of Bug Identification
article by Rick Foster appeared in "American Vegetable Grower," March
Proper identification is the first and most essential
step in making appropriate pest-management decisions. Trying to manage insects
without identifying them properly would be like asking a doctor to treat an
illness without describing your symptoms. If you don’t identify the pest
correctly, you may apply an insecticide that is not necessary, apply the wrong
insecticide, or apply it at the wrong time. One way to increase your skill in
identifying insect pests is to collect as much reference material as possible.
Most extension entomologists have numerous publications that will assist you.
Many are available free of charge or for a nominal fee. There are also a number
of good reference books that have excellent pictures of many insects.
Most insects have one of two
distinctly different types of life cycles. The first is called incomplete
metamorphosis. With this type of life cycle, the immature insect, called a
nymph, looks very much like the adult, except that it is smaller and lacks
wings. Insects with incomplete metamorphosis usually feed in much the same
manner and on the same food in the immature and the adult stages. Some examples
of pest insects that have this type of life cycle include the true bugs,
leafhoppers, and aphids.
The other type of life cycle is
complete metamorphosis. These insects have an immature stage, called a larva,
that looks nothing at all like the adult. The larval stage often feeds in a
completely different manner than the adult. There is also a pupal stage which
occurs between the larval and adult stages. It is during this stage that the
remarkable transition from caterpillar-to-butterfly or maggot-to-fly takes
Some examples of insects with this
type of life cycle include ants, caterpillars, maggots, and beetles. An
important point is that once these insects become adults, they do not grow
Mites are not insects. Therefore, the best way to identify them is by counting
their legs. They have eight, while insects have six. However, mites are very
tiny; counting their legs requires the use of a hand lens. Mites have sucking
mouth-parts and generally feed on the underside of leaves. Leaves that have had
considerable mite feeding will appear off-color, and may show symptoms of
wilting. There may also be some very fine silken webbing associated with the
mites on the underside of the leaves. Mites tend to be more of a problem during
hot, dry weather, so if these conditions occur, be prepared to look for mites.
To do this, shake a leaf over a sheet of white paper. If the dust spots walk,
you probably have mites.
Aphids are soft-bodied insects that occur in colonies on the underside of
leaves. They also have sucking mouth-parts with which they remove plant juices
from the leaves of most vegetable crops. Leaves that have been fed upon by
aphids may become curled. Aphids are rather small insects, usually reaching on a
length of 1/16-inch. They may have wings, but most will be wingless. Aphids are
somewhat unique insects in that they can reproduce without mating, and give
birth to live young. Scout for aphids by looking on the underside of leaves.
Although many people call all insects ‘bugs’ there is only one group of
insects that are truly bugs. Most bugs can be recognized by the presence of a
triangle on the back directly behind the head. Several different types of bugs
damage vegetables, including the tarnished plant bug, some species of stink
bugs, chinch bugs, and squash bugs. Bugs range in size from quite small, like
the 1/6-inch-long chinch bug, to fairly large, like the 5/8-inch-long squash
bug. Bugs tend to be fairly mobile, so scouting for them may require a sweep
net, depending on the specific insect and the crop. Some bugs are predators of a
number of pest insects.
There are a number of sizes, shapes, and colors of leafhoppers. Adult
leafhoppers are relatively small, rarely exceeding 1/4-inch in length. Some
leafhoppers that are important pests of vegetables include beet leafhopper,
potato leafhopper, and aster leafhopper. Adult leafhoppers are very mobile, and
often must be sampled with a sweep net or some sort of sticky trap. The nymphs
cannot fly, and therefore are much easier to find, usually on the underside of
Thrips are slender, very tiny insects that feed with rasping or scraping
mouth-parts. The wings of adults are fringed with long hairs, but these may be
difficult to see. Some species are capable of transmitting serious diseases of
vegetables. Thrips also feed on small grains and weeds, and may move to
vegetables in large numbers when these hosts dry down in summer. They tend to be
more of a problem during hot, dry weather. Thrips tend to hide in secluded
locations on the plant. On some crops, such as onions and cabbage, plants may
have to be taken apart to assess populations. Some thrips that are important on
vegetables are onion thrips, western flower thrips, and tobacco thrips.
Beetles can be readily distinguished by the hard covering that is formed by one
of their pairs of wings. They vary greatly in size, shape, and color. The larvae
may be grub-like or almost caterpillar-like. Both adults and larvae feed with
chewing mouth-parts. The larvae of some species, such as bean leaf beetles,
root-worms, cucumber beetles, and wire-worms, feed on underground portions of
plants. Others, such as Colorado potato beetles and Mexican bean beetles, feed
on plant foliage in much the same manner as the adults. Many species of beetles
are pests of vegetables. Fortunately, most can be easily identified with the aid
of the type of publications mentioned previously. Depending on the crop and
species of beetle, sampling can be accomplished by direct observation, scouting
for damage, and using a sweep net or beat cloth or sticky traps.
Maggots are the larval stage of flies. They are usually white, legless,
soft-bodied insects that feed in moist locations. Most of the maggots of
importance on vegetables feed on the roots or other underground plant parts. The
most important maggot pests of vegetables are the cabbage maggot, onion maggot,
and seed-corn maggot. The adult stages of these pests are flies, slightly larger
than houseflies. Damage is usually more severe during cool, wet weather, and in
fields with lots of decaying organic matter. These insects cannot be scouted for
because once they can be found, it is too late to do anything. Preventive
control must be practiced.
Probably the largest group of pests on vegetables are caterpillars, which are
the larval stage of butterflies and moths. They may vary in size from the
diamondback moth larvae (3/8-inch long) to the tomato and tobacco horn-worms (4
inches long). Correctly identifying which species of caterpillar is difficult.
Again, make good use of the reference materials you have available. The first
step in identifying a caterpillar is to note which crop it is feeding on. This
will frequently reduce the number of possibilities to a half dozen or fewer. The
size may not be very helpful, since all species start off as very small larvae.
However, determining the maximum size may help. Look for distinguishing
characteristics such as color, spots, stripes, and the number of prolegs.
Prolegs are fleshy appendages on the abdomen that look like legs. Often the
number of prolegs can help identify the insect. For example, most species of
loopers have two pair of abdominal prolegs. The best ways to sample for most
species of caterpillar is by direct observation on the plant, scouting for
damage, or by using a sweep net or beat cloth. However, some insects, such as
corn earworms on sweet corn, cannot be controlled after they are found. The
moths must be trapped with pheromone traps to predict when the eggs will be laid
so that protective control measures can be taken before the eggs hatch.
There are more than three-quarter
of a million species of insects in the world. Correctly identifying which
species are feeding on your crops can be a challenge. However, proper management
usually depends on how well you identify the pests you are trying to control.
article appeared in the July 2000 issue of Vegetable Production &
Marketing News, edited by Frank J. Dainello, Ph.D., and produced by
Extension Horticulture, Texas Agricultural Extension Service, The Texas A&M
Univerisity System, College Station, Texas.
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