Plant Health: Aphids

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This common pest has several control options, including natural enemies.

Oleander aphid is sometimes called the milkweed aphid. Photo: Suzanne Wainwright-Evans
With the flush of new growth in the spring, many insects arrive. One of the more obvious arrivals is the aphid—in high numbers and assorted colors. In northern regions they are found mostly outdoors in the spring and summer, but in southern regions they may be present throughout the year.

Aphids are small (1-10 mm) and pear shaped. Colors vary by species, but the more common colors are green, red, yellow or black. They can be smooth or some of the more ornate, “woolly” aphids may have white fluff similar to mealybugs. To distinguish the aphid from another pest like a scale, look for cornicles—two “tail pipes” that extend from the end of its body.

Sometimes aphids have wings, which can throw off a positive identification. This happens when populations get too large for the plant to support the aphid colony. To alleviate this overcrowding, some aphids will develop wings, helping them to fly to new food sources and, unfortunately, also potentially develop new colonies.

Newly born aphids will feed and molt four times before becoming an adult. When they molt, they leave behind their old skins on the surface of the plant, which look like white flakes. This can easily be mistaken for whiteflies or other pests, so use a 10x hand lens to confirm identification.

Aphids do no cause chewing damage like some other insects, such as caterpillars. They have mouth parts that are somewhat like a turkey baster, piercing the plant material then sucking out phloem sap. If there are just a few aphids in a crop, they usually do not do too much damage. But as a population grows, they can cause leaf curling, stunted growth and leaf yellowing. This can happen quite quickly, so when aphids are found, they should not be ignored. Almost all aphids in a colony are parthogenetic (can reproduce without mating), and produce live offspring.

If aphid populations are allowed to grow, another symptom can develop. Leaves and stems below the aphid population develop the appearance of being wet, and when touched, very sticky. This is honeydew. As aphids feed, they take up large amounts of sugar, not all of which can be used by the insect, so the excess sugar is excreted as droplets. If the aphids go untreated, this honeydew will build up and an opportunistic mold will begin to grow on these excreted sugars. This sooty mold does not harm the plant, but is an unsightly aesthetic problem. With treatment of the aphids and stopping the source of the sugars, this black mold can simply be washed off the plant, and you do not need a fungicidal treatment.

Aphids come every year, so growers should be ready for them. Because some aphid species can quickly cause distorted leaves when feeding (such as foxglove aphids), a proactive approach may be necessary.

The first step to control is prevention. A good start is inspecting your cuttings and plugs for pests when they arrive. If you are taking your own cuttings, keep the mother stock plants clean. Keeping weeds under control is also an important step to managing many pests, including aphids.

Natural enemies
Aphids have a lot of natural enemies, including ladybugs, lacewings, fly larva, wasps and true bugs. Tiny parasitic wasps in the genus Aphidius are one of the more commonly occurring beneficials for aphid control and are often misidentified. These adult female wasps are not much larger than an aphid. Once it finds its prey, the wasp inserts an egg inside the aphid’s body. The egg hatches and the immature wasp larva feeds on the aphid’s non-essential parts. After about seven days, the aphid begins to swell and turns light brown. This is called an aphid mummy. Approximately four days later, the wasp emerges from inside the mummy, chewing a hole in the back of the aphid to escape. Depending on the species, this cycle takes around two weeks. The adult wasps can live for several weeks and repeat the process.
Aphelinus abdominalis is another parasite that works in a similar way to Aphidius. The dead aphid victim turns black instead of light brown. It also will host feed on the aphids. The female wasp stings and paralyzes an aphid, then stabs holes in the immobilized body.  These holes will leak body fluids that the adult wasp drinks. This wasp, as well as several Aphidius sp., are commercially available. But if you are smart about your pesticide selection, you can conserve the native parasite species that are often naturally found in many nursery operations.

One naturally occurring beneficial I see all the time in nurseries is the hoverfly. The adults can mimic bees, but it is the larva that does the work for you. Often miss-identified as small caterpillars, these tiny predatory fly larvae have an amazing appetite for aphids. So when scouting, keep an eye out for the free beneficials that are working for you.

If you don’t have these beneficials naturally occurring in your nursery, they are commercially available from several different biological supply companies like Applied Bionomics, BioBest Biological Systems, Koppert Biological Systems or Syngenta BioLine.

Other products
If biologicals are not right for you, there are several pesticides labeled for aphids. Aphids do not have hard bodies like many other insects so they are easy to kill. Softer products like insecticidal soap and oils kill aphids on contact, but you must get complete coverage. Systemic pesticides like the neonicotinoids can be another option to provide a longer period of control, but can impact your populations of beneficials. There are several pyrethroids that are labeled for aphids. But be aware that if you are planning any kind of biological control program, once sprayed, these pyrethroids can impact some beneficial species for up to 60 days.

Some pesticides are softer on the beneficial and can be used to knock down aphid populations prior to the release of beneficials. Look for products that contain active ingredients like Beauveria bassiana or azadirachtin. Also products such as Endeavor (pymetrozine), which only kills aphids and whiteflies, can help preserve many of your beneficials.

Aphids are a very common problem. They can be easy to control, but before you treat, make sure Mother Nature has not already taken care of the problem for you. Look closely to make sure that what you are looking at are not the left behind skins or the dead bodies of aphids. Also check to see if naturally occurring biological control agents have moved in, like ladybird beetle larvae, lacewing larvae or parasitic wasps. If spraying is necessary, remember soaps and oils work great on aphids and have a minimal impact on your native beneficials but you must get good spray coverage.