Thrips are the most consistent, persistent insects to attack greenhouse plants throughout Canada, the United States and many Asian and European countries. The two species of thrips that commonly infect greenhouses are the western flower thrip (Frankliniella occidentalis) and Frankliniella tritici; they are so small that a compound microscope must be used to accurately identify them. There are mites, bugs, nematodes and fungi that will control thrips, as well as botanical and chemical insecticides to kill them.
Thrips suck out the contents of plant tissue with their mouths, causing silvery flecks and streaks on leaves; they deposit tiny greenish-black flecks of feces on the leaves. Feeding thrips pass along topoviruses, incurable, negative RNA viruses that cause numerous vegetable and ornamental species to develop rots, leaf spots and wilt--after which they die.
Most greenhouse thrips are adult females; they thrive in the warm temperature and high humidity of greenhouses. They deposit their eggs in the tissue of leaves or petals that offer them protection from insecticides. The larvae feed in protected buds or foliage and then move into the soil where they do not move about and are protected from insecticides directed at the foliage. It takes thrips from seven to 15 days to grow from eggs to adults when they can fly or be carried into greenhouses on clothing.
Blocking and Collecting
To eliminate thrips from a greenhouse they need to be blocked, if possible, and detected early so appropriate predatory insects can be introduced or insecticides applied. Thrips usually enter greenhouses through weeds. Weeds need to be controlled. Coarse gravel weighing down black plastic is used to control weeds inside greenhouses and in areas immediately surrounding them. Doors and windows should be kept closed.
Thrips need to be detected early. Collect them by tapping blossoming flowers over yellow or white paper. They are trapped by placing sticky blue cards near vents, doors and sensitive plants. Use a 10x magnifier to identify them.
Predatory mites that eat thrips larvae include Amblyseius cucumeris, Isphiseius degnerans, Hypoaspis aculifer and Hypoaspis miles. A bug, Orius insidiosus, eats thrips. A nematode, Thripinema nickewoodi and a fungus, Beauveria bassiana, kill western flower thrips in two to 14 days; they eat thrips from the time when the thrips are larvae until they are adults. Saccharopolyspora spinosa, a bacteria, can kill thrips. Three minute pirate bugs, Orius insidiosus, O. tristicolor and O. albidipennis eat thrips; they have voracious appetites and reproduce freely. These mites, bugs, bacteria and fungi are available commercially with instructions on how to apply.
For organic growers, thrips can be killed with insecticides derived from natural plant sources including those with the active ingredients of neem obtained from Azadirachta indica, a variety of mahogany, nicotine obtained from tobacco and pyrethrins, extracted from pyrethrum, (Chrysanthemum cineranaefolium).
Horticulturalists at the University of Maryland recommend chemical insecticides containing the active ingredients bifenthrin, chloropyrifos, cyfluthrin, diazinon, fenoxycarb, methiocarb, naled or sulotepp. The proper application of these insecticides vary; the law requires that instructions be printed on the label.