Unfortunately most plants have at least one type of pest that loves to feed on them. Controlling garden pests is a part of nature, and irises are no different.
The iris borer is one of the most frustrating and destructive pests there is for flower growers. There’s nothing more disheartening than to discover these nasty worms in iris bulbs.
Fortunately it’s a problem that is fairly easy to solve, as long as you are diligent. Plus, there are a few different treatment methods you can use.
In this detailed guide, I’ll show you all you need to know about getting rid of iris borers. Follow the control tips if you already have an iris borer problem, and then find out how to keep them from ever coming back!
Why Are My Iris Dying?
This is a common question I get asked a lot. Unfortunately, many times the reason why irises start dying is because of an all-too-common pest that we can’t easily see – the iris borer.
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So, if your plants are dying, and you have no idea why, then keep reading to find out if this could be your problem too.
What Are Iris Borers?
The iris borer is the larvae of a moth called Macronoctua onusta, which are most often referred to as the “iris borer moth”.
Macronoctua onusta is commonly found around North America. The larvae are a very destructive pest, though they are harmless in their adult form.
They get their name because the larvae bores into iris bulbs, and eats them from the inside out. This feeding habit makes them impossible to see, and difficult to get rid of.
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What Do Iris Borers Look Like?
In their adult form, iris borers look like any other moth you would see fluttering around at night. They are about 2″ long, with brown wings.
Iris borer larvae on the other hand are easy to identify, they look like pink caterpillars. They’re tan to light pinkish in color, with a dark red head, and tiny dots all along both sides of their body.
When you disturb them, they sometimes excrete a yellow-colored substance from their mouth. Of course, the most telling characteristic of all is the fact that they are found inside of an iris bulb.
Iris Borer Life Cycle
There are four phases to the iris borer life cycle: eggs, larvae, pupa, and adult. Female moths lay their eggs on the leaves in the late summer and early fall, which is where they overwinter.
The eggs hatch in early spring, and the borers emerge. These tiny larvae burrow into the bulb shortly after hatching, where they feed until they are large enough to pupate.
Once large enough (sometime in mid-late summer), the larvae move into the soil to pupate for a few weeks. Adults emerge sometime in the late summer or early fall, and begin laying their eggs.
Where Do Iris Borers Come From?
Since the moths can fly, they can come from anywhere. The moths are naturally attracted to irises, since that is their main host plant.
So unfortunately, if Macronoctua onusta moths are present in your area of the world, then it’s a pretty sure bet that they will eventually find your irises.
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What Do Iris Borers Eat?
Iris borers burrow into the bulbs, and feast on them from the inside out. They can eat any type of iris species, but bearded varieties seem to be their favorite.
Though they don’t eat the foliage, that is usually their main entry point into the bulb. So, you will likely see a hole somewhere at the base of leaves.
Iris Borer Damage To Plants
Unfortunately, these sneaky little devils do their damage underground. So, you won’t notice it right away.
The first sign you will probably see is that the leaves are turning brown, and the plant looks like it’s dying. But even then, it takes a keen eye to spot the signs of an iris borer infestation.
Signs Of The Iris Borer
If you have an iris plant that is turning brown, then get down on your hands and knees to take a closer look. Here are a few sure signs of an iris borer infestation…
Tan colored streaks on the foliage in early summer
Leaves that turn brown in the late summer
Holes in the leaves towards the base of the plant
Mushy stuff that looks like sawdust around the bottom of the plant, or the tops of the iris bulbs
Stem of the plant is mushy, and seems to be rotting
Soft or mushy bulb that appears to be rotting
Pinkish or tan colored worms inside of iris bulbs
Organic Iris Borer Control Methods
Once the iris borer has entered a bulb, pesticides will have no effect on them. But there are a couple of things you can try to kill them before they bore into the bulb.
Beneficial nematodes
These predatory parasites can attack and kill the iris borer larvae. There are lots of different species of beneficial nematodes, so look for one that preys on caterpillars.
In order to be effective, you must apply them in early fall, before the iris borer eggs hatch. Learn how to use beneficial nematodes here.
Spinosad Spray
If you’re looking for an all natural iris borer insecticide, then try using spinosad spray. Made from natural soil bacterium, spinosad can kill the larvae before they enter the bulb.
However, there’s a very short window of time that it will work. It must be sprayed when the eggs are hatching, before the larvae bores into the bulb. So it can be difficult to get the timing right.
How To Get Rid Of Iris Borers
Unfortunately, the only way to get rid of iris borers that have entered the bulb is to manually remove them. There are two ways to do that…
Dig Them Out By Hand
This is the most disgusting method you can use to get rid of iris borers, but oddly the most satisfying. Dig up the infested bulb, and find the spot where the borer entered.
Then carefully cut the hole larger, until you see the worm. You can either pluck it out of the bulb, or stick a sharp object in there to squish it.
I pull them out, and then drop them into a container of soapy water. They can’t swim, and will eventually drown, but it takes a long time in water alone. The liquid soap will kill them faster.
You can replant the bulbs if the damage isn’t too bad. But, if they are badly damaged or rotting, then it’s best to just discard them.
Soak Iris Bulbs In Soapy Water
If you don’t like the idea of digging the nasty worms out of your iris bulbs, then you can take more of a passive-aggressive approach.
Dig up the bulbs, and place them in a bucket of soapy water (this soap kills them faster). Some people like to use bleach for this method, but there’s really no need to use such a harsh chemical.
Once they’re submerged in water, the borers will exit the bulbs, and then drown. Soaking them overnight should do the trick, but you could leave them for a day or two, just to be sure.
How To Prevent Iris Borers From Coming Back
Getting rid of iris borers can be a very manual, gross, and frustrating process. So, it’s best to try to avoid them all together, right?
Preventing an infestation is actually very easy. In fact, my irises have been borer free for over 10 years!
Grow Resistant Varieties
Though the iris borer can feed on any type, Siberian varieties are more resistant to them. Once I learned this trick, I planted a bunch of Siberian irises in my yard.
They are just as gorgeous in the garden, and I’ve never had any issues with the borers attacking them. Woohoo!
Cut Back Your Irises In The Fall
This is my preferred method for preventing iris borer infestations in my garden, and it’s simple. As I said before, the moths lay their eggs on the leaves in late summer, where they overwinter until spring.
So, the best way to avoid iris borers is to cut down your plants in the fall. If you don’t have time to cut them back in the fall, then do it as soon as you can in late winter or very early spring. Here are the steps…
Step 1: Cut down the leaves – Using a sharp pair of pruners, cut the leaves all the way down to the tops of the bulbs. Take care to cut them back as far down as you can.
Step 2: Remove all the cuttings – Remove all of the leaves and debris that you can, so none of it is left in your garden. If you leave any of the cuttings in your garden, the eggs could still overwinter on them, and you don’t want that.
Step 3: Toss out or destroy the leaves – After you cut back all of your plants, throw the waste into the garbage, or burn it. Do not put iris foliage into your compost bin, because the borer eggs could just overwinter in there.
In this section, I’ll answer some of the most frequently asked questions about the iris borer. If you can’t find your answer here, then ask it in the comments below.
Will neem oil kill iris borers?
Yes, technically neem oil would work to kill iris borers. However, since the borers feed on the inside of the bulbs, rather than on the leaves, it’s not a very effective treatment method.
If you do want to try it, be sure to treat the base of the leaves several times before the borer is able to enter the bulb.
Getting rid of iris borers is not difficult, though it may be a bit cringe-worthy. That’s why it’s best to prevent an infestation in the first place. Now that you know the easiest way to control iris borers, you can feel confident that your flowers will be much safer.
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Share you iris borer control tips in the comments below!
