Greenhouse Insect and Mite Pest Management
Insect and Mite Pests Commonly Found in Greenhouses
Aphids
Aphids are small, soft bodied insects (See Figure 27.1). More than 20 aphid species can infest various greenhouse crops. They range in color from green to brown, red, black, or purple. Some species may even have different color forms in the same colony. Most have the soft exoskeleton, but some species produce waxy, cotton-like strands that cover the body. Aphids are identified by their sucking mouthparts, long, thin legs, long antennae, and pear-shaped body.
Life Cycle
Aphids reproduce quickly under warm conditions, often completing a generation in 6 to 7 days. Most aphids are female and reproduce without mating giving birth to live young called nymphs rather than lay eggs. This allows their populations to increase explosively. They shed their skin several times. Then the nymphs reach adulthood and soon produce new aphids.
Types of Damage
Aphids feed by inserting their stylet-like mouthparts through plant tissue directly into the phloem and removing plant sap. They tend to move to new host plants and actively search for soft, fresh plant tissue to feed upon. The insects usually feed on young terminals or the undersurface of leaves. Aphid feeding can cause leaf discoloration and/or leaf distortion (leaves curl downward or upward).
Monitoring
Yellow sticky cards can monitor when winged aphids are active and may detect a migration of aphids into the greenhouse, particularly in the spring and summer. Cards should be positioned 4 to 6 inches vertically just above the crop canopy. Additional cards placed near doors or vents can detect whether insects are moving into the greenhouse from outside. Cards should be counted at least weekly and changed weekly or when they are full.
Cultural Management Strategies
Sanitation is a vital part of aphid control. Carefully inspect plant material brought into the growing areas; do not purchase infested plants or cuttings. Eliminate all weeds in or near the greenhouse; they can serve as a reservoir for migrating insects or for insects carried by ants. Discard old stock plants, hanging baskets that have not sold, and don't keep “pet” plants.
Physical Management Strategies
Screen doors and vents to prevent migration into the greenhouse, especially during the Fall and Spring. Aphid control is much more successful when an infestation is detected and controlled early in a crop.
Biological Management Strategies
Biological control involves the use of predators and parasites to control the pests. Aphids are susceptible to many natural enemies, both predators and parasitoids. In general parasitoids are more effective than predators in reducing aphid populations, although parasitoids may fail to provide acceptable control under warm conditions when aphid populations tend to increase rapidly. Some effective species of biological control agents are commercially available. Examples include parasites such as Aphidius ervi, Aphidius colemani, and Aphelinus abdominalis and predators such as Aphidoletes aphidimyza and Chrysoperla rufilabris.
Parasitoids. Parasitoid wasps such as Aphidius colemani and Aphidius ervi can be used to control aphids. Aphidius colemani is used to control green peach aphids and melon aphids but not against the foxglove or potato aphids. This tiny parasitoid wasp lays one egg in an aphid. This egg hatches into a larva, which feeds on the internal parts of the aphid. When mature, the larva will pupate, and a new adult wasp will emerge from the shriveled aphid body (mummy).
Pedators. The predatory midge, Aphidoletes aphidimyza, resemble small mosquitoes or fungus gnats. The adult females of the predatory midge will emerge from pupae when placed in the warm greenhouse and lay their eggs near aphid colonies. After hatching, the orange midge larvae attack and feed on aphids. This predatory midge is a general aphid predator and will feed on many different aphid species. It can be used alone or in combination with a parasite for rapid knockdown of aphids.
Insecticidal Soaps. Applications of insecticidal soaps and highly refined horticultural oils can provide effective aphid control in some cases and should be evaluated by growers. These pesticides kill exclusively by contact.
Microbials. Insect-pathogenic fungi, Beauveria bassiana, have been used to control aphids too. It is usually best to use fungal pathogens while the infestation is small, not against a major infestation
Chemical Management Strategies
Systemic or translaminar insecticides tend to be more effective than contact insecticides, provided that a sufficient amount of insecticide reaches the aphid feeding sites. Systemic insecticides will be most effective against those feeding on new growth. Contact insecticides can be very effective with thorough spray coverage and good canopy penetration.
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