I first learned about ‘natural’ borer control from Nancy McDonald, co-editor of ‘Roots’, the bi-annual magazine of the Historic Iris Preservation Society (HIPS). HIPS works to identify, locate and preserve historic iris cultivars that are in danger of being lost forever. You can learn more about their work at www.HistoricIris.org.
When irisarians mention iris borers they’re usually referring to the larval, or caterpillar stage of Macronactua onusta. The iris borer has four life cycles, beginning as a tiny egg, typically laid on mature iris leaves in August and September.
Eggs will overwinter on the iris leaves and begin to hatch in early spring as soil temperatures reach 45 degrees to 50 degrees F. This is one good reason to clean your iris beds of old, dry leaves in late winter. Never use old iris leaves for mulch or compost. You should burn the leaves (if that’s allowed in your area) or bag them up and send them to the landfill.
When the borer eggs hatch the tiny larva move across the soil in search of new, succulent iris leaves. These larvae bore into the leaves and begin to tunnel through them. By mid-summer the caterpillars will have grown to their full size of 1½” to 2” in length and have worked their way down to the rhizome.
In late July to August the mature larva will exit the rhizome, move into the soil, and begin to pupate. In a few weeks, the adult borer moth will emerge from the soil to complete the life cycle, with female borers laying a new generation of eggs that will hatch the following spring.
The iris borer can introduce bacteria into the rhizome that results in soft rot. In addition, the borer holes left in the rhizome can also provide an entrance point for fungus or bacteria. The larval stage of the iris borer is the most destructive insect pest of irises.
The adult moths are a chocolate brown with a wing span of about 2”. Being moths, they fly at night so are rarely seen.
For those who don’t wish to use pesticides there are two species of nematode, a soil-dwelling roundworm, that are effective in controlling iris borers. The nematodes can only attack iris borers when the borer is on or in the ground. Thus, timing of your nematode application is critical.
The life cycle of the iris borer offers two opportunities for these ‘natural’ control methods, which are safe for people, pets and wildlife.
The first control opportunity occurs in early spring as the borer eggs hatch. The tiny larva will be crawling across the surface of the soil in search of iris leaves.
The second opportunity is in late July to August as the mature borer larvae exit the iris rhizome, move into the soil, and begin to pupate.
Two species of nematodes, Steinernema carpocapsae (Sc) and Heterorhabditis bacteriophora (Hb) are available for iris borer control.
According to the University of Maryland, Sc provides 100% control and Hb provides 87% control of iris borere.
Again, your first, best preventative measure is to remove and dispose of old, dry iris leaves in late winter. This will help eliminate iris borer eggs before they hatch.
Then, plan to order nematodes about the time your soil temperatures are reaching 40 degrees to 45 degrees F in spring. The nematodes are perishable and have a short shelf-life. You can keep them in the refrigerator for a week or two but plan to apply them as soon as possible after delivery.
The second application should be made soon after the time the larvae exit the rhizome and move into the soil to pupate in late summer to early fall. Again, the nematodes are only able to attack the borers when the borers are on or in the soil. That application should be made around late August to September in most locations.
Basic application instructions are as follows;
First you’ll need to determine how many nematodes are needed to treat your irises effectively. To do that, calculate the total square feet of your iris beds. You’ll only need to apply the nematodes in the immediate area of each iris clump. Nematodes are very tiny and, although quite mobile, they can only travel so far. Don’t include walking paths in your calculation, only the actual square feet of your iris clumps.
Also, don’t worry about extreme accuracy. Arbico Organics recommends the following:
· 5 million nematodes per 1,600 square feet
· 10 million nematodes per 3,200 square feet
· 50 million nematodes per acre (approximately 43,000 square feet)
You can apply nematodes using a hose-end sprayer, a pump sprayer or even a watering can if your garden is fairly small.
Warning! Do not apply nematodes to dry soil.
If the soil in your iris beds is dry be sure to water first. Nematodes require some moisture in the soil. If it’s too dry they could be desiccated before they are able to move below the soil surface.
As you apply the water/nematode mixture remember that the nematodes are “in suspension” in the solution. The nematodes will gradually settle out so it’s important to agitate the container during application to keep the nematodes fully distributed within the solution as you apply it.
Protect your nematodes!
Be sure to make your application during the early morning hours or late evening so the newly-applied nematodes are not exposed to hot, direct sunlight before they have time to move into the soil.
Make TWO applications for best results.
You don’t need to double the application rate though. Just apply half your nematodes in the first application, then wait about a week to make your second application. This staggers the nematode population and helps improve their effectiveness. (You’ll still want to make a spring application and a fall application, but this technique may improve your results.)
Only mix as much solution as you can apply within an hour or two. Leaving your nematodes in water for two hours or more can kill them. The species used for iris borer control are soil-dwelling, not aquatic.
I buy nematodes from Arbico Organics. I’ll post links below to their product pages.
(I do not have an affiliate relationship with Arbico-Organics. I use their products and believe they are high quality and reasonably priced.)
For Sc nematodes:
For Hb nematodes:
For both: (This is what I use.)
If iris borers have been a problem for you, or if you just want to ensure they don’t become a problem, these nematodes offer a safe and effective control method.
Then you can enjoy the beauty, and the fragrance, of your irises with fewer worries.
Best wishes and happy growing!
Randy Smith