Pest and disease damage on ornamental products is not tolerated by consumers requiring a sizeable investment in pesticide applications. These practices led to a total of 5.36 million pounds of active ingredients applied to nursery and floriculture crops in California, Florida, Michigan, Oregon, Pennsylvania, and Texas alone for the year 2000 (NASS, 2002). Of that total, 39% or 2.1 million pounds of active ingredient were applied to floriculture crops. Recognizing that these six states represent 55% of the national reported value of the nursery and floriculture industry (NASS 2003), one can project on a national scale, 9.75 million pounds of active ingredients, are applied to floriculture and nursery crops annually. Of that total, 3.8 million pounds of active ingredient are applied to floriculture crops. Understanding the investment that a greenhouse grower must consider when choosing a pest control strategy, proper pest identification is extremely important.
Aphids are small soft bodied insects that feed on plant sap through piercing and sucking mouthparts. The most common species in the greenhouse is the green peach aphid (Myzus persicae). Aphids feed in groups on stems and leaves, usually closely associated near phloem tissue along leaf veins. Some aphids transmit certain viruses that cause diseases in flowering crops. Aphids that feed on young leaves and buds will cause chlorotic pinpoints and distorted foliage. They also excrete honeydew, a sticky sugary sap. Honeydew if left unchecked will develop into a black sooty mold rendering ornamental plants unsalable.
Fungus Gnats, Shore Flies and Moth Flies
Fungus gnats, shore flies, and moth flies are often confused in that they are similar in appearance and appear under wet, over watered conditions. They are easily distinguished when they are at rest. A fungus gnat (Bradysia sp. and Sciara sp.), at rest holds its wings slightly spread when viewed from above. Shore flies (family Ephydridae, commonly Scatella stagnalis), are a bit more robust in appearance with bristle-like antennae, which are shorter than the head. Moth flies or drain flies (family Psychodidae), are grayish in color with many fine hairs.
Adult fungus gnats are commonly found on soil and other surfaces with standing water. They mostly feed on dead and decaying tissue being primarily a nuisance. The larvae typically feeds on soil fungi and decaying plant matter as well, but in high populations will attack roots and other soil-borne plant organs such as tubers and corms.
Shore flies typically feed on algae and not plant tissue. They do, however, appear under the same conditions as fungus gnats and are often confused. Adult shore flies are larger and more robust than fungus gnats. The adults are primarily a nuisance. Use the same cultural control strategies for shore flies as fungus gnats.
Moth flies also thrive under moist and over watered conditions. Excess fertility creating algal scum in and around drains and bench supports tend to attract drain flies. Moth flies have been reported to feed on plants, but little documentation is available. Like shore flies, moth flies are primarily a nuisance and cultural management is the best control.
Moths and flies from several families have larvae that cause damage by tunneling through leaves. The serpentine leaf miner (Liriomyzia trifolii) and the chrysanthemum leaf miner (Pytomyza atricornis) are the two most prevalent species in the greenhouse. The adult female punctures the leaf using an ovipositor, inserting her eggs. The larvae hatch within the leaf and tunnel through the leaf for about two weeks. Afterwards, they drop from the leaf and pupate in the soil.
The citrus mealybug (Planococcus), is the most common mealybug found on greenhouse crops, but the longtailed mealybug (Pseudococcus longispinus) is also found as well. Adult females are grayish, soft-bodied, wingless insects that are elongated, segmented, and coated with a whitish cottony wax. It is the wax coating that protects the mealybug making contact insecticides less effective. Contact sprays are most effective at the first instar nymphal stage. Surfactants can improve insecticide penetration through the waxy protection.
Mites are arachnids and not insects. Arachnids include tics and spiders. They do not have segmented bodies, wings, or antennae. Mites common to the greenhouse include the spider mites and red mites [carmine mite (Tetranychus cinnabarinus), privet mite (Brevipalpus obovatus), and two spotted mite (Tetranychus urticae)] tarsonmed mites [broad mite (Polyphagotarsonemus latus), cyclamen mite (Phytonemus pallidus), and bulb scale mite (Steneotarsonmemus laticeps)], bulb mites (Rhizoglyphus spp.), and bud mites [carnation bud mite (Aceria paradianthi)]. Of these mites listed, the two spotted mite is probably the most common; however, others are becoming more prevalent with crop diversity.
There are many genera of scale insects common to greenhouse crops, all from the order Homoptera. They feed on plant tissue by piercing plant tissue and sucking sap. They also inject toxins that stunt growth as well as spot and streak the foliage. Some species produce honeydew much as aphids, mealybugs, and whiteflies.
There are two groups of scale common to the greenhouse, armored and soft. Armored scale (family Diaspididae) has a plate like covering that protects the soft insect body underneath. Armored scales do not secrete honeydew. The surface of soft scales (family Coccidae) is smooth and brown. Soft scales are typically larger than armored scales.
Slugs and snails
Slugs and snails are not insects, but mollusks. Slugs are differentiated from snails in that they lack a shell. Slugs and snails feed at night and during daylight, they hide in cracks and crevices on the bench, beneath pots, or in plant litter on the ground. They prefer dark and moist hiding places. Slugs and snails are most active when the conditions are cool and moist.
Thrips (order Thysanoptera) commonly attack many greenhouse crops. The western flower thrips (Frankliniella occidentalis) is the most serious thrips problem in greenhouses. Thrips damage is often not apparent until sometime after feeding. They feed on flowers, buds, terminals, bulbs, and corms. Thrips hide deep in these tissues and the damage is not visible until after the tissue elongates.
Adult and larval stages of thrips feed on plant tissue by puncturing cells and feeding on the oozing sap. The tissue, as it grows, becomes silvery, stippled, blotched, streaked, or deformed. Western flower thrips also feed on pollen and some mites.
Whiteflies are in the order Homoptera and are not true flies, which is the order Diptera. The primary whiteflies that infest greenhouse plants are the greenhouse whitefly (Trialeurodes vaporariorum), and the silverleaf whitefly (Bemesia argentifolii).
Whitefly nymphs and adults feed on plants with piercing and sucking mouth parts and are associated with phloem tissue. Whitefly feeding can stunt plant growth and kill young plants. Whiteflies produce honeydew much as aphids, mealybugs, and scale.
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