Late Summer!
Old ways still work on old fashioned bearded iris and
their borers
Dear Janet,
My neighbor's giant black iris' that I gave her are all
infested with those iris borers. She suggested that I check mine.
Yes, some of mine are infested although not as bad as hers. Have
you had any experience with this? I checked the web and found
different ways to treat it. Some say to pull them all out and soak
in bleach or Epsom salts then to treat the soil with certain
insecticides. They are the biggest ugliest looking grub/worm thing
I've ever seen....and so many eggs!
C.
Dear C.,
Iris borers are big and ugly when they are old enough to have
eaten all the way down into the root but they don't require weapons
of mass destruction. Also, they're "normal" for iris. If you grow
bearded iris eventually you will deal with borers because the moths
whose young are iris eating caterpillars will find those
plants.
Each fall, these moths emerge from the soil, having rested there
since August as pupae -- that state in between caterpillar and
winged adult. The moths lay eggs on iris foliage. Where conditions
favor the borers or weaken the iris, many of the borer eggs survive
to hatch in spring and the borers eat without disturbance all
summer. Then, the damage to the plants is bad. In other places,
even just across the street, the irises may be tougher, natural
predators of the borers more active or the gardener more savvy so
there is less damage.
Borer damaged iris are small and weak, the leaves have dead
streaks in them and the plants fall over, destabilized by rotting
roots or chewed leaf bases. Late July and early August is a time to
deal with them. Lift the iris, then cut off and discard parts of
the root that have succumbed to a bacteria called soft rot that
enters through borer holes. Cut off parts with grubs in them,
squish the grubs or leave them where the birds can get them. You'll
know the parts to cut because soft rot stinks and the grubs at this
stage are 1-1/2 to 2 inches long so they and their holes are easy
to spot.
You can dip the roots before replanting in a 10% bleach solution
to clean inside remaining holes and cut edges. Alternatively, you
can leave the salvaged roots in a warm place in the sun for a day
or two -- gardeners where I grew up used to spread them on the
concrete sidewalk.
Don't put pesticide in the soil. Even if you can get an
insecticide to thoroughly penetrate the top few inches of the soil
so it reaches all the pupae, it's nigh impossible to kill them
because they're old, tough and in a resting state. In addition,
you'll kill other insects in the soil, many of which are beneficial
in this or other plant-insect systems working in your garden.
Instead, aim for the borer's weak link. In late fall or early
spring (before April when borer eggs hatch), cut off and discard
all iris foliage. That eliminates a lot of the eggs. If the
previous year's damage was very bad in spite of the fact that you
had done a good foliage clean up, you can use an insecticide to
spray the iris foliage when it's 4" tall, about two weeks later
when it's 10" tall and again just as you start seeing the flowering
stalks rising up. Almost any insecticide will do since you're
killing tiny larvae as they hatch, before they can start eating
into the leaf. Using a systemic insecticide such as Cygon can
reduce the number of sprays -- systemics stay inside the leaf to
kill the borers as they first start to eat.
Wildlife lovers, avoid systemics. These toxins become part of
the flower as well as the leaf and can sicken nectar drinkers,
including hummingbirds that feed at iris.
Some people cut off and hot compost the top half of each iris
leaf after bloom, in June. At that time of year borers are still
chewing their way down inside the leaf toward the root so this
practice may catch some insects that escaped earlier
preventives.
About the eggs you mention. There shouldn't be any eggs, which
only come from adult moths and are so tiny you can't see them
without a magnifier. You're probably seeing frass -- insect
excrement or bug poop.
More about iris borer...
Green thumbs up
to that spunky gardener who stood up to her tree trimmers and
refused to let them use climbing hooks to ascend and prune her
trees. Those so-called experts tried to tell her that's acceptable
practice but any certified arborist current even to 1990 standards
will confirm that hooks, which pierce the bark and open the tree to
infection and insect attack, should not be used unless the tree is
being removed.
Green thumbs down
to spraying insecticide when you don't know that insects are the
problem. Even when you're certain an insect species is seriously
threatening your plants, don't spray at any but the time of year
when that insect can be effectively controlled. Find that time in
pest control books. In the hot sun, many pesticide sprays can do
more damage to dry leaves and stressed plants than to dormant or
absent pests!
Originally published 8/23/03