Yesterday was our annual “spray for borers day”. This is an
event known mainly to irisarians in the USA who live east of the Rocky Mountains.
Quite why the iris borer has never crossed the continental divide to torment
iris growers in the West still puzzles me and seems entirely unfair. As we Easterners know so well, the
iris borer is a moth that lays its eggs in and around iris patches The little larvae (caterpillars) hatch
out in the first warm days of spring and start eating
the iris foliage. If that was all, it could be bearable, but very soon after they
start feeding, they chew their way inside the leaf and head down into the
rhizome, there to cause their destruction hidden well away from sight.
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| A young iris borer is revealed as it starts it's journey of destruction. |
Despite some “common knowledge” to the contrary, iris borers
quite happily feed on many beardless irises (including Siberians) as well as the
bearded ones. They grow to be large (2” long) and ugly, and they have enough appetite
to destroy whole rhizomes, particularly the smaller ones of irises like Siberians. Their feeding damage also opens up the rhizomes to
bacterial rot diseases. Not only that, they bred freely, are adept at locating
irises, and unless kept under good control can play havoc with iris plantings.
They are not an adversary to underestimate.
So how to keep on top of these insects? The usual advice is
to maintain a clean iris patch, removing old leaves and other materials where
eggs may be laid. Probably this helps but it is not sufficient alone. For one
reason, the small larvae spin silk threads and can travel quite long distances blown
on the wind – they are not just arising in your back yard..
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| Three months later - a fat worm and a rotten rhizome (U. Minnesota photograph) |
Now I have to make an admission here. I have spent my
professional career working with insecticides – trying to find out how they
kill insects, how insects become resistant to them, and generally studying their
toxic effects in both insects and mammals. I regard them as useful tools rather
than the devil’s work, but tools that demand respect – as you might be careful working
with axes, chain saws and tall ladders. With any reasonable care they are quite
safe to use (protect hands and eyes, and if any significant amount of the
concentrate gets on your skin, wash it off within a few minutes). So I have no problem
with using chemical controls for the borer. Others do have these concerns for
various reasons, but unfortunately, there are few, if any, effective alternatives
for anyone growing a significant number of irises.
I still use the old reliable Cygon (common name dimethoate), a plant systemic
insecticide from the older organophosphate class. “Systemic” means that the
insecticide is absorbed into the plant and kills the insect as it feeds on the
plant tissues. The borer caterpillar’s habit of rapidly heading inside the iris
leaves makes the use of non-systemic insecticides inefficient – the larvae are
only exposed on the surface for a short time and thereafter surface insecticide
residues don’t affect them. You can see this in the first photo where the larva was already feeding on the leaf margin inside the sheath before these were separated to reveal it at work. Their sneaky cryptic behavior probably helps to
protect them from natural controls like parasites and predators too. Cygon remains
very effective for us, but in 2002, in the overall drive by the US Environmental
Protection Agency to reduce the use of organophosphates, it was restricted to
use by commercial producers and it is not now available for homeowners. It
does have moderate toxicity to humans and it did have a record of causing
poisoning symptoms (no fatalities) in homeowners, primarily through contact
with the concentrated compound (and failure to remove it expeditiously), so that
limitation was justified, although I would rather have an accident with Cygon
than with a chainsaw where the average number of stitches required afterwards
is about 110 (that's not made up, it's Consumer Product Safety Commission data).
Mostly Cygon has been replaced for borer control by a newer and
safer compound, Merit (the common name is imidacloprid – these names are shown
in the list of active ingredients on the label). This is essentially a synthetic
analog of the old (and now defunct) natural insecticide, nicotine. Imidacloprid
is a very widely used insecticide worldwide. Its big advantage is that whereas
nicotine was a dangerous compound to use (I know two colleagues who spilled
liquid nicotine on themselves during class demonstrations and in both cases
ended up with a trip to hospital. That's why I use a video and not a real life demo of its effects). It is very active in affecting the human
nervous system and, as a liquid, is rapidly absorbed through
the skin. Imidacloprid is very poor at affecting vertebrate nerves but much
better at affecting those of many insects which makes it safe to the user and is
just what you want in an insecticide, and it has excellent plant systemic
activity. Unfortunately my experience after a couple of trials with
imidacloprid is that it isn’t all that great at controlling borers. This really
isn’t too surprising since its best activity is against sucking insects such as
aphids, whiteflies and thrips. It is generally less impressive in controlling caterpillars
such as our borers. Others seem to find it OK though, and use it regularly. There is
not a lot of choice.
However, the main problem with imidacloprid (and there are
several other insecticides in the same class) is that it may be implicated in
the ongoing decline of honey bee populations (colony collapse disorder). This is
a problem at several levels, but at the top of the list is that the loss of bees
as pollinators threatens the production of many crops in US.. This is not the
place to get into this complicated and contentious topic. Certainly imidacloprid
and like compounds are highly toxic to bees, but if they are centrally involved
in this problem, it is probably not through direct toxicity, but more likely by
altering bee behavior and navigation, or their sensitivity to pathogens. Most
experts feel that they are only a part of the problem and may not even be the major
one. For example, episodes of what looks like honey bee colony collapse were being
reported in the US and UK in the 19th century, long before
imidacloprid was ever used, and it appears to be occurring in other parts of
the world where these compounds have never been used. And in some countries
where these compounds are used, no reliable reports of colony collapse seem to
have been established. Viruses, other pathogens, mites that feed on bees, and stress
from the frequent movement of bee colonies around the US as pollinators, as well as these
insecticides, have all been invoked without establishing a clear smoking gun –
and so if there is a single reason for the decline it has yet to be
established. Perhaps there is a combination of stresses here with some insecticides
as one possible component of yet unknown significance. Some current thought
suggests that imidacloprid can make bees more sensitive to pathogens, but this
doesn’t explain all the observations. The mystery remains, as do suspicions
about these insecticides, and for that reason, their future use is under a cloud.
So, the possible involvement of imidacloprid and its relatives
in the honey bee decline phenomenon raises some concerns about its use for
borer control – are these insects safe as we enjoy watching them collect iris pollen or nectar? Currently there are no really effective alternatives available to the
iris grower, but there may be cause for hope around the corner. A new class of insecticides
that paralyze insect muscles (and not mammalian ones) has recently been
developed and they are just now coming into use. These have excellent activity against
caterpillars and seem quite safe to honey bees. The first, with the un-pronounceable
common name of chlorantraniliprole, is now showing up at garden centers in some
grub control products. In this use it replaces imidacloprid. Although I’m not
aware of any specific test that have been published, the more systemically-active
members of this group are likely to be excellent for borer control. Now we need
someone to run these studies to find out, and if they are effective, let's hope that they
are introduced for use in gardens in the near future to replace Cygon and imidacloprid. Although I have no real problems
with Cygon, safer is surely better, you can hopefully worry less about bees if you are now using Merit -- and
Cygon really does smell bad!
