The Rural Voice, 2002-08, Page 481
Gardening
Battling grubs in mg iris garden
Rhea
Hamilton -
Seeger and
her husband
live near
Auburn. She
is a skilled
cook and
gardener.
By Rhea Hamilton -Seeger
I have embarked on a crusade to
rid my garden of iris borers. My
grand show of lavender bearded iris
began to flop over about a week into
its blooming and I blamed the wind
and rain. As I clipped a few stems to
bring into the house to enjoy 1
noticed the base of the leaves were
goofy looking. That is a gardening
term for brown and rotten looking.
Upon closer inspection I realized I
had quite a few iris borers munching
their way down my iris leaves,
weakening and forcing the plant to
fall over. I encountered iris borers a
couple of years ago and was amazed
at the damage they can do in a
garden. They haven't changed.
Iris borers are the larvae of a
small, grey/brown night -flying moth.
The moth lays her eggs on iris debris
in the fall. The eggs overwinter and
hatch early in May and the 1/16 -inch
grubs burrow into the young
succulent iris leaves. You can see the
small pin holes in the leaves.
The larva gnaw out the soft tissue
between the upper and lower surface
of the leaf, crawl inside and become
leaf miners. The grubs will eat each
other until one super grub is left to
continue burrowing through the
leaves down into the rhizome. The
sap from the damaged leaves attracts
other insects.
By the time the grub enters the
rhizome, it will have grown to about
one and a half inches with a reddish -
brown head and a body that is
distinctly pink. It will eat out a cavity
and then exit into the soil to find a
place to pupate. In about five weeks
it becomes a moth taking to the skies
in September. The cavity left in the
iris root or rhizome becomes an ideal
place for bacteria and rot to set in.
The one striking impression I had
of the iris patch while it was suffering
from this ruthless grub was the smell.
The rotting material was very strong
and after 1 had cleaned out the patch 1
could still smell it when 1 walked past
a day or two later.
There are four key controls that
you can use against this pest. The
first, and most important, is
cleanliness in your garden. Iris, and
for that matter peonies too, should be
kept very clean. Keep all dried or
decaying leaves as well as any other
debris from surrounding plants
cleaned up. Plan on both
a fall and spring
clean up. Some
hybridizers
will go so far as
to lay old straw
on their iris
patch in the
fall and set it
aflame. I don't
think I would
recommend
that. Some
varieties of
iris won't survive.
The second control measure is to
sharpen your observation skills and
check your iris during their growing
season starting in mid-May. You
have to look for any signs of the
grubs in their early stages. Watch for
notched leaves, a shiny appearance at
the base of the leaves caused by sap,
or sawdust around the base. You can
run your fingers down the leaves
pinching as you go and hopefully
squishing the small grubs. or you can
use a knife and dig them out and
grind them into the dirt.
The third control is nematodes.
We are, by now, familiar with the use
of nematodes in controlling grubs in
our lawns. Those same parasites can
be used as a biological control of
grubs in your iris. Research from the
University of Nebraska indicates that
Sc (Steinernema carpocapsae) did the
best job in eliminating the grubs of
iris borers.
The fourth control is to set up a
spraying program with Cygon. This is
a systemic pesticide applied three
times in the spring, once the first sign
of iris growth (or when the tulips
bloom) and 10 days later and again
10 days later. This should rid you of
almost 100 per cent of the iris borers.
But with any chemicals in the garden
there are hazards. The active
ingredient in cygon is dimethoate
which is toxic to wildlife and aquatic
life so do not use near your pond. Be
careful of drift. It is toxic to bees.
those invaluable insects in your
garden. My first recomendation is to
use the first three controls and use the
chemical as a last measure in severe
circumstances.
Once you have diagnosed that you
have iris borer, dig up the rhizome.
August is the best time, and cut away
the infected area. You can let the
rhizome dry or heal for a few hours
out of direct sun and replant.
Supplement the soil with a healthy
dose of 5-10-10 fertilizer which can
also be applied early in the spring and
again about a month after blooming.
Never apply directly to the rhizome
but around them and then give them a
good soaking. Iris enjoy soil a bit on
the acid side. An addition of dry
wood ashes will add 5 to 25 per cent
of their required potash as well as 30
to 35 percent of lime. Never use
rotted animal manure as it will invite
all sorts of problems including more
grubs. When cleaning up your garden
this fall, don't be too ambitious when
you are trimming out brown or
spotted leaves. Leave the green
leaves as cutting them may affect
next year's blooms.
Happy grub hunting.0
BERNIE McGLYNN
LUMBER LTD.
BUYER OF HARDWOOD BUSHLOTS
Wholesaler - Hardwood Lumber
Box 385.11R 2.
Wingham, Ont NOG 2W0
BERNIE McGLYNN
Ph/Fax (519) 357-1430
SAWMILL -
R.R. #5, Mildmay, Ont.
(519) 367.5789
1\
AUGUST 2002 45