There are three general classifications of pests: chewing, sucking, and rasping.
Chewing pests, as ants, cutworms, leaf rollers, and leaf tiers, though not so prevalent as some others, do considerable damage by chewing seedlings, cutting off stems and foliage, and destroying flowers. Ants themselves do little damage to plants, but they carry aphids and mealy bugs from plant to plant throughout the greenhouse. Since chewing pests feed on foliage, they are most easily controlled by poisons applied to the areas of the plant on which they feed "stomach" poisons which the pests consume along with the greenery.

Sucking pests, as aphids, cyclamen mites, white flies, mealy bugs, and red spiders, pierce the foliage, generally from the underside, to suck the juices from inside leaves and stems. Control is accomplished by spraying "contact" poisons directly on the pests. Rasping pests, as thrips, rasp the foliage to obtain the juices from the injured section. These, too, are controlled by "contact" poisons sprayed directly on the pests.

Treatment Must Continue
The life cycles of these various pests differ greatly, and preventive or control measures must be taken accordingly. Aphids, which produce live young, have a life cycle of about 25 to 30 days during which time a female may reproduce several groups of 30 or more; a majority of the young will be females that will be mature enough in 10 to 12 days to produce another generation. The female red spider lays as many as 8 eggs daily; these hatch in 5 days and, depending on the temperature in the greenhouse, will produce another generation in from 10 days to a couple of months. Obviously, then, one treatment is not sufficient to destroy any particular pest infestation. Repeated treatments over a period of at least a month (once every ten days) are necessary to assure the destruction of not only the breeding generation but also of those maturing later and those still in the incubation period.

Pest Immunity to Chemicals
Another aspect of pest eradication is that some pests develop an immunity to certain sprays or dusts, either in the present generation or in succeeding generations hatched from eggs that have been subjected to treatment. When immunity is established, some different pesticide will have to be tried, and this is where up-to-the-minute information will serve you well.

Up to Date Bulletins
College research departments, agricultural experiment stations, and chemical companies have spent and continue to spend much time and money to research the habits of various pests that plague greenhouse plants and to develop controlling sprays and dusts. Most of these organizations publish bulletins (available to you for the asking or for a very small fee) that furnish valuable, up-to-the-minute information.

Recognizing The Pests

The important thing in controlling and eradicating greenhouse pests is to recognize them quickly so the proper treatment may be started without delay. These descriptions may be helpful:

Aphids may be yellow, green, or black. They are minute in size, with plump bodies, some winged and others not. They are usually found on new, tender shoots and leaves, and when aphids are present leaves will soon become distorted.

Cyclamen mites are invisible to the naked eye. Infestation should be suspected if new growth is distorted or twisted and new leaves become progressively smaller, or growth appears to stop.

Leaf tiers are small green or bronze caterpillars that spin webs around leaves, causing the edges of the leaves to roll under. These pests are sometimes called leaf rollers. Mealy bugs look like dabs of fuzzy white cotton at the onset, later developing an oval shape with tiny pointed appendages. The cottony coating has a waxy texture that is difficult to penetrate.

Nematodes (foliar) attack the leaves, leaving brown spots which are clearly visible. Wetting the foliage may encourage spread of this pest.

Nematodes (root-knot) are tiny eel worms that irritate roots in which they hatch and form knots or galls wherever tissue is injured. They especially attack African-violets, begonias, cyclamens, and chrysanthemums. Growth is seriously affected, and plants become stunted or deformed. Steam sterilization of soil is an effective control.

Red-spider mites are troublesome pests, since they are so small they cannot be seen without a magnifying glass. They usually attack the underside of tender new growth which becomes mottled, and turns brown and dies if the infestation is heavy. You will notice a fine web, which may eventually envelop the entire plant. This pest thrives in a dry atmosphere, so fine water misting, spraying, or washing of susceptible plants is recommended.

Scales are small white or brown sucking insects that hide themselves under a hard protective covering that looks like a dried oval scab.Snails and slugs may be a bit troublesome if you grow orchids or other plants that require high humidity. Plants that are moved outdoors for the summer sometimes acquire these pests. Before returning plants to the greenhouse, inspect pots carefully and flick out with a small stick and destroy any slugs or snails you find. In my many years of orchid growing I have been troubled only once with this problem. The pests were picked up in the outdoor garden, but a careful inspection revealed them before the pots were returned to the benches in the greenhouse.

Thrips are thin, winged when adult, and can be tan, yellow, brown, or black. They are difficult to detect when small, but affected foliage has a silvery appearance, and flower petals are streaked or deformed.

White flies, minute in size, are aptly named. If a plant infested with this pest is disturbed, cloud of hundreds of tiny white flies are visible as they fly from the undersides of the leaves, leaving behind countless tiny eggs that keep developing.