Thrips: The No-See-Sum Plant Pest

Onion thrips (Thrips tabaci), top, and western flower thrips (Frankliniella occidentalis), bottom are typical thrips, none to easy to tell apar! Photo: Thrips: www.gardentech.com

Thrips are tiny insects that are mainly active at night. As a result, you typically see the damage they cause well before seeing the insects themselves. 

They attack flowers and leaves, piercing their cells to suck out the liquid. The cells then fill with air, giving the damaged area a sort of silvery or bleached appearance. Since thrips tend to gather together, you’ll usually find irregular patches of silvery or beige tissue on broad leaves and flower petals and silvery stripes on lance-shaped leaves. The damaged parts look as if they have been rasped. You’ll also likely see their excrements before you see the insects themselves: small black deposits are readily visible on the affected parts. 

In addition to damaging the plant by sucking its sap, thrips can do even greater damage by transmitting plant viruses from one plant to another.

There are more than 6,000 species of thrips, the most common in homes and gardens being western flower thrips (Frankliniella occidentalis), eastern flower thrips (Frankliniella tritici), greenhouse thrips (Heliothrips haemorrhoidalis), onion thrips (Thrips tabaci) and gladiolus thrips (Taeniothrips simplex). They can be very hard to tell apart and even experts are often fooled.

Depending on the species, thrips may attack either foliage or flowers and flower buds. Some species harvest pollen (the sight of loose pollen on an African violet flower is usually a clear sign of a flower thrips infestation), while others attack roots and bulbs underground. 

There are also beneficial thrips that are predators on other insects, including other thrips, although plant-damaging thrips are far more numerous and better known.

? Helpful Hint
There is no such thing as a thrip: thrips, with an “s” at the end, is both singular and plural. So … one thrips, two thrips, red thrips, blue thrips.

Identifying the Culprit

Thrips are hard to see unless you have very good eyes, but try blowing on the infected plant part. This usually sends them scurrying about and you should be able to catch the movement. Why they react this way is a mystery: maybe they find humans have bad breath! This is certainly the best method of spotting flower thrips especially, as otherwise they often work sight unseen inside flowers and flower buds.

In spite of their wings, thrips are not great flyers, usually jumping from plant to plant or coasting with outstretched wings. Outdoors, they are carried far and wide by the wind.

Usually, thrips are more obvious and cause more damage when plants are already stressed something else: a chronic lack of water, low atmospheric humidity, hot temperatures, etc. On the other hand, rainy summer weather can seriously reduce their number.

Know Thy Enemy

The life cycle of a thrips depends on the species as well as the host plant, the weather and lots of other factors, but it’s still possible to draw a portrait of a typical situation. 

Outdoors, adult thrips generally overwinter in plant debris, bark or other material. They become active in early spring and can lay 60–300 eggs over several weeks, most often in plant tissue they pierce for that purpose. There may be female and males, but some species are parthenogenic, so females don’t require fecundation to produce viable eggs.

These eggs hatch after 3 to 5 days, and the nymphs then feed for 1 to 3 weeks before descending into the soil or leaf litter. There they molt and become prepupae, then pupae, a stage that lasts 1 to 2 weeks. During this period, they cause no damage. The adults then climb back up to the vegetation above to feed and reproduce. Adults rarely live more than a month.

As you can imagine, these leads to multiple stages being present at once on the same plant. There can be up to 15 generations per year in the open air; even more indoors where there is no off-season.

Host Plants

Many thrips are very picky about what they eat, but, other than the extremely specific gladiolus thrips, the ones most gardeners have to worry about have a wide host range. Here are some of the plants they are most likely to infect:

Houseplants: African violet, avocado, azalea, begonia, brugmansia, chrysanthemum, croton, crassula, cyclamen, dieffenbachia, dracaena, ficus, fuchsia, gerbera, gloxinia, hibiscus, impatiens, orchid, peace lily, pelargonium, poinsettia, streptocarpus, syngonium, yucca.

Edible plants: apple, asparagus, basil, bean, blueberry, carrot, cherry, cabbage, corn, cucumber, garlic, grape vine, leek, onion, pea, pear, pepper, potato, raspberry, strawberry, tomato.

Annuals, perennials and bulbs: aster, carnation, chrysanthemum, dahlia, datura, gladiolus, impatiens, iris, lily, peony, petunia, pelargonium, snapdragon, sweet pea, squash, verbena, zinnia.

Woody plants: aralia, birch, citrus, hydrangea, linden, maple, privet, rhododendron, rose, willow.

The only groups of plants that rarely seem to serve as hosts for thrips in home plantings are ferns and conifers.

Control

Controlling thrips is made extra difficult in that they are not accessible to pesticides throughout their entire cycle. At any given time, part of the population will be well sheltered in the tissues of the host plant as eggs while another part will be pupating in the soil where insecticides can’t reach them either. So, even if you carefully spray an infected plant from top to bottom with an appropriate product, there will always 2 generations that are out of reach. That’s why thrips always seem to pop out of nowhere just when you think you have them licked. 

You’ll therefore have to repeat your applications (insecticidal soap, neem oil, horticultural oil, pyrethrin or just about any other insecticide will do) every 5 to 7 days until you see no more of them … and that can sometimes take months!

African violet growers find they can eliminate flower thrips by systematically removing all flowers and flower buds … for a full 3 months.

You can also use commercial or home-made sticky traps to catch adults. Usually, these traps come in yellow and that color will work, but thrips are even more attracted to blue. Online and in bigger garden centers, you can often find blue sticky traps designed specifically for thrips. Thrips are also attracted to light and readily enter lighted insect traps.

Or practice exclusion. A floating row cover, combined with crop rotation, can serve as a thrips barrier in the garden. Indoors, isolate new plants for at least 40 days, then inspect them carefully before placing them with other plants. 

Covering the soil with a reflective mulch, usually a thin sheet of silver, gray or white plastic with a shiny surface, also seems to be effective in keeping thrips off. It has be installed beforehand, then punch holes into it so you can transplant seedlings. Since reflective mulch is aesthetically somewhat questionable, it’s usually reserved for gardens where appearance is of little importance, such as a vegetable garden.

Also, gladiolus thrips overwinter on gladiolus corms stored indoors. Storing the corms in an extra-cool place (between 35 and 40?F/2 to 4?C) for at least 6 weeks will help eliminate them, as they can only tolerate cold temperatures for short periods.

The Enemies of Our Enemies…

In outdoor gardens, beneficial insects often help to control thrips. Earwigs, ladybugs, lacewings and pirate bugs are efficient thrips predators as are several mites. Certain species of soil nematode are also known to attack thrips while they pupate underground. The presence of predators explains why thrips infestations outdoors sometimes seem to sputter out all on their own.

There are also thrips predators that can be released in a garden or greenhouse environment, including the beneficial mite Amblyseius cucumeris, but they generally adapt poorly to use on houseplants raised in typical homes, largely because the air there is too dry for their taste.

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Thrips: they may be tiny, but they can cause a lot of damage. The faster you react to them, they easier they are to control.