Description
Aphids are soft-bodied, pear-shaped insects about 1/16 - 3/8 inch long.
They are sucking insects that form colonies, often on tender plant
tissue such as newly-opened leaves, where they cause curling. They are
very prolific, especially when they lay live young instead of eggs.
Under greenhouse conditions, all aphids are usually females that give
birth to live offspring (3-6 per day). Young start feeding immediately,
and may mature and be ready to reproduce in a week. There are over 400
species of aphids; common colors are green, pink, red, orange, yellow,
black, brown, and gray. Aphids may be winged or wingless; new
infestations are usually wingless. Besides weakening plant growth by
sucking plant phloem, aphids secrete a sticky honeydew which can become
black and moldy. Aphids are also responsible for the transmission of
plant viruses
Identifying
The
key characteristic for distinguishing aphids from all other
groups is their "tail pipes" or cornicles on the rear
of the abdomen - all aphids have two there. It would be best to
have an entomologist that is familiar with aphids identify the
species, however, that is not often possible. The most common
species found in the greenhouse are the green peach (Myzus
persicae), melon or cotton (Aphis gossypii), potato (Macrosiphum
euphorbiae), and chrysanthemum (Macrosiphoniella sanborni)
aphids. To distinguish the green peach aphid (which
varies in color from light green to rose) from the melon
aphid, take a close look at the head. The green peach aphid
has an indentation which looks (in the words of Dr. Jim Price of
the Univ. of Fla.)like it "was struck between the antennae
with a 2 x 4".
Melon aphids may be yellow, green, dark green, or black. Potato
aphids are large, long-legged pink or green aphids that form
colonies which fall quickly off the plant when startled. Their
head structure looks more similar to the melon aphid drawing.
They can be differentiated from green or pink melon aphid by
their large body size, their long thin cornicles (melon aphids'
are short and stubby). Black melon aphids automatically are
differntiated by their color. Chrysanthemum aphids are
not as common. They are shiny, dark red-brown to blackish-brown,
and their colonies often line up uniformly along stems. |
Crop Management
Aphids are serious greenhouse pests mainly because they reproduce so
rapidly. Control aphids first by limiting introductions from outside the
greenhouse. Screen entrances, inspect plant introductions for aphids,
remove weeds that may harbor aphids, and dispose of plant debris
promptly. Avoid over-fertilizing with nitrogen, and control ants that
may protect aphids from predators.
Pest Monitoring
Plants must be monitored frequently for the first sign of aphids - check
growing tips and undersides of leaves. Yellow sticky cards are not
useful for monitoring aphids, since only winged aphids are attracted to
them. If winged aphids are found on sticky cards, they may be strays
from outside, which may or may not be crop pests. Or, if they are the
winged type of a common greenhouse aphid pest, this usually means that
aphid colonies are crowded - and thus a serious aphid problem already
exists, and it's too late for biological control.
Physical or
chemical controls
Syringing - knocking aphids off plants with a water spray - is a
time-honored method of control. Aphids frequently become resistant to
pesticides; using beneficial insects can delay development of
resistance. Insecticidal soaps and summer horticultural oil (when not
phytotoxic) are effective. Avoid using most residual insecticides for a
period of three months prior to planned use of biological controls.
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