Aphids are one of the most common insects found on trees and shrubs, vegetables, annuals and perennials in home gardens. There are over 300 species of aphids in Minnesota.
- In most cases they cause little or no damage to the health of trees and plants.
- Signs of aphid feeding are: twisted and curled leaves, yellowed foliage, stunted or dead shoots, poor plant growth, and weak plants/trees.
- In most cases natural enemies, like lady beetles and parasitic wasps keep aphid populations in control.
- Treating aphids for the health of plants is usually unnecessary.
- Aphids can be easily controlled with pesticides, but pesticides should be used only as a last option.
How to identify aphids
Aphids are small, 2-4 mm (1/16 – 1/8 inch) long, pear-shaped, soft-bodied insects. They can range in color from green, black, red, yellow, brown, or gray.
Fully mature aphids can be wingless or can have wings.
Adult aphids look like young aphids (nymphs) but are larger.
Winged adults are similar in color, but slightly darker because of their wings.
The best way to identify aphids is to check for the presence of two tail pipes (cornicles) found at the end of their abdomen.
All types of aphids have cornicles, but they might look a little different, based on the species.
Green peach aphids have long cornicles whereas cabbage aphids have short cornicles.
Aphids shed their exoskeletons (cast skins), as they grow. These white cast skins can be found on leaves or stuck in honeydew secretions of the aphid.
Wooly aphids are similar to true aphids, but have white waxy strands covering the pear-shaped bodies. The wax filaments make these aphids look fluffy and cottony, as if they are covered with wool. The wax also keeps predators away from aphids and helps aphids move easily around plant hairs.
Cabbage aphids are grayish-green with a waxy covering that gives them a grayish-white appearance. They have short cornicles and feed on the underside of leaves in large clusters, in the center of cabbage heads, or on the youngest leaves.
Adults are present in both wingless and winged form. Wingless females producing live young (nymphs), are most commonly seen.
Green peach aphids
The green peach aphid is commonly found on cabbage (and 300 species of plants) but does not cause serious damage to cabbage. Green peach aphids are yellowish-green, without a waxy coating, and have long cornicles. They can live through the winter, in the form of eggs.
- Aphid damage is generally minimal, especially when the plants are healthy and mature.
- Aphids gather where they can feed on new succulent growth:
- unopened flower buds
- the underside of young leaves
- developing stems, twigs, bark and roots.
- Signs of aphid feeding:
- twisted and curled leaves
- yellowed foliage
- stunted or dead shoots
- poor plant growth
- weak plants and trees.
- Wooly aphids leave accumulations of wax and shed skins on leaves, twigs and bark.
When aphids feed, they inject saliva into their host plant which helps digest the sap. This sap is sucked up by fine needle-like mouthparts of the aphid. After feeding, they secrete a sticky, shiny waste product called honeydew.
Honeydew is a sugar rich material that attracts ants, yellowjackets (especially during late summer and fall) and other insects that feed on it.
It will coat bark, leaves, and objects beneath the plant, including car windshields and lawn furniture, leaving a sticky mess.
Honeydew generally does not cause any harm, but can cause leaves to stick together and can encourage the growth of sooty mold.
Sooty mold is a fungus that feeds on honey dew and is often found on trees with aphids.
The fungus is not harmful to the tree, but can cause dark, fuzzy splotches on leaves and branches.
When fungal growth is thick, the leaves cannot produce energy (by photosynthesis), and can result in smaller fruit.
Aphids can also carry several different plant viruses including cucumber mosaic virus.
The virus can infect many vegetables including squash, cucumber, pumpkin, melon, beans, spinach, tomato, lettuce and beets as well as annuals and perennials such as impatiens, gladiolus, petunia, phlox and rudbeckia.
Viruses can cause mottling, yellowing, or curling of leaves and stunting of plant growth.
In some cases fruit can be misshapen.
How to protect your garden from aphids
Check your plants for aphids throughout the growing season. Make sure to check both leaves and stems for aphids.
Remove weeds from your garden to reduce potential sites for aphid attack. Weeds such as sowthistle and mustard can support large numbers of aphids.
Aphids can grow faster on excess nitrogen. So, pay attention the to soil fertility level. Apply less soluble forms of nitrogen in small portions, throughout the season, to keep aphid growth under control.
You can physically knocked aphids off of plants with a strong spray of water from a garden hose. This method will also help wash off any honeydew or sooty mold that may be present.
Aphids are not strong insects and even a good rainstorm can knock them off. You may notice more aphids in seasons with lighter rain.
When aphids are found in smaller numbers, they can be crushed to reduce the damage they can cause.
Aphids have many natural enemies that can be commonly found in our gardens. Aphids can multiply very fast and do not have strong defenses. This makes them a good food source for many insects.
It may take some time for the enemies to catch up to fast growing aphids. But, these enemies slowly reduce the number of aphids to tolerable levels.
The best known natural enemy is the lady beetle. Both adults and larvae feed very well on aphids. Other enemies include lacewing larvae, syrphid flies and aphid midges.
Stingless wasps live and grow inside aphids
Many species of tiny stingless wasps can attack aphids. Diaeretiella rapae is a typical wasp that lives and grows inside aphids, especially the cabbage aphid.
It is dark brown and approximately 1/8 inch in length.
The wasp deposits a single egg into each aphid nymph or adult.
The wasp larva matures inside the aphid, slowly killing it.
The aphid eventually turns into an aphid mummy (light brown hardened shell of the host aphid).
The wasp lives through the winter as a fully grown larva in the mummy.
Then it escapes from the aphid mummy by cutting an exit hole in the mummy.
Overuse of pesticides reduces natural enemy populations and can result in increases in aphid problems. Avoid unnecessary pesticide use.
To protect natural enemies:
Have nectar producing plants as a food source for adult syrphid flies and parasitic wasps.
Provide a water source for adult flies and wasps.
Provide shade for adult flies and wasps to hide from the heat of summer.
Only use low impact pesticides that do not affect natural enemies (paper wasps, parasitic flies and wasps) and pollinators such as bees and flies.
Low impact sprays are less harmful
Many pesticides do not hurt/affect people, pets, non-target insects and the environment.
Neem (azadirachtin) is a plant based pesticide that discourages aphid feeding. Insects are not killed quickly, but it makes them stop eating and they slowly die.
Insecticidal soap, horticultural oil and pyrethrum are also very effective at controlling aphids.
Remember to spray the underside of leaves as well as the top.
These materials will only kill aphids that they come in contact with. So, repeat applications may be needed.
Systemic pesticides will kill other insects
Systemic products are taken up by plants and transported through the sap to the leaves, stems and branches where the aphids are feeding. When the aphids consume the pesticide they will die.
This process can take between two to four weeks depending on the product that is used and the size of the tree. Most of these products will also kill other insects feeding on foliage in trees.
- These pesticides should be applied to the soil next to the trunk of trees or shrubs.
- They can also be sprayed on the trunks of woody plants.
- Professional landscape services have access to pesticides that are injected directly into trees.
Residual pesticides last longer and kill other insects too
These products remain on the plant for one week or more (see product label for recommended treatment intervals).
Broad-spectrum pesticides, are generally longer lasting but kill a variety of insects, including natural enemies. This can make aphid problems worse. Avoid spraying pesticides when possible.
Common examples of pesticides available to residents include: acephate, permethrin, bifenthrin, lambda-cyhalothrin, cyfluthrin and malathion.
You can also hire professional landscape companies to treat aphids on trees and shrubs. They have the training and experience to successfully manage an aphid problem.
CAUTION: Read all pesticide label directions very carefully before buying and again before using to ensure proper application. Be sure that the label specifies it can be used on the specific plant you wish to treat. The label is the final authority on how you may legally use any pesticide.
Whenever using any pesticide, including low impact, natural or organic pesticides, follow all label directions and use standard pesticide safety practices for transporting, storing, mixing, applying and disposing of pesticides to protect yourself, your neighbors and the environment.
Systemic pesticides are the most effective on wooly aphids, since they can reach the leaves through the soil.
Similar to other aphids, natural enemies (lacewings, lady beetles, hover flies and parasitic wasps) keep wooly aphid numbers low.
Wooly aphids are not affected by horticultural oils and soaps, like other aphids. This is because the waxy secretions of the wooly aphids and the distorted leaves do not allow the the pesticide to enter the leaves.
Contact pesticides also do not work on wooly aphids for the same reason.
Removing honeydew from plants is not necessary as it does not harm plants.
If honeydew is a major problem, treatment should be be applied after the leaves have started forming (or when honeydew is noticed) to kill the aphids and reduce honeydew problems.
Dish detergents and tar removers can be used to remove honeydew from structures and vehicles.
You will need to scrub with a wash cloth or sponge.
Tar removers and heavy duty dish washing detergents may damage painted surfaces and remove the clear coating from cars.
Test it on a small portion of the surface to make sure it does not cause additional damage.
Common aphids in Minnesota
Pictures and descriptions of common aphids found in Minnesota are given below.
Green peach aphid - The green peach aphid feeds on 100s of different plants including potato, pepper, cabbage, spinach, asparagus, aster, dahlia, iris and verbena.
Potato aphid - The potato aphid commonly infests potato, tomatoes and other solanaceous crops, such as (peppers, eggplant and morning glory).
Melon aphid - The melon aphid feeds on a variety of plants, especially cucurbits, e.g. watermelons and cucumbers, as well as asparagus, pepper, eggplant, aster, hollyhock and lily.
Cabbage aphid - The cabbage aphid feeds only on cruciferous plants, e.g. cabbage and mustard.
Brown ambrosia aphid - The brown ambrosia aphid is common on Rudbeckia, coneflower and sunflower.
Oleander aphid - This aphid feeds on milkweeds throughout the spring and summer, but does not cause any damage to milkweed. It is 1/8 inch long, pear-shaped and bright yellow with black legs. It has long antennae and two black tail pipes at the end of its abdomen.
All trees and shrubs can have aphids on them.
Aphids of landscape plants will feed only on a specific tree or shrub, and will not attack any other type of plant. For example, aphids infesting a quaking aspen would not attack an oak tree.
The most commonly found aphids in Minnesota landscapes occur on: roses (tea, cane, climbing and shrub), spirea, poplar (aspen and white poplar), willows, oaks, maples, white pine, lindens and fruit trees (especially apples and crabapples).
Wooly elm aphid
Wooly elm aphids (Eriosoma americanum) attack American elm and result in curled leaves with white cotton-like masses. These aphids complete a part of their life cycle on serviceberry trees and shrubs.
They do not cause major damage to mature trees and shrubs. But, they are damaging when found in large numbers on serviceberry shrubs younger than three years old.
Wooly alder aphid or Maple blight aphid
Signs of feeding by the wooly alder aphid (Prociphilus tessellates) are: dense, white, wooly masses on the leaves and twigs of silver maple (in early spring) and on alder twigs and branches (in the summer).
- When maple leaves are severely affected, the leaves fold lengthwise and cover the aphids inside.
- The white, waxy threads do not cause any damage, but can be mistaken for a fungus.
In the fall, the aphids produce males and females who mate and lay eggs on the maples. These eggs, and in some cases nymphs (young aphids) can live through the winter on branches of the alder tree.
Leafcurl ash aphid
Leaf curl ash aphid (Prociphilus fraxinifolli) feeds on white, black and green ash species in the spring and causes distortion of terminal leaves.
- This type of aphid is most commonly observed on green ash in urban and residential areas.
- These wooly ash aphids live and reproduce on the foliage of ash.
- Colonies of leafcurl ash aphid last until mid-summer.
- Winged forms of this aphid move to the roots of ash and they remain there for the rest of the year.
Other wooly aphids
Many other wooly aphid species are found on deciduous trees and shrubs in Minnesota. Two wooly apple aphid species (Eriosoma lanigerum and Prociphilus corrugatans) feed on serviceberry leaves and are commonly found in urban landscapes.
- The wooly apple aphid attacks apples, including crabapples, American and red elm, pear, hawthorn and mountain ash.
- Both of these species cause damage similar to that caused by the wooly elm and alder aphids.
More about aphids
The life cycle of aphids is complex and varies with each species. The rose aphid is a good example of a “typical” life cycle.
Female aphids do not need a male to reproduce. For most of the summer, females give birth to live young instead of laying eggs. The rose aphid can complete up to 15 generations per season because of:
- Rapid development time of aphids
- Asexual reproduction by aphids
- A long reproductive lifespan
In the fall, male nymphs are produced that mature and mate with females.
- These females now lay eggs that live through the winter on the plant.
- In the spring, the eggs hatch and new female aphids feed and reproduce.
When plants become crowded due to higher number of aphids, winged aphids are formed. These winged aphids are better at finding food than wingless aphids.
They are not very good at flying, but can travel for miles with the wind (due to their small size).
Wooly aphids usually alternate feeding between two host plants.
- They lay eggs on the primary host plant.
- The eggs live through the winter and female aphids hatch from the eggs during spring.
- On the same plant, females begin producing live offspring, without mating.
- The aphids are clones of the female aphids that laid the eggs.
After one to two generations on the primary host, the new aphids will develop wings when they reach adulthood. Winged female aphids will fly to a secondary host plant and begin feeding and producing additional clones.
- Wooly aphids spend most of the growing season on their secondary host.
- Each female produces hundreds of clonal offspring over several generations.
- The average lifespan of an aphid from birth to adult is approximately one month.
- They reach sexual maturity in four to ten days and then are able to produce their own offspring.
In late summer or early fall, another generation of winged females is produced on the secondary hosts. These females then fly back to the primary host.
- Here, the winged aphids produce clones that are both male and female.
- These aphids mate with each other.
- The eggs from this generation will live through the winter and start the cycle again in the spring.
Most wooly aphids have two hosts, but there are some species that can survive on just one host.
Reviewed in 2018