Answer: Thrips are the major insect problem of gladiolus. They are tiny insects that are difficult to see with the naked eye. They are slender black insects about 1/25 inch long. Thrips cause white streaks on the leaves. Flowers often are misshapen, streaked, discolored and occasionally will fail to open. To protect gladiolus spray or dust plants with malathion or carbaryl (Sevin) when plants reach 6 inches in height. Continue treatment every seven to 10 days. Thrips can overwinter on stored corms. After harvesting and curing, shake gladiolus corms in a sack with a small amount of carbaryl (Sevin) dust. Use 1 to 2 teaspoons for each 100 corms.
A non-chemical method of controlling thrip build-up is to soak the corms in very hot water (not boiling -- about 160F) for about 2 minutes. Try to plant them in a different location each season to help control thrip populations, but also to control Fusarium wilt and other viruses. Immediately lift and destroy any plant that turns yellowish and looks stunted.
Monarda, or bee balm, is a favorite of caterpillars and bees. Leaf cutter bees especially like the plant. I would caution you against using Sevin. It will kill bees, which defeats the purpose of planting bee balm to attract bees to your garden. Bees are important pollinators, both for your garden and for your neighbor's gardens.
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