Greenhouse Insect and Mite Pest Management
Insect and Mite Pests Commonly Found in Greenhouses
Mites
Several types of mites can cause serious damage to a variety of greenhouse crops (See Figure 27.6). Being in the class Arachnida, they are not insects but are more closely related to spiders and ticks. Adults have eight legs (except the eriophyid mites with four legs) and no antennae or body segmentation. Among the most common mite species are the broad mite (Polyphagotarsoeneumus latus), carmine spider mite (Tetranychus cinnabarinus), cyclamen mite (Phytonemus pallidus), the Lewis spider mite (Eotetranychus lewisi), and especially the two-spotted spider mite (Tetranychus urticae). There are many reasons why mites are problems in greenhouses. Mites are relatively small and they spend most of their time on the undersides of leaves, within tender buds, or deep within flowers, which makes them difficult to detect and effectively control.
Life Cycle
High temperatures of 70 to 80 degrees F (21 to 27°C) and lower humidity favor the development of broad mites. Female broad mites lay eggs on leaf surfaces over an eight to 13-day period. Larvae (nymphs) hatch in two to three days and begin feeding. Broad mites can complete their life cycle (egg to adult) in as little as seven days. High relative humidity (80 to 90%) and temperatures of 60 degrees F (16°C) favor the development of cyclamen mites. Severe outbreaks of cyclamen and two spotted spider mites typically occur in greenhouses in the fall and winter months. Adult female cyclamen mites may live for up to one month and can reproduce without mating.
Types of Damage
Mites feed on plant material by piercing plant tissue with their mouthparts and removing plant fluids. Some mite species inject a toxin in their saliva as they feed. Feeding damage varies depending upon the specific mite and plant species being attacked, but often appears on the upper leaf surface as a characteristic mottling or speckling. Most mite species have a wide host range, attacking a variety of greenhouse crops. Two-spotted spider mites are widely distributed and are common pests of many plant species. Theses mites produce a fine webbing on plants and the damage includes stippling or bronzing of leaves. Generally, they feed on the undersides of leaves, giving the upper leaf surface a speckled or mottled appearance. Leaves of mite-infested plants may turn yellow and dry up, and plants may lose vigor and die when infestations are severe. Depending on the type of plant attacked, cyclamen mites may infest the entire plant or be concentrated around the buds.
Monitoring
Plant inspection is the only reliable method to assess the presence of mites. Attention should be given to areas of the greenhouse where mites may be spread on the clothing of workers, such as along walkways and near entrances. Attention should also be focused on plant species or varieties that are particularly susceptible to mite infestations and in areas of the greenhouse that have a history of mite problems.
Cultural Management Strategies
Continuous vigilance on the part of the grower may prevent a costly infestation. The introduction of infested plant material into greenhouses, as well as allowing vegetation to grow near the outer perimeter of the greenhouse, may give rise to population explosions, especially during the summer. Infestations usually start and develop most rapidly on plants near steam pipes or other objects which radiate heat. It is much easier for the grower to prevent a mite infestation than to eradicate it once it is established in the greenhouse.
Two-Spotted Spider Mite. Cultural practices for managing two-spotted spider mites includes carefully inspecting incoming plant material, removing weeds in and around greenhouses and disposing of old plant material including “pet plants” which may harbor populations. Plants that are over-fertililized with nitrogen-based fertilizers promotes succulent new growth which is more susceptible to two-spotted mites.
Biological Management Strategies
Commercially available predatory mites for two-spotted spider mites include Phytoseiulus persimilis, Galendromus occidentalis, Neoseiulus (=Amblyseius) californicus, Amblyseius andersonii, Amblyseius fallacis, and Feltiella acarisug. Phytoseiulus persimilis is the most effective predatory mite for control on two-spotted spider mite. If predatory mites are used, early release at the first sign of mite infestation is critical. Unlike a miticide, predatory mites will take some time to control infestations. Without spider mites, the predators die. This means new spider mite infestations require new introductions of the predator.
Phytoseiulus persimilis. A predatory mite, Phytoseiulus persimilis, is the mainstay in spider mite control and can be used on many crops. They are active year-round and feed on spider mite eggs, larvae, nymphs, and adults. P. persimilis is suitable for use in short term crops such as bedding plants. Releases should be made when two-spotted spider mite populations are low or first detected. Two applications, one week apart may be needed. Make releases near infestations and concentrate releases near localized hot spots.
Chemical Management Strategies
Pesticide resistance can be a common problem in mite control. Unfortunately, many strains of mites exist, and many of these strains are resistant to certain miticides. Currently there are a variety of effective miticides, but unless these chemicals are used carefully and sparingly, it is only a matter of time before resistant strains evolve. Because mites primarily occur on the undersides of leaves, applications of contact miticides should be directed at both the lower and upper leaf surfaces. Good coverage is essential to good control, particularly when using contact miticides.
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