University of Vermont Extension 
Department of Plant and Soil Science

Summer News Article
line
BENEFICIAL NEMATODES
 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
           
Nematodes are very tiny, microscopic “worms” that mostly live in soils, although “foliar” nematodes live in leaves.  While some species harm plant roots, others are beneficial by attacking and killing pests such as grubs in our lawns and gardens.  They are easily applied, but the right timing and conditions are needed for them to be effective. 
           
Beneficial nematodes are becoming much more popular for soil pest control. They feed on over 200 pests from up to 100 insect families.  These pests are ones that either directly injure our desired lawn or plants, or immature grub stages of insects before they mature to adults that cause damage.
           
Being natural, beneficial nematodes are safe to use around humans, children and pets.  Being natural, they’re safe too for soils and wont harm non-target organisms such as bees or pollinators.  They’re easy to apply (no protective gear is needed), quick to work (often within a couple days), and cost effective.  While you may need 20,000 or so to treat a square foot, they are so small that for under $20 you should get about 5 million—enough to treat 1,500 square feet.
           
The “mode of action” of beneficial nematodes is like something from a horror film (at least for the target pests).  Once applied, the tiny juvenile stages seek out their specific prey.  One type (Steinernema) uses an ambush strategy, waiting for the prey to come close before attacking.  Another nematode (Heterorhabdtis) uses a “cruising” strategy, seeking out and destroying pests.  They either follow the trail of excrement of the pest, or seek it through changes in temperature and carbon dioxide levels.
           
Once found, the beneficial nematodes enter the grub or other pest through body openings, such as the mouth or openings to the breathing tubes (“spiracles”).   These juveniles carry a bacterium (safe for plants and the environment) that they release into the pest blood.  These bacteria multiply, killing the pest in two days or often sooner, and converting host tissue to food for the nematodes.  Since these bacteria work to help their host-- the nematodes-- they are called “symbiotic”. 
           
The grubs change from a beige-white to reddish brown when infected, die and turn slimy, and are soon hard to even find.  The nematodes feed on the dead pest from within (hence they are called “entomopathogenic”), and can go through several generations of adults in a couple weeks.  Once their food source is gone they exit and move on to new prey. 
           
There are a couple main types of nematodes you’ll find (Steinernema, Heterorhabdtis), and a couple or more species of each.  Each targets specific pests, so it helps to know what pests you have in order to buy the right nematode.  For instance, one species of the former (S. carpocapsae) is effective on larval stages of armyworm, weevils such as black vine, caterpillars, cutworm, and sod webworm.  Another species (S. feltiae) is used for larvae of cabbage maggot, onion maggot, raspberry crown borer, and thrips.  Both species are effective on larvae of codling moth, corn earworm, and cucumber beetle. You’ll want a species (H. bacteriophora) of the other nematode for larvae of tree and vine borers, European chafer, Colorado potato beetle, corn root worm, flea beetles, and grubs including those of the Japanese beetle and ones damaging lawns.
            
Your local garden center, master gardener programs, online resources, books, and state extension diagnostic clinics (www.nepdn.org) are all good resources to identify target pests.  Labels and online descriptions will then guide you to the appropriate nematode product.  If you’re not sure what pest you have, you may need to apply a couple different nematode products.
            
Soil type can affect the nematodes.  Some work best in cultivated soil (Steinernema) while others work best in undisturbed soils (Heterorhabdtis).  While one species works best in clay or silt (S. feltiae), another works best in sandy soils (S. carpocapsae).  Soil type isn’t crucial for the other main beneficial nematode (Heterorhabdtis).
           
Once you’ve chosen the right product, to be effective you must apply it properly.  Timing of the season is important, as the soil stages of your target pest need to be present for the nematodes to work on them.  Late summer into early fall is a good time for many grubs of plants, crops, and lawns.  This is a great time for lawn grubs, as then they are usually small, with thin skins the nematodes can penetrate, and they’re feeding on grass roots near the soil surface.  It is when armyworms fall to the ground to change stages (pupate) as well.
           
Remember that the nematodes are living organisms, so should be applied very soon after you get them.  They like moisture, so if it hasn’t rained, water thoroughly both before and after application.  You can apply during the rain too.  If they come on a wet sponge, rinse them from this into cool water.  Then use this as a concentrate to water or spray on according to label directions.  Then keep the soil moist for a week to 10 days after, if it doesn’t rain.
           
Rainy, overcast weather is good for another reason—nematodes are quite sensitive to the UV in sunlight, and can be killed within a minute or two if exposed to it.  So you’ll often see recommended to apply in early morning or late afternoon.  They’re sensitive to temperature too, with ideal temperatures between 45 and 100 degrees (F). 
           
For prevention, you can apply 2 to 3 times a year, such as in spring, summer, and fall.  For control of existing pests, apply every 2 weeks until the infestation lessens or goes away.
 

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