
necessarily disappear quickly. We have begun to expect instant
results in our world of microwave ovens and one-hour-photo shops,
but often biological control agents require weeks, months or
sometimes even years to bring populations of pests under control.
And a "good" predator or parasite never completely wipes
out its host or prey because it would go out of business. The idea
is to hold pest numbers below the level of damage that can be
tolerated by the consumer or gardener. Biological control agents
are often successful at that level.