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 insecticides 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" insecticides 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" insecticides sprayed directly on the pests.
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.
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.
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. Also, there are websites local to your area that provide specific bulletins as well. Use your search engine to find the relevant sites for your geographical location.