of Rhode Island GreenShare Factsheets
small, soft-bodied insects with long, slender mouthparts with which
they pierce stems, leaves, and other tender plant parts to suck
out plant fluids. Almost every plant has one or more aphid species
which occasionally feed on it. Many aphid species are difficult
to distinguish from one another; however, identification to species
is not necessary to control aphids in most situations.
be green, yellow, brown, red or black depending on the species and
the plants they feed on. A few species appear waxy or woolly due
to the secretion of a waxy white or gray substance over their body
surface. All are small, pear-shaped insects with long legs and antennae.
Most species have a pair of tubelike structures called cornicles
projecting backwards out of the hind end of their body. The presence
of cornicles distinguishes aphids from all other insects.
are generally wingless, but most species also occur in winged forms,
especially when populations are high or during spring and fall.
The ability to produce winged individuals provides the pest with
a way to disperse to other plants when the food source gets scarce.
Although they may be found singly, aphids often feed in dense groups
on leaves or stems. Unlike leafhoppers, plant bugs, and certain
other insects that might be confused with them, most aphids do not
disperse rapidly when disturbed.
many generations each year. Most pest aphids reproduce asexually
throughout much of the year with adult females giving birth to live
offspring (often as many as 12 a day) without mating or laying eggs.
Young aphids, or nymphs, molt about four times before becoming an
adult. There is no pupal stage. Some species mate and produce eggs
in fall or winter, which provides them a more hardy stage to survive
harsh weather. In some cases, these eggs are laid on an alternative
host, usually a perennial plant, for winter survival. When the weather
is warm, many species of aphids can develop from newborn nymph to
reproducing adult in less than two weeks. Because each adult aphid
can produce up to 80 offspring in a matter of a week, aphid populations
can increase with great speed.
Low to moderate
numbers of leaf-feeding aphids are usually not damaging in gardens
or on trees. However, large populations cause curling, yellowing,
and distortion of leaves and stunting of shoots; they can also produce
large quantities of a sticky substance known as honeydew, which
often turns black with the growth of a sooty mold fungus. Some aphid
species inject a toxin into plants, which further distorts growth.
A few species cause gall formations.
viruses from plant to plant on certain vegetable and ornamental
plants. Squash, cucumber, pumpkins, melons, beans, potatoes, lettuce,
beets, chard and bok choy are all common hosts of aphid-transmitted
viruses. The viruses cause mottling, yellowing or curling of leaves
and stunting of plant growth. Although losses can be great, they
are difficult to prevent through the control of aphids because infection
occurs even when aphid numbers are very low; it only takes a few
minutes for the aphid to transmit the virus while it takes a much
longer time to kill the aphid with an insecticide.
A few aphid
species attack parts of plants other than leaves and shoots. The
woolly apple aphid, for example, infests woody parts of apple roots
and limbs, often near pruning wounds, and can cause overall tree
decline if roots are infested for several years.
plants regularly for aphids--at least twice weekly when plants are
growing rapidly. Many species of aphids cause the greatest damage
when temperatures are cool (65-80 degrees F). Once aphid numbers
are high and they have begun to distort and curl leaves, it is often
hard to control them because the curled leaves shelter aphids from
treatment or natural enemies.
to be most prevalent along the upwind edge of the garden and close
to other sources of aphids, so make a special effort to check these
areas. Many aphid species prefer the undersides of leaves; be sure
to turn leaves over to check them. On trees, clip off leaves from
several areas of the tree to check for aphids. Also check for evidence
of natural enemies such as lady bugs, lacewings, syrphid fly larvae,
and the mummified skins of parasitized aphids. Look for disease-killed
aphids as well. Substantial numbers of any of these natural control
factors can mean that the aphid population may be reduced rapidly
without the need for treatment.
seldom kill a plant, the damage and unsightly honeydew they generate
sometimes warrant control. Consider the nonchemical controls discussed
below; most insecticides, if used, will destroy beneficial insects
along with the pest. On mature trees, such as in citrus orchards,
aphids and the honeydew they produce can provide a valuable food
source for beneficial insects.
Control: Natural enemies can be very important in the control
of aphids, especially in gardens not sprayed with broad-spectrum
pesticides (organophosphates, carbamates, and pyrethroids) that
kill natural enemy species as well as pests. Usually natural enemy
populations do not appear in significant numbers until aphids begin
to be numerous. Among the most important natural enemies are various
species of parasitic wasps that lay their eggs inside aphids. The
skin of the parasitized aphid turns crusty and light brown, a form
called a mummy. The generation time of most parasites is quite short
when the weather is warm; when you begin to see mummies on your
plants, the aphid population is likely to be reduced substantially
within a week or two.
also feed on aphids. The most well known are lady beetle adults
and larvae, lacewing larvae, and syrphid fly larvae. Naturally occurring
predators work best, especially in a small backyard situation. Commercially
available lady beetles may give some temporary control when properly
handled, although most of them will disperse away from your yard
within a few days.
very susceptible to fungal diseases in humid weather. Whole colonies
of aphids can be killed by these pathogens when conditions are right.
Look for dead aphids that have turned reddish or brown; they have
a fuzzy, shriveled texture unlike the shiny, bloated, tan-colored
mummies that form when aphids are parasitized.
Control: Before planting vegetables, check surrounding areas
for sources of aphids and remove them. Aphids often build up on
weeds such as sowthistle and mustards, moving onto crop seedlings
after they are planted. Check transplants for aphids and remove
them before planting.
populations are localized on a few curled leaves or new shoots,
the best control may be to prune these areas out, drop the infested
plant parts in a bucket of soapy water, and dispose of them. In
large trees, some aphids thrive in the dense inner canopy; pruning
these areas out can make the habitat less suitable.
In some situations
ants tend aphids and feed on the honeydew aphids excrete. At the
same time, they protect the aphids from natural enemies. If you
see ants crawling up aphid-infested trees or woody plants, put a
band of sticky material (Tanglefoot, etc.) around the trunk to prevent
ants from getting up. Do not apply sticky material directly to the
bark of young or thin-barked trees or to trees that have been severely
pruned; the material may have phytotoxic effects. Wrap the trunk
with fabric tree wrap or duct tape and apply sticky material to
the wrap. Alternatively, ant stakes, dusts, granules, or baits may
be used on the ground to control the ants without affecting the
aphids or their natural enemies. Prune out other ant routes such
as branches touching buildings, the ground, or other trees.
of nitrogen fertilizer favor aphid reproduction. Never use more
nitrogen than necessary. Use less soluble forms of nitrogen and
apply it in small portions throughout the season rather than all
at once. A urea-based, time-released formulation (most organic fertilizers
can be classified as time-release products as compared to synthetically
manufactured fertilizers) is ideal.
vegetables are primarily susceptible to serious aphid damage during
the seedling stage, losses can be reduced by growing seedlings under
protective covers in the garden or in a greenhouse, or inside and
then transplanting them when they are older and more tolerant of
aphid feeding. Protective covers will also prevent transmission
of aphid-borne viruses.
mulches have been successfully used to reduce transmission of aphid-borne
viruses in summer squashes, melons, and other susceptible vegetables.
They repel invading aphid populations, reducing numbers on seedlings
and small plants. However, as plants grow, aluminum foil mulches
give mixed results for aphid control; they seem to repel natural
enemies of aphids as well as aphids. The few aphids that do drift
onto plants grow and reproduce with greater speed than those landing
on plants growing in bare soil because temperatures are higher on
the aluminum foil-mulched plants. Yields of vegetables grown on
aluminum foil mulches are usually increased, despite higher aphids
numbers, by the greater amount of solar energy reflecting on leaves.
to reduce aphid populations on sturdy plants is to knock them off
with a strong spray of water. Most dislodged aphids will not be
able to return to the plant and honeydew will be washed off as well.
Using water sprays early in the day allows plants to dry off rapidly
in the sun and be less susceptible to fungal diseases.
Control: Many pesticides are available to control aphids around
the home. Check labels before using, as not all of these materials
are registered for all plant types. Selective insecticides such
as oils and soaps are safer to use where children and pets may be
present, and may provide more effective long term control because
they do not kill the natural enemies of the aphids.
from the University of California Statewide IPM Project, 1999
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program