Thrips—The Almost Invisible Garden Foe
Q: Mike: This past season, the leaves of my gladiolas developed stripes and the flowers were deformed (on the few that lasted long enough to get a flower stem!). I finally diagnosed the problem as thrips and want to know what to do now that it's time to put the bulbs in storage for the winter. One person said they should be soaked in insecticidal soap before they're stored; another said to dip the bulbs in boiling water before they're planted next year. Is either correct?
- ---Barb in Allentown, Pa
- ---Janice in Piney Flats, Tennessee
- ---Glenn in Jeffersonton, VA
Because they are so small (between one-twenty-fifth and one-fiftieth of an inch long) the best way to diagnose a thrips problem is by the damage they cause. (And yes, a single one is still called thrips, not thrip.) These minute menaces cause attacked leaves to have a distinctive streaky or speckled silvery appearance (the result of their having all the moisture sucked out of them). Flowers turn brown at the tips and don't open properly.
Now, thrips are also extremely thigmotaxic (a twenty-dollar word meaning they hide deep inside flowers and leaf crevices), so you probably won't see any unless you trick them into showing themselves. Hold a big piece of white paper underneath while you shake a plant with suspicious damage and if thrips are responsible, some will drop down onto the paper. They're fast moving, long and thin, but only about the size of a period at the end of a sentence.
They are also the rabbits of the insect world! At least 10 generations will trouble you in a typical Northern summer. You'll get double that number of dastardly descendants down South, and they're pestiferous 24/7 in greenhouses.
As with aphid infestations, sharp sprays of water are a good start. Cradle the affected part of the plant with one hand first thing in the morning, set your nozzle to a sharp stream and shoot it into the little crevices they're hiding in. Older gardening books also recommend the opposite; dusting diatomaceous earth onto dry plants in the middle of the day, being careful to get this all-natural desiccating agent into the petals and crevices.
Thrips are attracted to both bright blue and bright yellow sticky traps. In greenhouse situations, use both colors. Outdoors, stick to blue, says Dr. Linda, as yellow may also capture beneficials. Hang lots of those blue traps early in the season, to try and capture the first run of adults before their populations can explode.
Luckily, several beneficial insects prey on thrips. The best, says Dr. Linda, is the wonderfully named minute pirate bug ("Argh, Matey!"). Linda says she's seen these helpful creatures completely control gladiola thrips (that had destroyed the glads a year before) after the pirates' favorite plants—sweet alyssum, thyme and daisies—were planted in between the glads.
So if you see small (about an eighth of an inch long) dark bugs with a distinctive black and white pattern on their wings, leave them alone; those be pirates that will gleefully plunder your ships—eh, thrips. (You can also buy pirate bugs for release, but this is generally only done for greenhouse infestations.)
Other insects that prey on thrips include lacewings, ladybugs and the also-wonderfully named 'big-eyed bug.' Having lots of the plants known to attract these and other beneficials in your garden may prevent your ever having to deal with thrips. You can also buy ladybugs and lacewings for release in greenhouses or outdoors (follow the directions carefully) and you can purchase beneficial nematodes to water into your soil to control the thrips in their pre-adult stage.
OK, now that gladiola storage question. There may be a few eggs on those attacked corms, but the majority of next year's problem is overwintering outdoors in plant debris. So clean up all the plant trash in that area and collect and compost any mulch.
Brent Heath of 'Brent & Becky's Bulbs' grows a lot of gladiolas at their Virginia headquarters, and says that the only time they see thrips is when the glads have been stressed. He suggests you plant your corms a little deeper next season to keep the roots cooler (this also helps keep the top-heavy plants from falling over), give them a compost rich soil, lots of air flow, and protect them from heat and drought. Healthy, happy plants are seldom bothered by pests.
And please don't boil your bulbs, adds Brent. Just wash them in the kitchen sink with a little soap and gentle scrubbing next Spring. Then rinse the corms well before planting—which you should do in a different area of the garden if at all possible.