Though nearly invisible, thrips can cause enormous damage to gladiolus. Here’s how to identify and control them, with thanks to the experts at the Clemson Extension Home and Garden Information Center (hgic.clemson.edu), the University of Florida IFAS Extension (edis.ifas.ufl.edu), and growit.com.
What to Look For
Adult thrips are slender, winged insects about 1/20 to 1/16 inch long. Nymphs are even smaller. Most gardeners never even suspect they have them till their glads start looking sick.
“Both adults and nymphs . . . feed by scraping surface cells to suck plant sap. When they feed on flower buds, the flowers may die without opening. With light infestations, their feeding causes leaves to have brown or silvery speckles or streaks. With severe infestations, leaves and flowers are stunted and distorted and may turn brown and die.” (Clemson Extension Home and Garden Information Center)
“Flowers or leaves suspected of being infested with thrips should be shaken over a white sheet of paper to better observe the insects. A 10x magnifying glass or hand lens will aid in their detection. . . .
“Thrips may be seen on foliage during overcast days but hide in flower buds and beneath leaves on sunny days. [They’re found] wherever gladiolus are grown but also attack iris, . . . lily, narcissus, freesia, . . . tomato, begonia, snapdragon, chrysanthemum, and geranium. Females lay up to 200 eggs in leaves, and larvae develop rapidly. Nine or more generations may occur outdoors each year.” (University of Florida IFAS Extension)
Ladybugs and Sticky Traps
We usually recommend non-toxic methods first, but thrips can be tough to beat without chemicals.
“Several naturally occurring enemies feed on thrips, including green lacewings and ladybird beetles. To avoid killing these beneficial insects, which naturally reduce thrips populations, insecticides should be avoided as much as possible.
“Bright blue or yellow sticky traps will provide some protection from thrips. Paint cardboard or wooden boards and then coat with petroleum jelly. Attach them to stakes and place near the flowering bulbs. The adults are attracted to the yellow or blue and get caught in the petroleum jelly.” (Clemson Extension Home and Garden Information Center)
Rotating where you grow glads in your garden from year to year can help, too.
Insecticide Sprays and Systemics
Once thrips become established, most gardeners find they have to resort to insecticides to control them. And to protect the flowers, you must start treatment before buds emerge.
“If thrips are beginning to damage plants, apply [acephate (the active ingredient in both Ortho Orthenex Garden Insect and Disease Control) or carbaryl (in Sevin). Many experts recommend alternating these two to help prevent the thrips from developing resistance.] Treat foliage or flowers as soon as thrips are found. Weekly applications may be needed until control is achieved. Spray the plants to the point of run-off. Be especially careful to cover the undersides of the leaves. Continue to inspect the plants periodically and apply an insecticide if plants become re-infested. [Insecticidal] soaps are safe and effective.
“Several systemic insecticides [such as Bayer 2-in-1 Systemic Granules or Ortho Systemic Insect Killer] are applied as soil drenches, so that the roots take up the toxicant and spread it to where the insects are feeding. They may [take several weeks to achieve control] but are effective for a longer time than most contact insecticides. [Systemics can also kill pollinators, so avoid them once your glads start to bud.]
“Be sure to wear the appropriate protective clothing when using insecticides, as described on each container’s label. Read and understand each label before doing an application.” (University of Florida IFAS Extension)
Knocking Them Out in Storage
The best time to eliminate thrips is often during winter storage. Here are three simple and effective methods:
“l. Temperature: Low temperatures for storing the corms will kill thrips. Corms should be kept at 35-40° F [40-45° F according to other authorities] for at least four months. DO NOT freeze the corms.
“2. . . . Napthalene flakes: In treating large lots, sprinkle the flakes among the corms in storage using one ounce to every 100 average sized corms, or l pound to 2,000 corms. Cover the corms with a light canvas or wrapping paper. For small lots, place the corm in tight paper bags, scatter the flakes over them and fold the top of the bag to retain the fumes. Suspending the bags from beams in the attic or cellar helps to prevent rotting. DO NOT USE A CLOSED CONTAINER . . . , as it may cause the corms to “sweat” and sprout. The corms should be dry before adding the Napthalene flakes.
“3. Dipping or Soaking [Before Storage]: Lysol may be used; 4 teaspoons per gallon of water is the recommended rate. Soak the corms for 6 hours. [Other authorities recommend 1¼ tablespoons of disinfectant per gallon of water and soaking for three hours.] Dry corms [thoroughly!] before storage.
“If you dip in the spring, plant the corms immediately after treatment.” (growit.com)
We hope this helps! If you have other observations or advice for combating this challenging pest, please email us.