Are
colonies of little bugs covering the leaves and stems of your plants? Aphid
populations have been high recently in the state, which is somewhat unusual this
late in the season (normally the spring generations are more predominant). While
you may view them as just another nasty to be eliminated, they're also really
fascinating creatures, with bizarre lifestyles and eating habits and
they're also dinner for lots of other insects.
Aphids are primarily northern temperate zone insects, with about 1350 species in North America, although only a handful are pests (see below for some common pest species). Only 25% of all plant species are infested with aphids; the Compositae, Coniferae, and Rosaceae support the highest number of aphid species.
Most aphids are 1/8 inch (23 mm) long. Their pear-shaped
bodies have exhaust pipes (cornicles) protruding from the back end of the
abdomen (their function is unclear). Many
species are green, but others are white, yellow, red (pink), brown, black, or mottled.
And many species have two color types, such as the green peach aphid, which has
both a green and a red form. Some, such as the cabbage aphid or wooly apple aphid, secrete a white or gray
waxy substance that covers the body. Adult aphids may be winged or wingless.
They have sucking mouthparts and feed exclusively on plant sap. Aphids feed on stems,
leaves, and even roots! They
reproduce rapidly to form colonies or clumps,
particularly on new growth.
The life cycle of most species is rather complex. In
Wisconsin aphids spend the winter as eggs. When these hatch in the spring, they
produce only wingless females that give birth to live young (without mating =
parthenogenetic reproduction). Each female aphid reproduces for a period of 2030 days,
giving birth to 60100 live nymphs. The nymphs look like the adults but are
smaller. The nymphs mature and can produce offspring within a week when
temperatures are high. Eggs within these females start to develop long before birth
so that a newly born aphid can contain within herself not
only the developing embryos of her daughters but also those of her
granddaughters which are developing within her daughters. This 'telescoping of generations'
means aphids can build up immense populations very
quickly. Under ideal conditions, one cabbage aphid could produce 1,560,000,000,000,000,000,000,000,000
offspring by the end of a growing season. (Obviously this doesn't happen, since natural controls
such as weather and predators eliminate significant numbers of aphids.)
After several generations, when the food supply
becomes short, or the area becomes overcrowded, winged females appear. These
winged females fly to new host plants and produce many more generations of
wingless females. At the end of the summer, when temperatures begin to drop and
daylength decreases, both males and females are produced. These aphids mate and
the females lay eggs on the appropriate host plant.
Although
aphids are generally weak fliers, some species migrate considerable distances.
They accomplish this by flying upward to be carried by fast-moving, low level
jet streams. Several species, such as the English grain aphid, corn leaf aphid,
bird-cherry oat aphid and greenbug (as well as some pest leafhoppers and moths)
catch a ride on the prevailing winds in the spring to move from southern areas
to the Midwest each year.
Most
species of aphids feed on justone or a few species of closely related plants.
Some species, however, have a primary host where they spend autumn, winter and spring
and an unrelated, secondary host plant for the summer. For example, the rosy
apple aphid has apple as its primary host and plantain (Plantago lanceolata) as its
secondary host. Only a few species have a wide range of secondary hosts, but
these tend to be important pests, such as the green peach aphid.
Aphids feed by inserting their mouthparts into phloem vessels through leaves, buds, stems, or roots and sucking out the sap. Some species probe through stomata, but most just jab through the plant's cells. Their mouthparts are very thin, so to prevent damage during insertion, they secrete a special liquid that hardens quickly to form a protective sheath as they slowly push their mouthparts into the plant. (It can take anywhere from 25 minutes to 24 hours from starting to insert the mouthparts to actually getting something to eat.) Once the phloem is reached, the aphid injects saliva that probably helps prevent the plant cell from sealing the puncture.
Plant phloem has lots of sugars but not many amino-acids or nitrogen. In order to get enough nitrogen in
their diet, aphids must ingest far more sugar and liquid than they need. They
have special filters in their digestive system to help excrete the excess sugar
water, called honeydew. This sticky honeydew coats the plant's leaves, other
plants, picnic tables, cars or anything else below where the aphids are feeding.
Sooty molds may develop on the honeydew, covering the plant's leaves with
unattractive black growth, and reducing the photosynthetic ability of the
plant.
Some species of ants are attracted to and feed on
the honeydew. Ants will protect the aphids from natural enemies and will actually carry them to new plants when the food source is
depleted. Some ants even go so far as to build small shelters for the aphids or to keeping
root-feeding aphids inside their
own nests. A few species of aphids have become so dependent on their ants that
they won't even excrete honeydew unless stimulated by an ant! However, if aphid numbers
get too high the ants will feed a few aphids to their larvae. And the ants are
better at protecting their aphid herds from some natural enemies (such as
ladybugs) than others (such as lacewings or hover fly larvae). No only do they
fight off or kill the predators, but they also remove the eggs of some.
Of course, aphids are also pests because of their feeding. Although aphids are generally present at non-injurious levels present on most plants, large numbers of aphids can reduce the vigor of plants as a result of excessive sap removal. Chlorotic spots may develop where cell contents have been removed. Bud feeding results in distorted leaves and, at high densities, can cause stunting and/or wilting. Severe damage to the top of the plant may reduce the number of flowers produced. Some species inject chemicals into the plant that change the growth pattern, causing new plant growth to be distorted or stunted. And some species of aphids transmit plant viruses including cauliflower mosaic, cucumber mosaic, lettuce mosaic, plum pox, potato virus Y, tobacco mosaic, and turnip mosaic viruses.
Aphids are relatively common and often occur in large numbers, so there are many animals which use aphids as a food source. Most people think of lady beetles as the aphid predator, but there's actually a long list of natural enemies of aphids. Numerous parasites, predators, and pathogens attack aphids. Some of the most common ones that specialize on aphids include:
Aphid midge
Aphidoletes aphidomyza
(Cecidomyiidae)
The wimpy, fungus-gnat-like adults give rise to bright orange, slug-like larvae that inject a toxin into aphids leg joints to paralyze them, then suck out the aphid body contents through a hole chewed in the thorax.
( Hemerobiidae)These are similar to green lacewings, but tend to be smaller, the adults are brown and the eggs are not stalked. They are frequently found in trees.
(Chrysopidae)Chrysoperla carnea and other species are voracious aphid predators as larvae; only a few species feed as adults.
(Syrphidae)
The adult resemble honey bees or wasps, but it's the slug-like, pale green to yellow maggots that eat aphids.
(Coccinellidae)Numerous species occur throughout the Midwest, feeding as both adults and larvae on aphids. Some common species include Asian multicolored, two spotted, seven spotted and twelve spotted ladybeetles.
The tiny, distinctively patterned black and white adults and the pinkish-yellow to light brown nymphs and adults are very active and will feed on small aphids.
Most of these wasps have similar developmental processes. They are born in the aphid and, as larvae, consume it from the inside. They pupate inside the mummy and later emerge.
Several fungi, including Beauveria bassiana, Paecilomyces fumosoroseus, and Verticillium lecanii, will infect and kill aphids.
There are many other arthropods which will eat aphids if given the chance:
You can monitor for aphids in your yard by visually inspecting your plants. Cast skins, honeydew droplets, and sooty mold are indications of an infestation. Aphid infestations are often localized, and different aphid species prefer different parts of the plant, so you should check the entire plant rather than just the terminals.
But should you do anything about the aphids? That
depends their absolute numbers, whether any natural enemies are present, if
honeydew is causing problems, if the plant's growth is being affected, and your
tolerance for their presence. Natural controls usually bring aphid populations under control shortly after they become
noticeable. If you find numerous lady bird
beetles or lacewing larvae on the plant, you should allow the natural controls to do the job.
Avoid using broad spectrum insecticides that could kill these natural enemies.
If natural control is not working and aphids are overrunning your plants, there are several options:
Some common or interesting aphids found in Wisconsin include:
Myzus persicaeThis is the most common aphid in the home garden, with over 500 host plants. The adults are yellowish-green or reddish, with three dark lines on their back. It is often found on vegetables (especially potatoes), fruit trees (cherries and plums), and ornamentals including carnations, chrysanthemums, English ivy, iris, lilies, poppies, roses, snapdragons, tulips, and violets.
Brevicoryne brassicaeThis is a small gray-green aphid with a powdery, waxy covering, found in dense clusters on the underside of leaves. Broccoli is frequently infested. Other hosts include cabbage, cauliflower, brussels sprouts, kohlrabi, collards, kale, and radish.
Aphis gossypiiA very small dark green aphid with black cornicles. It can be found on all the common home vegetables, especially melons.
Macrosiphum euphorbiaeThis aphid is a vector of virus and mosaic diseases of potato and tomato. It attacks many garden and ornamental plants as well as weeds. Interestingly, this aphid uses the rose canes in the fall and hatch in the spring into pink and green aphids that feed on rose buds and young leaves. In early summer the aphids migrate to other host plants.
Macrosiphum rosaeThis host-limited species is common to all rose gardens. Adults are light green to pink with black appendages.
Eriosoma lanigerumThese aphids feed on the bark and roots of apple, causing a swollen gall to form on the stems where aphids have fed. They also produce waxy filaments on the abdomen, creating a wooly appearance.
This is the largest aphid known, and has a virtually cosmopolitan distribution.
Susan Mahr, University of Wisconsin - Madison
Cabbage and Turnip Aphids from North Carolina State University
Cotton and Cowpea Aphids from North Carolina State University
Green Peach Aphid from North Carolina State University
Honeysuckle Witches' Broom Aphid a Colorado State University Fact Sheet
Honeysuckle Witches' Broom Aphid a University of Minnesota Yard & Garden Brief
Pine Bark Aphid Michigan State University Home Horticulture series
Potato Aphid from North Carolina State University
Rosy Apple Aphid a Michigan State University Fruit IPM Fact Sheet
Spirea Aphid and Apple Aphid from Mid-Atlantic Orchard Monitoring Guide
