Gardeners are always being challenged to come up with easy, effective methods to deal with pest problems in their gardens. As we learn more about the world, we are also looking for earth-friendly solutions to these problems. Beneficial nematodes can be part of the solution when they are used correctly. Nematodes are microscopic threadlike creatures that live in the soil almost everywhere. In fact, there are more than a half million distinctly different species of nematodes worldwide, and in numbers, they account for the vast majority of all multicellular organisms. So it isn't surprising that some of these organisms are of use to us. Research has made it possible for gardeners to enlist the help of a type of nematode that actually attacks many of the soil pests that plague our gardens. These nematodes are known both as beneficial and parasitic nematodes. There are quite a few books, magazines, catalogs and web sites devoted to organic gardening practices and many of them can provide you with more detailed information about beneficial nematodes.
How do you know which nematodes are beneficial and which ones are garden pests themselves? Scientists have spent years studying and identifying the life cycle of different types of nematodes. While there are nematodes that parasitize garden plants, the species being sold as beneficial nematodes have been thoroughly studied and their life-cycles are well understood. We can even buy specific species of beneficial nematodes for a special need. The most effective nematode for our garden problems is a combination of two species, Steinernema and Heterorhabditis.
Can nematodes hurt me or anything else? Studies have shown that these beneficial nematodes only target very specific pests and neither the nematode nor the toxins they produce can harm vertebrates. That leaves out all of us as well as our pets, birds, squirrels, etc. They don't hurt earthworms either.
What types of pest problems will beneficial nematodes help control? The specific type of nematode you buy for garden use targets any garden pest that has a larval stage in the soil. In our area, that would be cutworms, grubworms, many borers, root weevils, cabbage maggots, sod webworms and others. Many garden pests such as rose chafers have a larval stage in the soil, so they would be affected too.
What about iris borers? There has been some exciting work done on using beneficial nematodes to control iris borers. In a study done at the University of Maryland, beneficial nematodes applied by their guidelines did as good a job as any other treatment, including the now banned Cygon. For gardeners who want to avoid using harsh chemicals such as Cygon, beneficial nematodes may be a great alternative.
If nematodes are living organisms, how can I buy them? One of the advantages of using these organisms is that they can be grown in labs on artificial media and with proper handling, they can stay viable for several months. They are usually sold on a piece of sponge or in a small shaker that contains thousands of nematodes.
Does it matter when I buy or use the nematodes? That is one of the keys to success. Beneficial nematodes will not live very long in the soil if there are no target hosts. If you are applying them to deal with a specific problem such as iris borers, they should be applied while the borer is in the larval stage in the soil. For iris, this means as soon as the foliage is up about 2-3 inches. We'd suggest making one application then and another about two weeks later. To split an application, simply cut the sponge in half or use half the container. For other specific pests, you will need to know their life cycles. Beneficial nematodes can also be applied "just in case" several times early in the season to catch any larval insects in the soil.
Would it be better to buy them through mail order or locally? If they have been shipped and stored properly, it's o.k. to buy them either locally or by mail. They should be kept refrigerated if they are stored for more than a few days and should not be allowed to get above 80º. If you buy them locally, be sure not to leave them in the car on a sunny day while you shop for groceries! They can be safely stored at home in the refrigerator.
How many nematodes will I need? Measure the area where you've had a problem. In the vegetable garden or a flowerbed, a single package will probably be enough. If you are treating lawn, you may need more.
How should I apply the nematodes? There will be instructions on the package for you to follow. Most are on small sponges that you add to a watering can or sprayer full of water. Once they are released into the water, they can be spread onto the area you want to treat. The package should give recommendations on how much liquid to use and how far to spread that solution. Generally, a million nematodes will treat at least 2000 square feet of soil. Be sure to water the soil thoroughly just before you apply the nematodes. It also helps to water again lightly right after you spread the solution, being sure to rinse any of it off the foliage and mulch. They cannot live out of the soil.
Why is water so important? Nematodes actually travel in the water between the particles of soil so it is essential that the soil be moist so they can find a host.
When should I use the nematodes? Ideally they should be applied when the temps are neither hot nor cold and the soil temperatures are between 50 and 75º. If it's a humid day, even better.
How quickly will they work? Beneficial nematodes can kill their hosts within 24 to 48 hours. Nematodes kill their pests by infecting them with a bacterium that is toxic to them.
How often will I have to use the nematodes? If there are hosts present and the soil stays moist, beneficial nematodes will keep working in the garden all season. In our climate, many of them are killed during the winter, so it helps to reapply early each season.
Since I can't see the nematodes, how will I know they are working? They are microscopic and the pests they are targeting are usually hidden from our view too, so this might worry you. For the most part, we just have to wait and see if the target plants are being protected from the borers or weevils.
Can I still use other pesticides at the same time? No. Insecticides will also affect the nematodes. In fact, nematodes are most effective on lawns and gardens that have been kept organically.
How does using traditional pesticides compare with the nematodes for effectiveness? Studies that have been done show nematodes to be equally effective when they are used according to directions. Problems arise when they are used in soil that is too dry or too cold.
This sounds fairly complicated. Is it really worth all the extra effort? All gardeners will have to make that decision for themselves, but it does offer us a viable, organic, earth-friendly alternative to chemical pesticides.
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