Plant Health: Aphids in Greenhouses

Pansy and green peach aphid

That famous philosopher Yogi Berri was not only perceptive when it came to the foundations of scientific research, but he apparently also understood how observations benefit from field experience. Producing and managing crops in the greenhouse is highly intensive and depends not only on the knowledge of the grower, but also requires a certain watchful persistence and day-to-day familiarity with the plants themselves. This is most critical in the management of plant pests and diseases.

Aphids are among the greenhouse pests that count on only a minimal level of neglect to succeed at what they do, namely to reproduce and spread. Our greenhouses provide them with an almost ideal environment of well-fed plants at comfortable temperatures, free of their natural enemies. Often hidden or well camouflaged by their green or dark colors, blending in well with the background or settled under leaves and flowers, aphids can be hard to detect until populations are already too high. It doesn’t take many to start an infestation either: aphids don’t need to mate to reproduce, but are born pregnant as one entomologist has put it, so a population can start with just a single individual.

Helpful Websites BioBest www.biobest.be/home/3 Koppert www.koppert.com Rincon Vitova www.rinconvitova.com Syngenta Bioline www.syngenta.com/global/Bioline/en/Pages/home.aspx Foxglove aphid and spring floral crops www.greenhousecanada.com/content/view/2696/38/ Greenhouse Aphid Management www.greenhouse.cornell.edu/pests/pdfs/insects/Aphids.pdf

Our objection to aphids on ornamental plants is mostly on aesthetic grounds: their presence, their residues or the sooty mold that develops, spoils the ornamental value of the plants and the insects themselves are seen as contaminants. Some aphids distort or stunt growth or cause leaf spotting. Very heavily infested plants can even be killed. In propagation situations they’re insecta non grata as some can transmit viruses affecting how the final plants grow or look.

According to one source, there are more than 4,400 different kinds of aphids known, but fortunately greenhouse growers will encounter a mere handful of these. Only a half-dozen or so keep popping up as the most troublesome because of their wide host range, adaptability to greenhouse production, and the difficulty growers sometimes have in controlling them. Some strains or biotypes of aphids have the unfortunate ability to detoxify certain insecticides, melon and green peach aphids being the most notorious in this regard. The good news is that we now have effective tools for controlling aphids, including biological control. European and some American growers have taken this further, using so-called “banker plants” that host grain aphids (which don’t go to other crop plants) as a way of feeding or sustaining the natural enemies when the pest aphids are no longer present or at very low levels.

Following are some of the more common aphids seen in greenhouses. Particularly for those interested in using biological control, it is important to identify which species is present, as the biocontrols can be somewhat particular. Learn more about each kind of aphid by searching the Internet using its Latin name.

Green peach aphid (GPA), Myzus persicae: It’s usually pale green or pinkish, distinguished by the indentation on the head between the antennae seen under magnification, and enjoys many greenhouse crops. It is the most common aphid we see in the diagnostic lab.

Melon aphid (MA), Aphis gossypii: The black aphid many complain about, it is usually very dark green, but often with a waxy bloom or with pale yellow individuals mixed in. The ‘tailpipes’ (cornicles) are entirely black. It also has a wide host range and, until recently, it was the second most common species encountered. It has been among the more difficult ones to control with insecticides, though newer products are working well against it.

Foxglove aphid, Acyrthosiphon solani: Larger than either GPS or MA, this pale green species is somewhat larger and thrives under cooler temperatures. It can also be found on many kinds of plants. Easily recognized by the darker green spots that surround each cornicle on the back, infestations are often associated with distorted growth or yellow spotting on some plants. It has become more common in greenhouses in recent years and we have even seen damage on zonal geranium. An excellent article on this up-and-coming species can be found at the end of the side bar.

Other less common aphid species include rice root aphid on foliage and other plants usually seen around the base of stems or under lower leaves, cabbage aphid on foliage of flowering cabbage and kale, tobacco aphid and potato aphid on foliage and flowers of Calibrachoa, and crescentmarked lily aphid on lower stems and foliage of lilies.

ABOVE: Tobacco aphid. BOTTOM: Melon aphid and mummy nice.

Among the most effective insecticide controls for aphids include the neonicotinoids Flagship, Marathon (and generics), Safari and TriStar; also the products Aria, Endeavor and Kontos. Pyrethroids include Talstar (and generic bifenthrin products), Decathlon, Tame, Mavrik, Scimitar and Astro; insect growth regulators Distance and Enstar; and other labeled products Avid (and generic abamectin), Acephate, DuraGuard, paraffinic horticultural and other oils (SuffOil-X, Ultra-Pure Oil, Triact), and M-Pede. Read labels for restrictions, sensitive plants and application information.

Biocontrol can be an effective way to control aphids, but it will be important to match the biocontrols to the present or expected aphid species. For example, the parasitoid Aphidius matricariae is particularly good for GPA and tobacco aphid and species other than MA, while A. colemani can be used especially for MA, but also for GPA and other small species. A. ervi and Aphelinus abdominalis are other aphid parasitoids available. These are more appropriate for the larger species like foxglove and potato aphids, and the predatory midge Aphidoletes aphidimyza can be used for a variety of species though may work best in combination with Aphidius especially for melon aphid. If using insecticides and fungicides in the same crop where biocontrols are used, check for possible impacts on the natural enemies – some products are incompatible with natural enemies, others have only minimal or transient impacts. More information on this and on biocontrols can be found in the side effects sections of the Biobest and Koppert websites below. Be sure to note favorable conditions (daylength, temperature, humidity) for the biocontrols and plan to release them early before the infestation is noticeable. Look for the sesame seed-like mummies of Aphidius or the black mummies of Aphelinus as evidence the parasitoids are active in the crop.

Have a Question? You can contact Dan at dog1@cornell.edu.