Aphids hiding between daylily leaves
Photo: Dr. Steven D. Frank

Aphids are common pests of perennials grown indoors and outdoors. The most common species are generalists such as green peach aphids, cotton aphids, potato aphids, and foxglove aphids. These species may feed on dozens or hundreds of plant species. Other aphid species are specialists and may only be a concern on particular crops such as chrysanthemum aphids on chrysanthemums, rose aphids on roses, or oleander aphids on milkweed and oleander. Perhaps the most sinister are root aphids that feed on many plant species but do it underground where you can’t see them.

There are hundreds of aphid species, but for management purposes, you primarily have to identify them as aphids. Aphids are soft and round with long, thin legs. A uniquely aphid characteristic is cornicles, which are two narrow projections from their abdomen that look like tail pipes. Only aphids have these and they are easily visible on most species.

Aphids pierce plants with needle-like mouth-parts and suck out phloem. Aphids damage plants in four primary ways. First, by removing phloem — the sugary solution produced by photosynthesis — aphids reduce the amount of energy available to plants reducing plant growth. Second, aphids often feed on newly expanding leaves, apical buds, and other areas of active growth. This damages the tissue so leaves become deformed or meristems may die, creating an ugly plant. Third, by piercing the plant, aphids can transmit diseases such as cucumber mosaic virus that can affect many ornamental crops. Finally, since aphids feed on sugary phloem day and night they excrete a lot of sugary waste called honeydew. This sticky solution is noticeable on its own but worse, it is a favorite substrate for black sooty mold. This black mold is not a plant pathogen but is ugly and reduces growth and sales.

A green peach aphid gives birth to her clonal daughter. Notice the cornicles on her abdomen.
Photo: Matt Bertone, NCSU

Monitoring

Aphids can live in greenhouses year-round. Outdoors, they often overwinter as eggs on plant stems or buds that hatch in spring. So as long as plants are growing, aphid populations can be growing. Aphids can grow wings when plant quality deteriorates or when they are too crowded with other aphids. This is an escape mechanism but most aphids are wingless most of the time.

Therefore, yellow sticky cards are not a useful way to monitor aphid populations. You actually have to look at the plants. Since many aphid species congregate on new growing tips, you can scan the crop for aphids this way. You can also scan your crop for shiny or sticky spots of honeydew on leaves or black sooty mold. Finally, look for ants. If you see a trail of ants into your crop, it is probably because they are collecting honeydew from aphids or other phloem-feeding insects. The ants drink honeydew from the aphids and in return protect them from predators and parasitoids.

Here, rose aphids and honeydew are present on rose buds and leaves.
Photo: Dr. Steven D. Frank

Prevention

Aphids are everywhere. If you grow perennials outdoors they are everywhere in the environment. If you grow indoors, there are probably a few lurking around most of the time.

To prevent aphid outbreaks, sanitation is important. The most common and generalist aphid species feed on hundreds of plant species. So, if you have weeds under greenhouse benches or within your field production, chances are aphids can live and reproduce on them (lots of other pests can, too).

Aphids also love nitrogen. Fertilizer makes plants grow but also makes plants more nutritious for aphids so they reproduce faster. Of course, fertilizer is necessary for most crops but don’t add more than is recommended or you are just feeding pests.

Ants tend aphids on weeds growing in the corner of a greenhouse.
Photo: Dr. Steven D. Frank

Treatment

Aphid populations can quickly get out of hand, but luckily they can be easier than other pests to kill. Since they are soft-bodied and slow-moving, they are very susceptible to insecticidal soaps and oils. This can often be your first line of attack to reduce aphids and protect beneficial organisms and workers.

Aphids are also heavily attacked by many biological control organisms that can be released into your greenhouse or that find them naturally in outdoor crops. These include predators like lady beetles, green lacewings, hoverfly larvae but especially parasitoid wasps that lay eggs inside of the aphids. Look for parasitized aphids — called mummies — when you are scouting. They are usually bloated and light brown. If you see mummies, you know parasitoids have found the colony and are helping out with control.

There are also many insecticides that will kill aphids, but keep in mind when dealing with flowering perennials that pollinators and other beneficials are often present.

Steven is associate professor and extension specialist at North Carolina State University. He conducts extension and research related to greenhouse, nursery and landscape pests.