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A New Wrinkle in Iris Borer Control

© Mary Henry

Now that it's August, it's time to do the Iris thing again. Here in Minnesota (and I suspect elsewhere in the colder zones), the bearded Iris need to be dug, inspected, divided and reset during late July or early August each year. The reason for this annual chore is the Iris Borer, a nemesis I never met in the South. Only the German bearded iris, Iris germanica, its hybrids and other species and hybrids that have surface rhizomes such as Iris cristata are susceptible. This pest comes to our gardens in spring as a moth that lays eggs on the emerging Iris foliage. The eggs hatch into larvae that tunnel into the new leaves and feed like leaf miners early in the season. As the ugly beasts grow, they migrate down into the rhizomes and continue to feed. They are now nearly an inch long so they can do a great deal of damage. As August turns to September, the now fat, happy Iris destroyers leave the rhizomes and pupate in the soil preparing to emerge and start the cycle over in spring.

Up until now, there were not many alternatives for control. Most people opted for the chemical method as the easiest, but the chemical of choice is cygon, a systemic insecticide that has many environmental implications and a host of warnings for its use. It must be sprayed in spring on the young foliage when it is 4 to 6 inches high. The natural method of control is to watch vigilantly for signs that there are borers tunneling in the leaves early in the season and to smash them by pinching the leaf in the appropriate place. At this time each year, the rhizomes need to be dug, checked for borer damage, divided and reset in a bed that has been closely raked and checked for any pupa that can be found. Any rhizomes that were damaged but seemed salvageable should be trimmed carefully to remove rotted material and soaked in a 10% solution of chlorine bleach for a few minutes. After they are reset, Irises should not be mulched so that the over wintering insects have as little protection as possible.

This summer, I gave up. In early July I found that more than half of my plants had borer infestations that had gotten beyond the pinch 'em stage. I ripped them all out and made compost of them. Then I found out about a better way!!!

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