Pests & Diseases
Aphids are small plantlice that feed on plant parts. There must be about
300 species in midwestern US, each of them having a preference for a specific
family or genera of plants to feed on, and they are also specific to a certain
part of the plant, such as roots, or stems, or twigs, or leaves, or flowers,
or fruits, or some combination of those parts.
Aphids are characterized by tiny size (1 to 5 mm), a pair of tubular appendages
located near the tip of the abdomen, antennae with 4 to 6 segments, and sometimes
displaying transparent membranous wings. Body colors may be a single
color, or spotted or striped, and some species have different head and/or
antennae colors from the body. Many species produce honeydew, and are
therefore tended and protected by ants. The recreational gardener is
usually plagued with aphids, some of which must be controlled, and some of
which cause no serious harm. It is convenient to describe aphids by
their apparent color for convenience, however, there are so many species,
that color alone cannot lead to accurate identification.
Dark blue green with green head, winged
or wingless: This is probably the corn leaf aphid which can be found
in large colonies on the tassels, or ear tips, but mostly in the leaf whorls
of corn plants. This aphid can usually be ignored as the damage is
minimal, and the corn is usually harvested before the plant is seriously
Greenish yellow with dark spots on top
of the abdomen, and black appendages: This aphid can be found on a great
variety of plants, including melons, cucumbers, vegetable plants, ornamental
plants, and weeds. This aphid has not been much of a problem in my garden.
Green or pinkish with black antennae,
head, thorax, and appendages: The rose aphid is a serious pest and
must be controlled with systemic pesticides. This is one of the reasons that
I don't grow roses.
Pale green with long black antennae,
marked with brownish or blackish on the thorax: This is the grain aphid
and is usually found on grasses, cereals, and small grains. The grain
aphid has not been much of a problem for me.
Pale to dark green with stripes in
the summer and pinkish in the fall; winged individuals are dark brown with
a brown patch on an otherwise yellow abdomen, and having two pimplelike bumps
on the front of the head: This is the green peach aphid which can be
found on all types of fruit, vegetable, and ornamental plants. Perhaps
the most common aphid in my garden feeding on the growing tips of apple trees,
etc. Damage is not serious, probably because I have a large population
of ladybugs and lacewings to feed on them. Fortunately, this aphid is not
present every season.
Carmine (dark red) aphids are usually
seen feeding in large colonies on Silphium perfoliatum (cup plant): this
aphid appears to have a very high reproductive rate, but a short life cycle,
and there is no serious damage.
Entirely black aphid colonies have been found on my Levisticum
officinalis (Lovage) and on Viburnum opulus (Highbush cranberry). I
usually cut off affected stems and discard with the trash (not in the compost
Gold aphids are the most voracious
and persistent that I have seen. They only colonize on Asclepias species
(milkweed), but are quite capable of destroying the plants without close
attention. Plant parts must be cut off and discarded. Some aphids will
drop to the ground and climb back onto remaining stubs further weakening
the plants. Standard soap solutions do not affect these critters, and if
you grow any of the milkweeds you may have to treat them like roses.
Pink aphids regularly appear on the
growing tips of tomato plants. These can usually be controlled by washing
them off with a strong water spray. I have also seen the American goldfinch
feeding on them, perhaps to feed their babies.
Very pale green aphids: This
is the tansy aphid of which I am quite fond, because ladybugs like them more
than any other. In fact, I grow a 4 x 8 ft patch of Tanacetum vulgare just
for this purpose. In years in which this aphid finds my property, the
lady bug population grows exponentially.
White aphids covering themselves with
white or gray threads that make them look like wool or cotton are called
woolly aphids. These feed on trees mostly, and some other woody plants.
The first line of defense is to wash off aphid colonies with a strong water
spray. Usually, they do not reappear in the same places. This
treatment is suitable for some species.
For more persistent species, a fine spray of soapy water solution made by
mixing a teaspoon of dishwashing soap with a quart of water will interfere
with their ability to breathe through pores in their abdomens.
Aphids which hide in curled up leaves are usually best treated systemically,
but watch for the presence of ladybugs. There is no real advantage
to destroying aphid colonies that are supporting ladybugs and their larvae.
Overwintering eggs of fruit tree aphids can be destroyed with the application
of a dormant oil spray after fall leaf drop has been completed.
To summarize, the organic gardener should not be overly concerned about
aphids, except for the gold ones. Observe the level of damage, and
give the treatments described above in order from weak to strong as necessary.