Thousands of Nematodes
Launch An Iris Borer Attack

Ten million worms can be a good thing!

Nematodes are a microscopic round worm, many species of which are parasitic on insects. Steinernema carpocapsae is a species that attacks the larval form of several ugly plant killers. My interest is in their ability to attack the dreaded Iris Borer.

This enemy of my favourite flower, overwinters as eggs on plant debris and then hatches and migrates across the soil until it finds a juicy Iris plant and then it moves in and starts to hollow it out. If the Nematodes are wandering around the garden during this migration then they will find the little borers, infest and kill them. There are a few suppliers of these creatures in the U.S. but now there is one in Canada that makes my life easier. They have sent me a few million of these little worms to try and they have been applied to the soil as per instructions. I will continue to update this page as I chart the success of these beasties in eliminating my resident Iris Borers.

When Do I Do This? I applied them in late May when the Iris where well developed and I’m worried that it might be too late. The borer’s eggs should have hatched and migrated to my Iris already and I don’t know about the efficacy of these little parasites once the borer is comfortably ensconced inside the Iris leaves. They may be able to find the large older larva that hollows out the rhizome but by then a considerable amount of damage has already been done to the Iris plants. We will wait and watch carefully. They need to have a soil temperature of 50 F to become active so cannot be applied too early.

How Do They Get Here? They arrived on a sponge. A relatively small sponge considering the number of little worms that were supposed to be present. The sponge was dropped into a gallon of water and the little creatures migrated into the water to make a stock solution. The directions said they would cover 2000 sq ft and that works out to 20 sq ft for each of my 100 Iris plants. Seems to be more than enough. I filled a watering can with water and soaked around a few Iris clumps to see how many I could do with one can of water. Four seemed to be a good number. Therefore I needed 25 watering cans full of Nematodes to cover all of my Iris. A little quick math told me how many ounces of stock solution was needed in each watering can of water and away we went mixing and sloshing and distributing nematodes around the entire garden. As they are microscopic, we can only assume that each Iris now has a reasonable population surrounding it and we will wait and see what happens.

What Else Will They Kill? This company, Natural Insect Control, also sells a box with a mixture of two different nematodes in it, that should be an effective and totally pesticide free method of controlling the white grubs in your lawn. As those grubs spend at least a year and often two as larva in the soil, the timing of the application should be much less critical. My small turf areas are not my garden’s best feature so I really don’t have a grub problem and therefore can’t accurately assess the effectiveness of this control. I do know that people whose opinions I trust say that it really does work.

Huge disclaimer here. I didn’t actually do any of this. The Nematodes arrived at my garden when I was away playing Grandpa to my twin grandchildren and the "Assistant Gardener," my long suffering wife, was left to communicate with me by phone and then go out and do all of the steps that I have described above. Luckily she has an affinity for Iris as well. My local garden center sells this product and it is available from the company website. $30 to protect all of my Iris or to rid your lawn of grubs with no pesticides seems a pretty good deal.

Did It Work?
Yes! The vast majority of my Iris grew better then they have for several years. 2 or 3 varieties were still attacked by the Borer. Whether they were missed during the nematode application or whether they are just very susceptible varieties, I really don't know at this point. As I had to dig them out anyway to get rid of the mess and to look for borer pupa, I only replanted one of the attacked varieties. We'll see how it fares next year.

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