Aphids can be a scourge in the garden. There are over 4,000 species of aphids in the world, but fortunately, only(!) about 250 species are attracted to the plants we grow in our gardens and landscapes.
Aphids are small, slow-moving insects that come in a variety of colors—green, yellow, brown, red, or black. They sometimes match the color of the plant they feed on, but not always. They have long legs and antennae and usually have a pair of cornicles (tube-like structures) sticking out of their hind end. No other insects have cornicles. Some adult aphids have wings—this can occur when populations are high or during spring and fall—that can allow them to disperse to other plants.
Aphid reproduction is interesting. Adult females can give birth to up to 12 live offspring each day without mating. During warm weather, many aphid species can grow from newborn nymph to reproductive adult in a week. No wonder it seems like they appear overnight. Watch this Deep Look video from PBS: https://www.youtube.com/watch?v=vrzalLssomg
Aphids are attracted to plants on which they can feed, particularly those experiencing active growth. During this early Spring season, they primarily target plants in our winter gardens, such as kale and broccoli, as well as ornamental landscape plants such as roses. Additionally, aphids are also attracted to the winter and spring weeds growing abundantly in many neighborhoods.
Aphids have sucking mouthparts, somewhat like little straws. They use them to suck sap (which is a plant's sugar source). What they excrete is commonly referred to as honeydew. Ants are attracted to this honeydew and will actively “farm” the aphids and protect them against natural predators.
Most plants, especially trees and shrubs, can survive just fine with low to moderate numbers of sap-feeding aphids. However, when aphid populations grow too large, they can cause noticeable damage, including yellowing leaves and stunted growth.
Aphids can also transmit viruses from plant to plant on many vegetable crops and ornamental plants. Symptoms can include mottled, yellow, or curled leaves, along with stunted growth.
Controlling aphids
Understanding their life cycle is key to their management. By checking plants twice a week while they are growing rapidly, you can catch infestations early. With small populations of wingless aphids, hosing them off with a strong stream of water works well. Once off the plants, aphids are unlikely to be able to return. You can also prune out the most infested parts. Make sure you also remove nearby weeds that can serve as a reservoir for new infestations.
Natural enemies are important for aphid control. Parasitic wasps lay eggs inside live aphids, causing them to turn into mummies. You can recognize these by light-colored aphid bodies with circular holes on top where the wasp has emerged. Lady beetles (especially the larvae), syrphid fly larvae, and green lacewings are voracious eaters of aphids. Explore the website! to discover more about natural enemies of pests.
If spraying with water is not effective enough, insecticidal soaps and oils are the best choices for most situations. These include petroleum-based horticultural oils or plant-derived oils such as neem or canola oil. These products kill primarily by smothering the aphids, so it is crucial to thoroughly coat the infested foliage. Soaps and oils kill only those aphids present on the day they are sprayed, so applications may need to be repeated. Although these products can kill some natural enemies that are present on the plant and hit by the spray, they don't leave a toxic residue.
The key to effective control is tackling aphids early—before their populations explode. Some species even cause leaves to curl around them, creating a natural shield that makes sprays and predators less effective. Acting fast can save your plants from serious damage!
Prevention
- When you purchase new plants, inspect them carefully.
- Don't over-fertilize. Excess fertilizers, especially too much nitrogen, can stimulate plants to put out lush new growth that will entice aphids to set up home.
- Excessive pruning can also stimulate plants to put out attractive new growth.
- Use a row cover, especially on seedlings and new plants. This will keep out aphids but also protect them from other pests.
- Control ants that can protect aphids.
For more information, check these websites:
https://ipm.ucanr.edu/PMG/PESTNOTES/pn7404.html#TABLE1
https://ipm.ucanr.edu/QT/aphidscard.html