Greenhouse Insect and Mite Pest Management
Insect and Mite Pests Commonly Found in Greenhouses
Thrips
Several species occur in greenhouses. Of the many species of thrips commonly found in greenhouses, flower thrips (Frankliniella tritici), western flower thrips (Frankliniella occidentalis), and chilli thrips (Scirtothrips dorsalis) are most problematic. The most distinctive external feature of these thrips is found on the adults (See Figure 27.9). They have 2 pairs of featherlike, long, narrow wings, which have few or no veins and bear fringes of long, fine hairs along their margins. The wings are held parallel along the back when at rest. Immature forms of thrips are wingless. Colors vary from white to straw yellow to brown. They feed with raspingpiercing-sucking mouthparts, resulting in deformed flowers and leaves.
Life Cycle
Thrips are gregarious with large numbers often concentrated on the same leaf or flower. The length of the life cycle is strongly influenced by humidity and temperature. Thrips in warm greenhouses have a shorter generation time than thrips outdoors. The pest’s rapid developmental time (egg to adult in 7 to 15 days at fluctuating temperatures), high reproductive rate, and preference for protected areas can make early detection difficult. In the greenhouse, thrips development may continue uninterrupted throughout the year if suitable crops are available.
Types of Damage
Thrips feed by piercing plant cells with their mouthparts and sucking out the cellular contents. The damage to plant cells caused by thrips feeding can result in deformation of flowers, leaves, and shoots. There is often silvery streaking and flecking on expanded leaves. Thrips often deposit tiny greenish-black fecal specks on leaves when they feed.
Monitoring
Early detection is critical for thrips control and minimizing virus transmission. Place cards just above the crop canopy, at about one per 500 square feet and near doors, vents, and over thrips-sensitive cultivars to monitor the movement of thrips. Research has shown that blue sticky cards catch more thrips than yellow ones. To monitor for a wider range of flying greenhouse pests such as winged aphids, whiteflies, and fungus gnats, use yellow sticky cards. Thrips are usually the smallest insects found on sticky cards. The number of thrips per card should be recorded weekly and graphed to detect trends.
Cultural Management Strategies
Weed control is essential for a successful thrips control program. Weeds can serve as a refuge both for thrips and tospovirus. Eliminate weeds inside the greenhouse and also from areas immediately outside, particularly near vents and doors. Black plastic mulch covered with coarse gravel can be used for weed control and to reduce thrips habitat around the greenhouse and under the greenhouse benches. Coarse gravel on plastic is an unfavorable site for prepupal and pupal stages of thrips. Ongoing problems with INSV are often the result of poor sanitation.
Biological Management Strategies
Several biological control agents are available for thrips: for foliar life stages, the predatory mites Neoseiulus cucumeris, Amblyseius swirskii, Amblydromalus limonicus, Iphiseius degenerans, and the predatory bug Orius insidiosus; and for the soil-dwelling pupal stages, the predatory mite Hypoaspis (also called Gaeolaelaps or Stratiolaelaps) and the nematodes Steinernema feltiae.
Predatory Mites. The predatory mites, Amblyseius (Neoseiulus) cucumeris and A. swirskii, appear to be well suited for immature thrips control on greenhouse crops. A. swirskii feeds on thrips and whiteflies and has demonstrated to be effective in greenhouse grown peppers, cucumbers, and several ornamental crops. A. swirskii works better at warmer temperatures while A. cucumeris is better at cooler temperatures. A. cucumeris feed only on the first instar nymphs so must be released early in the crop cycle before thrips populations are high
Minute Pirate Bug. The minute pirate bug, O. insidiosus, is a winged predator that consumes all mobile stages of thrips. The youngest nymphs are yellow and the oldest are a dark mahogany brown. O. insidiosus also feeds on pollen, spider mites, aphids, whiteflies, moth eggs, and young caterpillars.
Nematodes. S. feltiae is a beneficial nematode that attacks many soil-dwelling insects such as pupating thrips or fungus gnat larvae. Nematodes enter the insect hose through body openings and release a bacterium that kills the host insect. They are sensitive to UV light and dessication and are best applied in the evening or at dusk or on a cloudy, overcast day.
Biorational Management Strategies
Hoticultural Oils. Horticultural oils, neem oil, and other low-toxicity insecticides such as insecticidal soaps or pyrethrins can be somewhat effective for temporary reduction of thrips if applied when they first appear. These materials allow at least a portion of the natural enemy populations to survive because they don't leave toxic residues.
Microbials. Beauveria bassiana, is a strain of fungus that is pathogenic to insects. To be lethal, fungal spores must directly contact the pest. Spores attach to the insect cuticle, where they germinate and the fungus infects the body cavity, killing it after 7 to 10 days.
Chemical Management Strategies
Chemical control of western flower thrips can be difficult. Resistance to certain organophosphate, carbamate, and synthetic pyrethroid insecticides has been documented in populations of western flower thrips. They also tolerate most insecticides and feed either deep within the flower head or on developing leaves. This makes them a difficult target for insecticides, so thorough coverage is essential. Western flower thrips feeding on leaves (both nymphs and adults) tend to be more susceptible to systemic insecticides than when feeding in flowers. Leaf-feeding more easily results in the insects imbibing toxic concentrations of the active ingredient of systemic insecticides. If insecticides must be used, rotate insecticides among different chemical classes to help delay the development of insecticide resistance. Maintain the use of an effective insecticide for more than one generation of a pest before rotating to another class of insecticide.
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