My niece recently sent me a text with a photo of her shrub covered with aphids, asking what to do about them. And I know she’s not alone in battling the tiny pests.
Aphids are pear-shaped insects with relatively small bodies an eighth of an inch or less in length. They are referred to as soft-bodied insects because they do not have a hard exoskeleton like many other insects.
Aphids seldom dine alone, feeding instead in clusters on the soft tissue of young plant stems, leaves and buds. They feed by piercing tender plant parts and sucking out plant juices.
This can cause wilting and dieback of leaves and shoots or some types of aphids also inject toxins as they feed, causing curled leaves, plant galls or swollen growth. Certain aphids also transmit plant virus diseases when they feed.
What can you do when faced with an aphid infestation? Let us consider the different approaches gardeners can take.
Live and let live
You may not need to do anything if the aphids’ natural enemies are keeping their population in check. Become familiar with these beneficial insects and refrain from applying broad-spectrum insecticides that kill both aphids and beneficial insects.
Early in the season, a forceful spray of water to clusters of aphids can knock them off plants and render them incapable of returning. Where aphid infestations are localized, just snip off the few leaves or shoots where they are clustered.
Control weeds in and around gardens because they can be sources of infestations. Inspect garden transplants before planting to make sure they are not already harboring aphids.
Because high nitrogen levels within a plant increases aphid reproduction rates, avoid using excessive amounts of nitrogen fertilizer to encourage lush plant growth.
If you decide to use a pesticide spray for aphid management, select one that is least harmful to potential beneficial insects. Insecticidal soaps, petroleum-based horticultural oils and plant-based oils are the most benign choices available.
Be aware that these insecticides only kill the aphids that they come in contact with.
When using these sprays, thorough coverage to both surfaces of the leaves and stems is critical. Aphids protected by curled leaves will not be affected and repeat applications may be needed.
Note: Some products should not be used when the weather is hot because they can damage plants, so check and follow label directions.
Also, only use soaps manufactured for use on plants. Soap sprays made from “dish soaps,” which are primarily detergents can harm plants.
When large trees are badly infested with aphids, it is difficult to gain control during the growing season with any type of insecticide spray. A better approach with most types of aphids, is to apply dormant oil sprays just as the buds begin to open in the spring to smother overwintering aphid adults and baby aphids hatching from eggs.
For ornamental trees there are some systemic products containing the chemical imidacloprid that can be applied to the soil as a drench at the base of the tree. This is taken up by the roots and moved upward into the branches and leaves.
Current recommendations indicate that drenches of imidacloprid are most effective before aphid populations have burgeoned out of control and should applied be in the fall or early spring when soil moisture is available. It takes time for adequate levels of the chemical to reach the portions of large trees where aphids are feeding.
Do not expect a drench of imidacloprid to quickly control an aphid problem, but once effective it will remain so for a year or more.
For more on aphid control go to: bit.ly/aphidcontrol
For information on beneficial insects go to: http://bit.ly/goodinsects
Marianne C. Ophardt is a retired horticulturist for Washington State University Benton County Extension.