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Extension > Garden > Insects > Aphids on Minnesota trees and shrubs

Aphids on Minnesota trees and shrubs

John E. Lloyd, Plant Health Doctors, LLC
Jeffrey Hahn, Professor and Asst. Extension Entomologist

Aphids are one of the most common insects found on trees and shrubs. There are over 300 species of aphids in Minnesota. Fortunately, in most cases they cause little or no damage to the health of landscape plants.

Identification

Aphids are small, 2-4 mm (1/16 – 1/8 inch) long, pear-shaped, soft-bodied insects. Adult aphids look like young aphids (nymphs) except that they are larger. Fully mature aphids can be wingless (apterous) or can have wings (alate) (Fig. 1). They can range in color including green, black, red, yellow, brown, or gray. All aphids, regardless of age, have two protruding tailpipes at the end of their abdomen called cornicles (Fig. 2).

aphid nymphs

Photo: University of Massachusetts Extension

Figure 1. Aphids go through simple metamorphosis. Nymphs look similar to adults.

aphid cornicles

Photo: David Cappaert, Michigan State University, Bugwood.org

Figure 2. All aphids have characteristic tail pipes called cornicles. They release an alarm pheromone to warn other aphids when predators attack or when the plant is disturbed.

Common tree and shrub aphids in Minnesota

Essentially all trees and shrubs have aphids that infest them. The most commonly noticed aphids found 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) (Figs. 3, 4, 5, 6).
Aphids on most landscape plants are host specific, meaning they will feed on a particular tree or shrub and are not interested in others types of plants. So for example, aphids infesting a quaking aspen would not be expected to colonize an oak tree.

rose aphid

Photo: Jeffrey Hahn, University of Minnesota Extension

Figure 3. Rose aphid (Macrosiphum rosae)

spirea aphid

Photo: Daily Pics, Web Community (http://dailypics.ning.com)

Figure 4. Spirea aphid (Aphis spiraecola)

oak aphids

Photo: Jeffrey Hahn, University of Minnesota Extension

Figure 5. Oak aphids (Myzocallis punctata)

white pine aphids

Photo: Lacy L. Hyche, Auburn University, Bugwood.org

Figure 6. White pine aphids (Cinara strobe)

Biology

Rose aphid

Photo: Shipher Wu, National Taiwan University

Figure 7. Female aphids give live birth to their young. They are clones of the mother since they are not fertilized by a male.

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 require a male to reproduce, i.e. they reproduce asexually. Females give birth to live young instead of laying eggs for most of the summer (Fig. 7). Because of their rapid development time, asexual reproduction, and extended reproductive lifespan, the rose aphid is able to complete up to 15 generations during the growing season!
However, in the fall of the year, male nymphs are produced that mature and mate with females who then lay eggs that overwinter on the infested plant. In the spring, the eggs hatch and new female aphids begin feeding and reproducing.
Winged forms of aphids develop when populations increase and plants become crowded. These migratory forms are better adapted to seek new food sources. These insects are poor fliers, however, and will typically only move within the canopy or to adjacent trees. On the other hand, they are so small that if they are picked up by wind, they can move for many miles.

Damage

Aphid damage to host trees and shrubs is usually minimal, especially when the plants are healthy and mature. Aphids will cluster where they can feed on succulent growth. This includes unopened flower buds, the underside of young leaves, and on developing stems. Feeding by aphids can sometimes discolor leaves, distort or curl foliage, and form galls. When large populations feed for an extended period of time, aphids can cause wilted leaves, stunted shoots or shoot dieback.

As aphids feed, they inject saliva into their host plant which helps digest the sap. The pre-digested sap is sucked up by fine needle-like mouthparts of the aphid. A large portion of this undigested material is excreted through the anus of the aphid, a waste product called honeydew. Honeydew is often described as a "clear, sticky liquid that rains down from trees." It is a sugar rich material that attracts ants, yellowjackets (especially during late summer and fall), and other insects that feed on it. Honeydew will coat bark, leaves, and objects beneath the plant, including car windshields and lawn furniture, leaving a sticky mess.

Honeydew can cause leaves to stick together and can create a growing environment for sooty mold but is otherwise innocuous. Sooty mold is a saprophytic fungus that feeds on honey dew and is often found on trees infested with aphids. The fungus can cause dark, fuzzy splotches on leaves and branches. When fungal growth is thick, sooty mold can reduce the trees ability to do photosynthesis but is otherwise not harmful to the tree.

Once the honeydew dries on an object it becomes difficult to remove. Tar removers and heavy duty dish washing detergents will remove honeydew, but they may also remove the clear coating from cars. When using these products, follow the labeled directions and test it on a small portion of the car or other object to make sure it doesn’t cause additional damage.

Aphid nymphs

Photo: Jeffrey Hahn, University of Minnesota Extension

Figure 8. Ladybird beetle larvae feed on aphids and other soft bodied insects

Aphid nymphs

Photo: Clemson University - USDA Coop. Ext. Slide Series, Bugwood.org

Figure 9. Maggots of syrphid flies break the legs of aphids and suck their insides out through the broken leg.

Management

Natural

Let nature takes its course. Typically aphid problems are managed by their natural enemies and diseases. Because of their high reproductive rate and their lack of defenses, aphids are used as a food source by many other insects. They are the “cattle” of the insect world for predators, such as lady bird beetle adults and larvae (Fig. 8), lacewing adults and larvae, and syrphid fly (Fig. 9). They are also attacked by parasitic wasps (Fig. 10). When aphid populations grow quickly it may take a while for the predators and diseases to catch up. These natural control agents eventually will reduce aphid numbers to tolerable levels.

The indiscriminant use of pesticides will reduce natural enemy populations and can result in increases in aphid problems after treatment. Avoid making unnecessary pesticide applications!

In addition to reducing pesticide use, researchers at Maryland and Wisconsin have demonstrated that you can help maintain good predator and parasitoid populations by 1) having nectar producing plants as a food source for adult syrphid flies and parasitic wasps 2) providing a water source for the adults and 3) providing shade for the adults to hide from the heat of the summer.

Aphids are not strong insects, and even a good rainstorm can knock them off the foliage and reduce populations significantly. It is not unusual to see higher aphid numbers during seasons that lack heavy rain storms.

Aphid nymphs

Photo: Alex Wild, University of Georgia

Figure 10. This braconid wasp (a type of parasitic insect on aphids) needs nectar and water sources to survive in the landscape.

Cultural

Aphids feed on succulent tissues on trees and shrubs. Using excessive amounts of fertilizers can create lush trees and shrubs that are more attractive and susceptible to aphids. Avoid unnecessary fertilizer applications.

Physical

Physical techniques such as crushing the aphids and using a high pressure water spray from a garden hose are good options for aphid management on smaller plants. The water spray acts much like a heavy rain, knocking the aphids off plants.

Insecticides

Aphids on trees are easily controlled using insecticides.

Low impact sprays: There are several insecticides that are low risk to people, pets, non-target insects, and the environment. Horticultural oils, insecticidal soaps (listed as potassium salts of fatty aphids), and pyrethrum are very effective at controlling aphids and other soft-bodied insects and spider mites. However, they only kill what they contact and do not have any residual activity. Azadirachtin is also low impact product but does have some limited residual activity. Repeat applications may be necessary.

Systemic insecticides: Insecticides with systemic action can be applied to the soil adjacent to the trunk of trees or shrubs or sprayed on the trunks of woody plants. Professional landscape services also have access to insecticides that are injected directly into trees. Systemic products are taken up by plants and transported through the sap to the foliage, stems and branches where the aphids are feeding. This process can take between two to four weeks depending on the product that is used and the size of the tree. When the aphids consume the pesticide they will die. Most of these products will also kill other insects feeding on foliage in trees.

Common examples of systemic insecticides available to residents include imidacloprid and dinotefuran.

Residual insecticide sprays:These products remain on the plant for a period of one week or more (see product label for recommended treatment intervals). Keep in mind that insecticides kill predators and parasitic wasps as well as aphids, and can make aphid problems worse if used carelessly. Avoid spraying insecticides when possible to preserve these natural enemies of aphids, and to maintain a good long-term strategy for aphid management.

Common examples of insecticides available to residents include: acephate, permethrin, bifenthrin, lambda-cyhalothrin, cyfluthrin, 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.

All pesticides in this publication are listed by their common name, not by trade name. Please read the label carefully to be sure you are using the correct pesticide. The University of Minnesota Extension does not recommend or endorse any particular commercial brand of the products listed.

CAUTION: Read all label directions very carefully before purchasing and again before using an insecticide. Information on the label is the final authority.
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