Dr. Bugs: Managing Aphid Populations
Answer: Aphids are major insect pests of greenhouse-grown horticultural crops such as vegetables and ornamentals. Species encountered in greenhouses include green peach aphid (Myzus persicae), cotton or melon aphid (Aphis gossypii), potato aphid (Macrosiphum euphorbiae), foxglove aphid (Aulacorthum solani) and/or chrysanthemum aphid (Macrosiphoniella sanborni).
Aphids are 1.0 to 2.5 mm long, and green to black or yellow to pink depending on the host plant fed upon. In addition, aphids have cornicles (tailpipes) protruding from the back of the abdomen (Figure 1, pictured at top). All aphids in greenhouses are females with each female producing up to 100 live female nymphs within 30 days. Their high reproductive potential allows aphid populations to build up quickly on horticultural crops in greenhouses.
Aphids use piercing-sucking mouthparts to feed on new growth and leaf undersides (Figure 2), withdrawing plant fluids from the food-conducting tissues. Feeding by aphids results in leaf yellowing, distorted leaves and plant stunting. During feeding, aphids excrete honeydew, a clear, sticky liquid that serves as a growing medium for black sooty mold. The presence of black sooty mold on leaves inhibits the ability of plants to produce food by means of photosynthesis. The presence of white molting skins on plants (Figure 3) indicates that you are dealing with an extensive population of aphids.
Aphid management strategies include scouting, cultural and physical practices, insecticide applications, and releasing biological control agents.
Scouting: Scouting for aphids involves inspecting plants, especially those susceptible to aphids, at least once per week during the growing season. Be sure to check new growth and leaf undersides where aphids tend to reside.
Cultural and Physical Practices: Do not overfertilize plants with water-soluble, nitrogen-based fertilizers because overfertilizing plants encourages soft, succulent growth that is easy for aphids to penetrate with their mouthparts. Furthermore, young plants contain an abundance of essential amino acids, which are the components of proteins necessary for aphid reproduction and population growth. Weeds or undesirable plants located within the greenhouse and around the greenhouse perimeter should be removed because many weed species serve as hosts for aphids. Plants heavily infested with aphids should be disposed of immediately by placing them into refuse containers with tight-sealing lids or in garbage receptacles located outside the greenhouse.
Insecticide Applications: Systemic insecticides are applied preventatively, before aphids are present on plants, early in the production cycle. Systemic insecticides, in general, are applied to the growing medium as a drench or granule, providing up to 8 weeks of protection against aphid infestations. Contact insecticides must also be applied early in the production cycle to prevent aphid populations from reaching plant-damaging levels. All plant parts should be thoroughly covered with spray application solutions. In most cases, repeat applications will be required. Allow for ample spacing between plants to ensure thorough coverage with insecticide spray applications.
Releasing Biological Control Agents: There are a number of biological control agents (e.g., parasitoids and predators) commercially available that can be purchased and released into greenhouses to manage aphid populations. Parasitoids include Aphidius colemani, Aphidius ervi, Aphidius matricariae, and Aphelinus abdominalis. Predators include Aphidoletes aphidimyza, Chrysoperla carnea, Hippodamia convergens, and Adalia bipunctata. All biological control agents must be released early in the production cycle before aphids establish on greenhouse-grown crops. Be sure to have aphids identified to species before purchasing parasitoids because not all parasitoids will attack the same aphid species.