Greenhouse Insect and Mite Pest Management
Insect and Mite Pests Commonly Found in Greenhouses
Scales
There are several species of scale insects that can occur on greenhouse plants (See Figure 27.9). Most of them, however, are from two families, the Coccidae (soft scales) and the Diaspididae (armored scales). The distinction is important because behavior and management of the two groups are different. Both groups live beneath waxy covers that protect them from predators, parasitoids, and pesticides. Soft scales secrete a waxy layer over themselves that cannot be separated from their body. Soft scales also excrete sugary honeydew and may move from branches to leaves during their life cycle. In contrast, armored scales live beneath a waxy cover that is not attached to the adult body. Thus, the cover (also called a “test”) can be removed to reveal the scale insect hidden below. Armored scales typically do not move once they begin to feed and do not produce honeydew. Armored scales are also some of the toughest greenhouse pests around, because they cause severe plant damage and their “armor” makes them difficult for growers to manage effectively.
Life Cycle
Scale insects have a three-stage life history: egg, nymph, and adult. Adult females lay eggs underneath their protective covering which hatch over a period of one to three weeks. The eggs hatch into the mobile nymphs, called crawlers. The crawlers are the active stage that can move between plants.
Types of Damage
A large infestation of scales can cause leaves to tum yellow and drop, and it may stunt new growth. Soft scales excrete a waste product, honeydew, which is a thick, sugary liquid. The honeydew falls on lower structures and leaves, coating them with a shiny, sticky film.
Monitoring
Early detection is very important for effective control. Greenhouse workers should be trained to recognize these pests and the early symptoms of damage. Ants climbing on the plant can indicate a problem with scales.
Cultural Management Strategies
Scale insect management, like all pest management, begins with clean plants. Inspect plants and cuttings when they arrive for signs of scales, exoskeletons, tests or honeydew. Unfortunately, scales cannot be monitored with sticky cards or other techniques like tapping foliage against a clipboard. Therefore, inspection of the most susceptible plants and watching for the sheen of honeydew can help identify small populations.
Biological Management Strategies
Parasitoids. Metaphycus helvolus, a parasitic wasp is very effective when used as a preventive agent. Due to their minute size, adult wasps may get caught in honeydew if it is heavy. The adult wasp places one egg inside each scale, which, upon hatching into a larva, consumes the scale from within. A new parasitoid emerges from the empty scale cadaver.
Predators. Cryptolaemus montrouzieri is a predatory ladybird beetle used to contro1 mealybugs and scales. They hide under leaves and in plant crevices where scales are usually feeding. Their eggs hatch into voracious larvae with long, white, waxy projections resembling those of a mealybug.
Biorational Management Strategies
Insecticidal Oils and Soaps. Insecticidal oils and soaps can sometimes be effective, killing more life stages of these pests than many contact insecticides, but they provide no residual control. Again, thorough coverage is critical.
Insect Growth Regulators. Insect growth regulators disrupt insect development preventing eggs and nymphs from developing into reproductive adults. Two insect growth regulators used for armored scale control are buprofezin and pyriproxyfen.
Chemical Management Strategies
If contact insecticides are used, they should be applied during the crawler stage of scales. Repeated applications are therefore necessary to contact the susceptible stages as they are produced. Spray intervals will depend on the residual effectiveness of the insecticide used, which may vary from 0 to 3 weeks. The inclusion of a spreader-sticker can improve coverage, penetration, and residual activity, although the risk of phytotoxicity may be increased.
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