Aphids can cause serious problems in vegetable and ornamental greenhouses. Most common are the green peach aphid, the cotton or melon aphid, the potato aphid and the foxglove aphid. Proper identification is critical to the effective use of biological controls to manage aphids, especially in organic greenhouses where insecticide options are limited. The following information and photos come from an Ontario Ministry of Agriculture fact sheet by Gillian Ferguson, Graeme Murphy and Les Shipp.
Cornicles can be found on all aphids. They are a pair of tube-like structures projecting from their posteriors. Sometimes called honey tubes, for obvious reasons, cornicles can be used to identify different species, if you have a 10x hand lens.
On green peach aphid, cornicles are the same color as the body, slightly flared and darker at the tip. In melon aphid, cornicles are shorter and dark throughout their length, regardless of body color. In potato aphid, cornicles are long and thin and may be curved outward at the tips; they are the same color as the body. The foxglove aphid has cornicles of medium length, darkened at the tips.
Body color alone is not an accurate way to identify aphids, though it may be useful with other features. The green peach aphid is commonly light green-yellow, but can be darker green or sometimes pink/rose. Melon aphids are smaller, often dark green, almost black, but can be green, yellow or mottled. The potato aphid is a large, active aphid, usually green, but variable in color, often with a darker stripe down its back. The foxglove aphid is green, often quite shiny, with two darker patches on its abdomen at the base of the cornicles. The less common chrysanthemum aphid is shiny and dark brown.
The life cycle of aphids outdoors is quite complicated, at times involving sexual reproduction and egg laying. In the greenhouse, however, the life cycle is usually simple. All individuals are female. They give birth to live young, which in turn can reproduce within seven to 10 days. Individual aphids can give birth to 60 to 100 young, depending on host plants and nutritional status over a 20-day period. Obviously, aphids can rapidly build up large populations. Aphids are usually wingless, although adults may develop wings if the population density is high, allowing rapid dispersal.
Plant damage by aphids results from them piercing plant tissue with their mouthparts and sucking out the sap, causing deformed leaves and flowers. They also excrete a sugary, sticky substance called honeydew, which promotes the development of black, sooty mold fungus on the leaf surface. Sooty mold is not pathogenic on the plant, but in severe infestations it can interfere with photosynthesis. Aphids can transmit plant viruses (like cucumber mosaic virus). In ornamentals, the presence of the aphids themselves, their cast-off skins, honeydew and sooty mold reduceplant marketability.
Monitoring is essential to assure early detection of aphids and timely implementation of management strategies. Monitoring involves two different strategies: visual observation of the crops for wingless aphids, and yellow sticky cards for those with wings.
Inspect your greenhouse crops on a regular schedule for wingless aphids, and the small white flakes of cast-off skins produced as they molt. Initial infestations are usually isolated within the greenhouse, but can rapidly spread. Heavier infestations can result in the presence of honeydew on leaves, making them shiny and sticky. Ants are often attracted to honeydew, so their presence suggests an infestation. Growing some varieties or crops known to be attractive to aphids can help with early detection.
Place yellow sticky cards throughout the greenhouse to monitor for winged aphids. They can move into the greenhouse from outside, typically in the spring and fall as migrating aphids begin flying. If winged individuals are produced within the greenhouse, there is an advanced infestation where adults are dispersing to find new host plants. Effective crop monitoring should identify infestations before they reach this stage.
Biological controls are readily available for aphids. These include the parasitic wasps Aphidius (various species) and Aphelinus abdominalis; the predatory midge Aphidoletes aphidimyza; and lady beetles (Hippodamia convergens, Harmonia axyridis). Lace-wings are more generalist predators available for aphid control. Aphidoletes and lady beetles are usually used to supplement the activity of Aphidius and for reducing aphid populations in “hot spots.”
Aphidius species do not enter diapause and are usually more effective during winter, early spring and fall. During summer, other parasitic wasp species can parasitize Aphidius, reducing their impact on aphid populations. Optimum conditions for Aphidius are 65 to 77 degrees Fahrenheit and 80 percent relative humidity (RH). Aphidius completes its development from egg to adult in about 10 days at 77 degrees, and 14 days at 70 degrees.
Three species of Aphidius are commercially available. One is Aphidius matricariae, which can parasitize about 40 aphid species, including the green peach aphid. Aphidius matricariae has been largely replaced by Aphidius colemani, which is effective against both the green peach aphid and cotton aphid. Aphidius ervi is a larger species used against potato and foxglove aphids. Aphidius wasps lay their eggs inside the aphid. As the wasp develops, the aphid changes color and appearance, becoming swollen with a bronze color and a papery texture. This parasitized aphid is known as a mummy. The new adult wasp emerges from the mummy.
Aphidius species are best used when aphid numbers are very low. To facilitate continuous release of low numbers of these species, many growers use “banker plants” that essentially consist of seedlings of a cereal species like rye. These seedlings are host to cereal aphid species that do not attack non-cereal crops, and the cereal aphids in turn are hosts or food for the parasitic wasps. Research indicates that for optimum results, evenly distribute banker plants throughout the greenhouse, with a distance of not greater than 130 feet between each banker plant.
Aphelinus abdominalis primarily attacks potato and foxglove aphids. This wasp prefers to parasitize the second and third nymphal stages, while the first and small second nymphal stages are used for host feeding (i.e. as food by adults). To feed on an aphid, the wasp first pierces the aphid with its stinger or egg-laying body part, and then feeds on the aphid’s body fluid through the opening.
In contrast to A. colemani, egg-laying activity is low during the first few days of this wasp’s life. By the fourth day after emergence, an adult female can lay 10 to 15 eggs per day for the rest of its life of 15 to 27 days. During this time, an adult female may parasitize more than 200 aphids and kill about 40 by host feeding. Because adults prefer to walk or hop rather than fly over the crop, they tend to remain localized. Studies have shown that dispersal by this wasp is poor in the greenhouse, and that most remain close to their points of release. This means these wasps should be released as close as possible to aphid infestations for best results. Note that aphids parasitized by A. abdominalis appear black, while those parasitized by Aphidius species are bronze.
Aphidoletes aphidimyza adults resemble small mosquitoes whose larvae are at the predatory stage. Females lay eggs close to aphid colonies so that upon hatching, the orange-colored larvae have a readily available food source. Eggs usually hatch after two to three days; the larval stage lasts five to seven days, after which they drop to the floor to pupate. The pupal stage usually lasts about eight to 10 days. Adult A. aphidimyza feed on honeydew and are non-predatory. The larvae can kill between 10 to 100 aphids in total.
A positive characteristic of Aphidoletes is that, unlike parasitoids, it causes little disturbance in aphid colonies. Because of its furtive behavior, it triggers little defensive reaction by aphids. This means aphids attacked by Aphidoletes are less likely to disperse, escape predation, and start new colonies. When aphids are attacked by parasitoids, they defend themselves by kicking and producing alarm pheromones, resulting in their own escape, as well as many other members of their colony.
Under natural day lengths, A. aphidimyza enters reproductive diapause between September and March, because the larvae require at least 15.5 hours of light to prevent the pupae from diapausing. However, there is some evidence to suggest that regular preventative releases of Aphidoletes can be made throughout the winter to control aphids. The Aphidoletes adults lay eggs and the larvae feed on aphids; however, there is no second generation of midges produced. If lighting can be supplemented, even low-light intensities, such as from incandescent bulbs, are sufficient to prevent diapause.
Adults are nocturnal and require a period of darkness for mating and egg laying. Therefore, continuous lighting from a bright source will prevent reproduction. Likewise, lighting that eliminates dusk can also interrupt mating. It is also important to note that larvae drop to the ground and use grains of sand and possibly soil debris to form cocoons. If the larvae fall on plastic or concrete that is dry and free of debris, mortality of this predator will be high. Repeated or continuous release using banker plants is necessary under such situations to achieve acceptable suppression of aphids.
Lady beetles are also used for control of aphids. Harmonia axyridis, also known as the multicolored Asian lady beetle, is an introduced species that can provide excellent control. However, it has developed a bad reputation because it has developed huge populations in the outdoor environment, become a pest on some crops such as grapes, and it has displaced native ladybird beetle populations. Some biocontrol producers have stopped supplying this species. The second species, Hippodamia convergens, is a native North American species collected in the wild in California.
Both adult and larval lady beetles feed on aphids. When day lengths are suitable, lady beetles must feed on aphids to maintain egg production. Eggs are torpedo-shaped, orange-colored, laid in circular clusters on the underside of leaves and hatch in two to five days. The larval stage lasts for about three weeks after which they pupate. Adults emerge from pupal cases after three to five days. To increase the percentage of lady beetles remaining in the greenhouse, make releases late in the evening and sprinkle a sweet liquid (diluted soda) over the beetles. This provides an immediate energy and water source.
Lacewings also feed on a variety of prey including aphids, thrips, spider mites, young caterpillars and moth eggs, mealybugs, scales and whitefly larvae and pupae. However, they do have a preference for aphids over thrips, and then spider mites. Older larvae (third instar) are particularly voracious and can eat eggs, other larvae and even adults if food is scarce. A larva can consume 300 to 400 aphids and are usually best suited for high aphid populations. The adults feed only on honeydew, nectar and pollen.
Other control strategies include removing weeds from within, and immediately outside, the greenhouse. Aphids can develop on many different species of weeds. Remove plants in isolated areas of infestation that are detected early to prevent aphids spreading to the rest of the greenhouse. Consider the use of insect screening to eliminate the movement of aphids from outside into the greenhouse.
The author is Vegetable and Berry Specialist with University of Vermont Extension based at the Brattleboro office.