Aphids
Aphis spp.
Figure 1. Aphids on
Tomato Plant
Description
Aphids may be green, black, brown,
red, pink, or some other color depending on the sap color of the host
plant. They are usually slow-moving insects with somewhat pear-shaped
bodies ranging from 1 /16 to 1 /8 inch long. There may be various sizes
of wingless aphids (nymphs and adults) in a dense colony on a stem, on
the underside of the foliage, or on the flowers. Later in the season,
some may have relatively large, transparent wings. The slender antennae
are conspicuous, and near the rear end of the abdomen there are two
tubes called cornicles.
Figure 2. Aphids
on Pittosporum
Life History
The life history of aphids is
somewhat complicated and varies with the species. One of the less
complicated life histories proceeds as follows: Overwintering eggs on
branches and stems hatch in the spring to produce a wingless form known
as the stem mother. The unfertilized stem mother gives birth to living
young in great numbers. Several generations may occur in this fashion,
but in due time some individuals will develop wings (alates) and migrate
to another host. Here they may deposit eggs for the winter or, after a
few generations, migrate back to the original host to lay eggs.
Figure 3. Aphids on
Milkweed
Damage
Aphids are sucking insects that feed by
thrusting a long beak into the plant tissue. They withdraw great
quantities of sap, some of which they execrete as "honeydew". The
honeydew makes the plant sticky. When trees are heavily infested with
aphids, sidewalks, lawn furniture, and automobiles may become wet with
honeydew. A sooty mold often develops with the honeydew blackening stems
and foliage. This fungus is not parasitic to the plant, but reduces the
amount of photosynthetic area. Leaves of plants may be distorted by
aphids feeding on the undersides. Succulent stems may wilt or growth may
be arrested by colonies of aphids. On the other hand, damage caused by
aphids feeding on the bark of trees or woody shrubs cannot be readily
seen.
Figure 4. Green Peach
Aphids on Mustard Leaf
Nonchemical Control
Plants that are well
established and vigorously growing usually can tolerate low to medium
aphid population levels. Newly transplanted trees or stressed plants are
more vulnerable to aphid damage and control should be considered. Aphid
populations are usually controlled later in the season by natural
enemies such as parasites, predators, and pathogens. In some cases,
populations may crash literally overnite due to weather conditions.
Figure 5. Aphids on Spirea
Chemical Control
Apply an insecticide when aphids are numerous. Spray with high pressure and throroughly cover the foliage