Biological Control of Aphids
Aphids can be serious and persistent pests in the greenhouse. They are difficult to control due to their high reproductive capability and short development time which can quickly lead to their developing resistance to many different insecticides. The presence of aphids, their white shed skins and honeydew can reduce the aesthetic quality of a wide range of greenhouse crops. Aphids can also vector viruses such as cucumber mosaic virus and many different potyviruses.
Biology and Life Cycle
Most types of aphids found in greenhouses do not mate. All of the aphids present are females which can give birth to live nymphs. There is no egg stage. An adult female may live for up to one month. During this time, she may give birth to 40 to 100 live nymphs. About one week is sufficient for young aphids to reach maturity. Migratory winged aphids may appear when the colony becomes overcrowded or when the food supply is depleted. Outdoors, in the fall, winged aphids which mate and lay eggs appear so that aphids overwinter in the egg stage.
Aphids are small (less than 1/8 of an inch long), soft-bodied, pear-shaped insects with long legs and antennae. In greenhouse ornamentals some of the important species include the foxglove aphid (Aulacorthum solani), green peach aphid (Myzus persicae), the melon or cotton aphid (Aphis gossypii) and the potato aphid (Macrosiphum euphorbiae). See Managing Aphids in the Greenhouse for more information.
Inspect incoming plants for aphids. Avoid high nitrogen fertilization that promotes lush growth that is favorable to aphids. Remove weeds and “pet plants” that can be sources of aphid infestations. See Greenhouse Weed Control for more information.
Aphids are susceptible to many natural enemies, including aphid parasitoids or parasitic wasps, predators and entomopathogenic or insect killing fungi.
Parasitoids (parasitic wasps) develop in a single host and kill the host as they grow and mature. In general, parasitoids are more effective than predators in reducing aphid populations. However, it may be difficult for some parasitoids to search effectively on some crops with hairy or sticky leaves. Aphid parasitoids are host specific. If you are unsure of the species of aphids you may have or have multiple species, mixtures of different aphid parasitoids are commercially available. Parasitoids are shipped as either adults or “aphid mummies” from which the adults emerge.
Aphidius lays its eggs in aphids and the larvae develop within the aphid. The aphid is killed as the developing larvae feed upon it. The swollen exoskeleton of the aphid remains and is referred to as an “aphid mummy.” As the adults emerge from this mummy, one can see the small round exit hole.
Figure 1 : Aphid mummies. Photo by L. Pundt
Figure 2: Aphidius emerging from an aphid mummy on the right. Note the round exit hole.
Young green aphid nymph is on the left. Photo by L. Pundt
Aphidius colemani is a tiny (2 mm.) long wasp that is used against green peach aphids and melon aphids. The adult wasp lays one egg inside an aphid. This egg hatches into a larva that feeds inside the aphid. When mature, a new adult wasp will emerge from the tan aphid mummy. This active searcher is not as effective at temperatures above 86 °F.
Aphidius ervi attacks larger aphids such as the foxglove (Aulacorthum solani) and potato aphids (Macrosiphum euphorbiae). It resembles A. colemani but is about twice as large and darker in color.
Aphidius matricariae attacks green peach aphids, (Myzus persicae) including the closely related tobacco aphid (Myzus persicae subsp. nicotianae).
Aphelinus abdominalis attacks foxglove (Aulacorthum solani) and potato aphids (Macrosiphum euphorbiae). Adults feed on the small aphid nymphs and parasitize the larger aphids. Apelinus is better able to withstand higher temperatures than Aphidius sp. This species works more slowly for a longer period of time than the other parasitic wasps. Look for the elongated black mummies that are less swollen than Aphidius mummies.
Figure 3: Black mummy from which Aphelinus will emerge. Photo by L. Pundt
Tips for Use
- Release preventively
- Remove yellow sticky traps.
- Temperatures should be between 65 and 77° F and relative humidity between 70 and 85%
- Release at the end of the day in shaded locations.
- Look for aphid mummies after about 2 to 3 weeks, depending upon greenhouse temperatures.
- Consult with your supplier on release rates.
Aphid Banker Plants
One way to reduce the costs of biological control is to rear or grow your own natural enemies. This helps to reduce your shipping costs with releases of fresh, newly emerged biological control agents. Some biological control suppliers are selling “Aphid Banker Plants” consisting of wheat or barley plants with bird cherry oat aphids (Rhopalosiphum padi); (these aphid species feed upon cereal grains, and are only used in the banker plant system). The bird cherry oat aphid is a small, brownish to olive green aphid. Aphid banker plants need to be well watered, distributed thoroughout the greenhouse with new replacements started every two weeks. See references at the end for more information on how to use aphid banker plants.
Figure 4: Starter Aphid Banker Plant with bird cherry oat aphids. Photo by L. Pundt
Predators consume many prey during their lifetime. Repeated releases of aphid predators are often needed in order to keep pace with the aphids' high reproductive rate in the greenhouse. Their effectiveness depends upon their predation rate, ability to locate prey and increase in number.
The predatory midge, Aphidoletes aphidimyze, can feed on more than 60 different species of aphids. This gall midge is nocturnal, and prefers dark and humid areas near the lower plant canopy. They require a period of darkness for mating and egg laying. Only the larvae stage is predacious. Adults feed primarily upon pollen and honeydew. The bright orange larva kills aphids by biting their knee joints, injecting a paralyzing toxin and then sucking out their body fluids. Aphidoletes aphidimyza is typically sold as pupae in bottles or blister packs. Adults that emerge from the pupae lay their eggs near aphid colonies. Larvae move to the ground to pupate and use organic debris to make their pupal cocoons. Plastic or concrete floors that are free of debris will not provide sufficient pupation sites. So sawdust, peat or holes in the weed mat barrier on the ground are needed. Some growers also place their aphid banker plants in trays of moist sand to provide pupation sites.
Adults are short-lived and tend to be active at night, so are rarely seen. This midge is most effective in the summer and will go into diapause (period of resting) between September and March. This is because the larvae need at least 15.5 hours of light to prevent the pupae from diapausing. The adults will lay eggs and larvae will feed upon aphids, but there is no second generation of midges produced. Low light intensities, are sufficient to prevent diapause.
Tips for Use
- Place in greenhouse away from direct sunlight.
- Release in the early morning or evening near aphid colonies.
- Temperatures should be between 60 and 80° F, relative humidity between 50 to 85%
- Look for fed upon aphids that will appear shriveled, and turn brown or black.
- Can be used with Aphidius parasitoids.
The convergent ladybird beetle (Hippodamia convergens) and the two-spotted lady beetle, Adalia bipunctata, are commercially available from many biological control suppliers. .
Lady beetles feed on many different types of aphids and other soft bodied insects. Both larvae and adults feed upon aphid nymphs and adults. Adult lady beetles feed upon pollen, fungi and nectar in the absence of prey. Eggs are laid near prey and the larvae may consume from 500 to 1000 aphids. Older, fourth instar larvae are more efficient at capturing prey than adults.
Figure 5: Ladybird beetle adult. Photo by L. Pundt
Figure 6: Ladybird beetle larvae. Photo by L. Pundt
Tips for Use
- Adults can be refrigerated until released.
- Release in the evening or early morning, near aphid colonies when the vents are closed.
- Repeated applications may be needed.
- Look for aphids that have been fed upon and for ladybird beetle adults, larvae or their bright yellow eggs.
- Flowering, pollen producing plants will attract the beetles.
The green lacewing (Chrysopa rufilabiris and C. carnea) adults are active at night and feed on nectar, pollen and honeydew. The predatory larvae (also known as”aphid lions”) feed upon many different species of aphids as well as mites, whiteflies, mealybugs, scales and thrips. Because larvae will feed upon each other, they must be released as far apart as possible to discourage cannibalism. Green lacewings may be less effective on plants with hairy leaves.
Green lacewings are commercially available as eggs on cards, or as larvae shipped with a food source in an inert material in a small container or as larvae shipped in separate cells. Larvae may survive better than eggs and are quicker acting. A reduction in aphid population should occur after approximately two weeks.
Green lacewings also available as adults shipped in a small cardboard container. Look on the underside of leaves for the eggs laid on extended stalks.
Figure 7: Releasing lacewing larvae. Photo by L. Pundt
Figure 8: Lacewing larvae. Photo by L. Pundt
Figure 9: Green lacewing eggs. Photo by L. Pundt
Tips for Use
- Spread larvae over an area because they are cannibalistic.
- Look for clean, new growth as a sign that aphids have been killed.
- Ants and slugs will eat lacewing eggs on the cards so need to be controlled.
- If it’s too warm (above 95° F), they will leave the greenhouse.
Naturally occurring predators
Hover flies, also known as syrphid or flower flies are naturally occurring beneficial, predatory insects that may enter the greenhouse from outdoors. Adults begin emerging in April and May about the same time that aphid populations start to increase outdoors. Hover flies are so named because of the ability of the adult to hover in mid-air, dart a short distance very quickly, and then hover again. Adults are small (3/8 to ¾ of an inch long) and look like small bees or wasps. But, they in the fly (Diptera) family, so only have two wings and also have short antennae and large eyes. Hover flies lay their eggs (resembling a small grain of rice) near aphid colonies. Eggs hatch into small, legless larvae with a tapered head that feed upon aphids.
Figure 10: Hover fly adults. Photo by L. Pundt
Figure 11: Close-up of hover fly larve. Photo by L. Pundt
Several types of entomopathogenic (or insect-killing) fungi have been developed for use against greenhouse pests. The entomopathogenic fungus, Beauveria bassiana, is commercially available for use against aphids. However, because aphids have high reproductive rates and molt rapidly, so repeat applications are typically required. Beauveria bassiana is most effective when aphid populations are low. The fungus works best with a relative humidity >90%. Beauveria may not be compatible with the convergent ladybird beetle (Hippodamia convergens) depending on the concentration of spores applied. The entomopathogenic fungus, Isaria (=Paecilomyces) fumosoroseus is most effective when the relative humidity is 80% or higher for 8 to 10 hours.
Tips for Use
- Thorough spray coverage is needed so that the fungal spores contact the targeted insect pest and begin the infection process.
- Repeated applications (three to five) may be needed for effective control.
- High relative humidity is needed
In summary, aphid parasitoids, aphid predators and entomopathogenic fungi can all be incorporated into a biological control program against aphids.
Ferguson, G, G. Murphy, and L. Shipp. 2006. Aphids in Greenhouse Crops. Ontario Ministry of Agriculture, Food and Rural Affairs Fact sheet.
Gill, S. and J. Sanderson. 1998. Ball Identification Guide to Greenhouse Pests and Beneficials. Ball Publishing. Batavia, IL. 244 pp.
Heinz, K.M., R.G. Van Driesche, and M.P. Parella (Ed). 2004. Bio Control in Protected Culture. Ball Publishing, Batavia, Ill. 522 pp.
Jandricic, S. and S. Frank. 2014. Boosting bankers. Greenhouse Canada.
Jandricic, S. and J. Sanderson. 2011. Early Season Pest Threat. Greenhouse Canada.
Malais, M.H. and W. J. Ravensberg. 2003. Knowing and recognizing: The biology of glasshouse pests and their natural enemies. Koppert Biological Systems and Reed Business Information. The Netherlands. 288 pp.
Skinner, M., C. E. F. Sullivan and R. Valentine. 2014. Aphid Banker Plant System for Greenhouse IPM, Step by Step. University of Vermont & BioBest USA, Inc.
Smith, T. and L. Pundt. 2014. Greenhouse Pest Guide Web App.
Sullivan, C. F. and M. Skinner. 2012. Hyperparasites of Aphid Parasitic Wasps. University of Vermont Extension System.
Stack, Lois Berg. (Ed). 2014-2015. New England Greenhouse Floriculture Guide. A Management Guide for Insects, Diseases, Weeds and Growth Regulators. New England Floriculture Inc. and the New England State Universities.
Thomas, C. 2005. Greenhouse IPM with an Emphasis on Biocontrol. Publication No. AGRS-96. 89 pp. Pennsylvania Integrated Pest Management Program.
Valentin, R. 2011. Using Banker Plants in an IPM program. Greenhouse Management.