At first blush, greenhouses may seem a safe haven from the pests that infest outdoor garden plants, but in fact they can often harbor just as many harmful insects. Insect pests can infest greenhouse plants via pre-infested stock, meaning seedlings or cuttings with pests on them from the nursery, or as "fly-ins," which are pests that enter the greenhouse through doors and cooling or air vents. Many of these pests are difficult to see with the naked eye, so you may need to use yellow sticky paper to pick up the insects and a magnifying glass to study them better.
Look for tiny white, mothlike flies invading your greenhouse, to identify the whitefly, particularly the greenhouse (Trialeurodes vaporariorum) and silverleaf whiteflies (Bemisia argentifolii). Look on the younger leaves of your greenhouse plants to find whitefly eggs laid in circular patterns.
Spot spider mites infesting your greenhouse plants by looking for stippling of leaves and fine, weblike silks on the foliage. Look on the undersides of the leaves to see the perfectly round mite eggs, six-legged mite larvae, eight-legged mite nymphs and adults, as well as shed mite skins.
Identify mealybugs on your greenhouse plants by looking for tiny, spiny oval insects. Longtailed mealybugs are the easiest to identify, because of their long protuberance that grows from their hind ends. You're most likely to see mealybugs on your chrysanthemums, geraniums, hollyhocks, poinsettias, begonias, amaryllis and cyclamen.
Spot soft scales, specifically hemispherical, black, nigra and brown soft scales, on your greenhouse plants by looking for their miniscule round or oval bodies that sometimes take on a waxy-coated appearance.
Identify flower, greenhouse and banded greenhouse thrips by looking for discoloration of greenhouse plant foliage, including streaks on the flower petals, blanching of the leaf tissues and little "tar" spots of excrement on the foliage. You can identify adult thrips by their black bodies and bladelike wings that have long, hairlike fringe.
Look for winged insects feeding on your greenhouse plants' foliage to spot leafminers, which are yellow with black markings. The adult female leafminers make distinct "pinholes" in the plant leaves, while the larvae create winding mines in the leaves that slowly widen as the larvae grow.