Aphids - First Pest of Winter

As winter progresses, the seed catalogs start showing up and excited growers rush their orders to get a jump on the 2017 season. Whether you are starting plants under artificial lights or in a greenhouse there is a real excitement at seeing the first young plants pop out of the soil. Aphids are also excited about these new crops, and they are easily lured to your lush, fresh young plants.

Where do these aphids come from? In a greenhouse, aphids overwinter as adult females. You can usually find them on weeds or in leaf debris under the benches or old flats. If you maintain stock plants for taking cuttings, or have “pet plants” in the greenhouse or in your house, these can be reservoirs for aphids.

If you’re starting transplants in a greenhouse, you need to get your rear in gear and set up a regular monitoring program so insects and disease don’t eat into your profits. Greenhouse and high tunnels are great and really empower a cut flower grower, but with great power come great responsibility (quote from Spider-Man 2, I think). Anyhow, bugs love greenhouse plants as much as you love to grow plants in the greenhouse. You don’t need to live in fear of insects consuming your plants if you conduct regular monitoring, and combine this with good sanitation practices.

Cleanliness is Next to Aphidlessness

Sweeping or vacuuming plant litter and other debris are excellent sanitation practices in the greenhouse. If you have a spun-down polypropylene weed barrier on the floor, a power washer can be used to clean the floor. Afterwards use a hand sprayer to apply a disinfectant such as a quaternary salt (Greenshield) or Clorox (one part Clorox to ten parts water) to benches and growing areas. Notice I said “Clorox”. This is because Clorox is the only sodium hypochlorite that has an EPA label as a disinfectant for use in greenhouses. Store brands do not have this EPA labeling.

Do not leave any weeds on the greenhouse floor because these will serve as reservoirs for insects and mites that will easily migrate onto your cut flower transplants. This is where you get your winter exercise, bending over to pull weeds. You can use glyphosate to kill the weeds if the greenhouse is completely empty and you have the vents closed. Hand-weeding is usually not too much work unless you let weeds get out of hand during the fall and winter.

If hand-weeding is not appealing, chemical options include Diquat (Reward), or short-chained fatty acids (Scythe). Reward and Scythe just burn back the tops of the weeds and are not as systemic as glyphosate. A relatively new pre-emergent called Morengo is labeled for use in greenhouses to keep weeds from germinating. It is rather pricey, but it lasts a long time.

Aphids are usually among the first pest samples submitted in the spring by cut flower growers. They’re usually one of the “group of four” that I receive samples of at our CMREC lab. This very trendy and popular group includes green peach aphid, melon aphid, potato aphid, and foxglove aphid. Foxglove aphids appear to be growing in numbers in several operations. All four of these species can reproduce rapidly in a greenhouse and can really cause much frustration and anxiety. The key is early detection and prompt action. In most cases, aphid populations build up as little epicenters in your greenhouse. Find these epicenters and deal with them quickly before they get a chance to spread.

In spring, aphid populations can virtually explode in greenhouses. When greenhouses become packed with tight spacing of plants, as is often the case in spring, populations of aphids can build and spread rapidly. Growers’ attention is often on producing the plants and getting the outside fields ready for spring planting, and aphid buildup may go unnoticed until it’s out of control.

In greenhouses, most aphid species are able to reproduce asexually (parthenogenesis), with females giving birth to approximately 100 live female offspring (viviparous). This high reproductive capacity and short development time means that more frequent applications of insecticides may be required, thus increasing the probability of aphid populations developing resistance.

Now On To the Monitoring Part

Certain plants are highly susceptible to aphids and should be checked frequently in spring, so the problem may be dealt with quickly. Plants that are big draws for aphids include sage (just about any sage is a magnet for aphids), dianthus, coleus, basil, snapdragons, zinnias, and any of the ornamental peppers grown for cut stems.

Early detection is critical to prevent an aphid population from spreading throughout the greenhouse. When using biological control organisms, it is important that the release of natural enemies be initiated early on in the cropping cycle. What is just as important is which plants tend to be susceptible to aphids so that potential problems can be avoided when a crop is moved into a greenhouse.

During the feeding process, aphids extract between 11 to 12% protein, and large quantities of water and sugar from plants. The water and sugar initially ingested is excreted from the aphid’s anal opening. This excrement, which covers leaves, is a sweet, sticky substance called honeydew. Honeydew is an excellent growing medium for black sooty mold fungi, which can cover foliage and flowers. Look for shiny foliage, which is easily spotted, even if your eyes just are not what they used to be when you were a novice in this cut flower business.

Aphids shed their old skin or cuticle when they molt. The old skins or cast skins, which may be present on leaves, appear white. These are easily seen as you move through the greenhouse.

Aphids differ in their distribution on plants, and dispersal capability, which can influence monitoring procedures. Melon aphids tend to be located in the plant interior, aggregated on the stems and flower buds, whereas green peach aphids tend to be less aggregated. Green peach aphids are typically located on the terminal growth of plants. They may blend in with the foliage due to their light green color. The green peach aphid is also more mobile than melon aphid, often dispersing to adjacent plants. The potato aphid congregates in large numbers on plants, and the distinct stripe extending down the middle of body is noticeable.  

Invest in a good 10-20 X hand lens, and use it to examine foliage and stems closely for presence of aphids.

When aphid populations build up to very high levels, females will start to birth out young with wing buds, which will fully develop at the adult stage. The wings can be twice the length of the body of the aphid. The winged forms can disperse in your greenhouse, spreading the populations to new plants. If you reach this stage, you better take action quickly or things will get out of control rapidly.

Yellow sticky cards will capture winged aphids. Although yellow sticky cards may indicate the presence of winged forms of aphids, it is still important to inspect plant foliage at least weekly to detect aphid populations early enough, so that the appropriate controls may be implemented.

Biological Control

Aphidius colemani is a parasitoid we have successfully used in a Maryland grower’s greenhouses. It attacks green peach aphid, but it is more successful in parasitizing the melon aphid (Aphis gossypii). Females of both species lay eggs into aphids, and the eggs hatch into larvae that consume the internal contents of an aphid. As the parasitoid larvae mature, the aphid body expands and turns brown to tan. The emerging adult parasitoid creates a round exit hole in the dorsal (top) side of the aphids’ abdomen. Emerging adults will mate and then females of the next generation will search out and parasitize any aphid’s that are present.

Aphidoletes aphidimyza is a predatory midge that attacks over 50 species of aphids including melon aphid, green peach aphid, and potato aphid. The larval stage feeds on aphids, whereas the adults feed on honeydew produced by aphids. Adult females locate aphids using visual cues and olfaction (odors), and then lay eggs adjacent to aphid colonies. A single larva consumes 10 to 25 aphids during its life.

Insecticide Options

Several new insecticides have come onto the market in the last ten years that provide good levels of aphid control. The newer products tend to be safer materials used at very low rates. The somewhat bad news is that these new products cost a lot more than some of the older pesticides.

Some of the materials we tested at the University of Maryland Extension with good success in aphid control include Aria (flonicamid) and Endeavor (pymetrozine). Both materials block the stylet of the aphid, resulting in starvation of the insect. In our trials it took 5 to 10 days before you really see the impact of the application but it is very effective in controlling aphids. It is very difficult for insects to develop resistance to a material that does something physical like blocking the stylet.

Another material we trialed in 2009 is Kontos (spirotetramat). Kontos is a systemic insecticide, very effective in controlling green peach and melon aphid for 20 to 30 days. It is also good on spider mites. In 2016 we tested Mainspring and Acelepryn for aphid control. Both of these products are true systemics and sold by Syngenta Company. They both worked very well for aphid control.

To help determine insecticide efficacy, several aphid-infested plants can be marked with flags or flagging tape, and an estimate of the aphid number on each may be recorded. Several days after an insecticide application, the number of live aphids may be recorded. It is important to examine plants carefully and frequently to determine whether additional applications are required.

In 2016, several researchers including Karen Rane (pathologist), Andrew Ristvey (horticulturist), Joyce Latimer (horticulturist) and myself (Stanton) collaborated to revise an extension publication for greenhouse production: “Total Crop Management of Greenhouse Production”. It is $30 a copy and you can obtain a copy by sending an email to my technician Suzanne Klick at [email protected].

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]