Plants love the warm, moist conditions of a typical greenhouse; unfortunately many pests also thrive in the greenhouse climate, as well. A few species make up the majority of pests usually found in many greenhouses, but finding and identifying these pests is challenging. Prevention is worth a great deal, as eradication can be difficult once pests are established in a greenhouse.
Insects with piercing mouth parts, such as aphids and mealybugs, are the most common types of insect pest found in greenhouse settings. Other common greenhouse pests include scale insects, named for their flat, scale-like appearance; whiteflies, which suck sap from plants; thrips, elongated insects that feed on leaf tissue and cause premature leaf drop; and mites, near-microscopic pests that feed on leaf tissue, causing foliar spotting.
Many greenhouse pests are tiny and are difficult to spot with the naked eye. Looking for the damage they cause may be the main method of identifying greenhouse pests. Though the damage caused by the commonest greenhouse pests usually does not result directly in the demise of a plant, pest activity causes plants to become spindly, have shriveled and spotted foliage, or have flowers that are malformed or which fail to open completely. Many insect pests also secrete a clear, sticky substance known as honeydew, which encourages disease such as sooty mold. Several insects, especially thrips, are known vectors for several types of viruses, including tomato spotted wilt virus, which usually manifests as brown or yellow rings on leaves, black streaks on stems or tip dieback.
There are literally hundreds of species of thrips in a wide range of colors and identifying marks. A common characteristic of thrips is their elongated appearance, though they are only several millimeters in length. Adults are flightless and can be black- or yellow-bodied with white legs. Larvae are yellow with red eyes, and the presence of both adult and larval forms is indicated by black specks of fecal matter the insects drop on the undersides of leaves.
Scale insects appear to be exactly that: flat, waxy shield-shaped insects that adhere closely to the undersides and stems of plants. Visible to the naked eye, scale insects can be observed moving slowly across the plant surface. More than 150 different species exist. The majority are "soft scale" insects, which tend to be larger, up to ½ inch in diameter.
Mites are among the tiniest greenhouse pest, but among the most destructive. Spider mites are probably the most common type, though they are difficult to see with the naked eye. Adult spider mites can be identified by their long, spider-like legs as well as the two green or brown dots on the abdomen. The most obvious sign of a spider mite infestation is the presence of webbing on and under the leaves; feeding damage appears as small holes or stippling on the undersides of leaves.
The most effective way of keeping low pest populations in the greenhouse is to prevent them from becoming established. Though some level of insect population will always be present in a greenhouse setting, thoroughly cleaning all equipment and inspecting new plant material for evidence of pests are the two main methods of pest prevention and reduction. Washing pots, tools and plant trays with a mild bleach solution and soapy water not only kills pathogens but also any insect eggs that may be present. Any new plant material that is to be housed in the greenhouse should be thoroughly inspected for evidence of adult insects, larvae or eggs. Plants that show evidence of insect damage should be rejected or sequestered. Sterilizing plant growing media also reduces pest populations, as well as keeping the indoor growing area free of weeds and other debris.