OSU Extension offers tips for controlling aphids with less chemicals this spring

As spring arrives and the weather begins to warm up, gardeners need to be on the lookout for aphids – tiny, soft-bodied, plant-sucking insects.

In a February 2003 article, Gail Gredler, a home horticulturist with the Oregon State University (OSU) Extension Service, says aphids especially love tender young plant growth, which is prevalent this time of year.

Aphids are born pregnant and multiply quickly, and most aphids are “naked” – without a hard exoskeleton – but some species have a soft cottony substance over their bodies and can be just about any color. 

She notes, “Some aphids have wings, and some do not. They are about one-tenth of an inch long with long, hypodermic needle-like mouth parts, adapted to pierce and suck out plant juices.”

OSU Extension offers strategies to keep aphid damage at a minimum without resorting to toxic chemicals because there are so many kinds of aphids with varying life cycles. Gredler recommends diverse aphid control strategies, including sticky traps, smart landscaping and natural aphid predators.

Non-toxic strategies

The first line of defense would be to squish a few aphids, releasing a chemical signal to attract natural enemies like lacewings, ladybird beetles and parasitoid wasps. 

OSU Extension suggests following up with a strong spray of water from the hose to wash remaining aphids to the ground, blast the bottom of the leaves as they often congregate there and repeat every few days until numbers go down.

Periodically spraying water can work wonders with aphids on rose shoots and buds, bean plants, young broccoli and cabbage shoots and other tender garden foliage.

“Aphids are poor climbers,” Gredler states. “They are less likely to reestablish because they run a high risk of getting eaten by ground-roving predators, such as spiders and beetles.”

Aphid numbers can explode on nitrogen-rich plants, and it’s a good idea to use organic fertilizer or a slow-release synthetic fertilizer that will provide pests less readily available nitrogen.

“Keep plants healthy and prune off damaged foliage,” Gredler adds. “Plants with adequate supplies of nutrients, water and light can fend off aphids more easily than sickly or stressed plants but avoid over fertilizing.”

Tender new growth attracts aphids, so OSU Extension suggests using slow-release or organic fertilizers to avoid an overdose of nutrients to plants.

Most aphids secrete honeydew – a sweet, sticky substance which is a food source for ants, bees and flies – and some forms of aphids spread plant viruses from dripping honeydew, leading to sooty mold growth on many plants.

Gredler recommends gardeners utilize a smart landscape design to deter aphids from the garden and encourage predatory insects to eat aphids.

“Do not have aphid-attracting plants where aphids or their honeydew will do harm. For example, birches are notorious aphid-attractors,” she points out. “Don’t plant birches near driveways or decks, or vehicles and decks will be sticky with honeydew.”

Additional methods

Gredler mentions using sticky, yellow aphid traps, which are sold in garden stores and trap flying aphids in a non-toxic, sticky substance, and to quarantine aphid-infested plants if possible.

OSU Extension recommends introducing or encouraging natural aphid predators and to avoid the use of broad-spectrum pesticides which kill aphid predators such as ladybugs and green lacewings. 

“Do not purchase adult ladybird beetles, as they tend to disperse on release. A better predator to purchase may be the green lacewing, available for sale as eggs or larvae,” she says.

According to Gredler, the best strategy is to grow plants which attract and foster natural predators.

These include yarrow, wild buckwheat, white sweet clover, tansy, sweet fennel, sweet alyssum, spearmint, Queen Anneʼs lace, hairy vetch, flowering buckwheat, crimson clover, cowpeas, common knotweed and caraway.

If all else fails

If the above strategies don’t seem to do the trick, Gredler recommends trying the least-toxic method of chemical control of aphids – commercial insecticidal soaps. 

These soaps are available at most lawn and garden stores and eliminate only the insects that come in direct contact with the soap.

“This means you have to spray the soap solution directly on aphids to eliminate them,” says Gredler. “Make sure to check the underside of leaves and other hard-to-see areas for aphids. And remember, the soap spray is only effective as an insecticide until it dries. For plants in the sun, test an inconspicuous part of the plant first to see whether it will cause leaf burning. Always follow label instructions.”

Utilizing insecticidal soap on aphids allows predator insects with harder bodies to survive and naturally control aphids. 

Gredler adds commercial formulations of these types of soap have been extensively tested on plants, so they are safer than homemade solutions.

Melissa Anderson is the editor of the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net.