You Are Here: Management Updates 2008 Archive July 23, 2008

Date: July 23, 2008
Category: Insects
Subject: Nematodes to Control White Grubs

Entomopathogenic nematodes (EPNs) are tiny roundworms that can deliver pathogenic bacteria to certain kinds of insects. The nematodes either actively seek out the insect or lie in "ambush", waiting for an insect to pass nearby, depending on the nematode species. There are several species of nematodes currently available commercially, but they tend to be fairly specific as to the kinds of insects they attack.

Several people have asked recently about nematodes. The two most frequently asked questions are: 1) Which species is most effective against white grubs? and 2) When should I apply the nematodes?

1. Which nematode is most effective against white grubs?

Based on information from colleagues of mine in other states, the most effective species of nematodes against white grubs that is readily available is Heterorhabditis bacteriophora (HB). We have conducted trials in previous years that achieved acceptable levels of control with HB nematodes against Japanese beetles, oriental beetles, and European chafers. However, we were able to do everything "just right" - time the application when grubs were present and most susceptible, and water in very thoroughly.

Dr. Albrecht Koppenhofer, a turf entomologist at Rutgers University, knows more about nematodes and their interactions with white grubs than anyone else in the United States. He tells me that he has a very different experience - that HB works quite well against Japanese beetle grubs but not the other species. He has also tested other species of nematodes and some show promise, but most are limited in their availability. He also discovered a new species of nematode, Steinernema scarabaei, which shows tremendous potential against many species of white grubs, including all the ones that are problematic in New England. But that nematode has proven difficult to produce in large volumes. Dr. Koppenhofer's field and laboratory trials with S. scarabaei have been very well designed and have demonstrated the effectiveness of that nematode. (In fact, it appears that a single application may last for more than one season in some parts of New Jersey.) But because of the production issues, it will be a while before we see that nematode available on the market.

So for now, the best bet regarding nematodes on white grubs appears to be HB. Keep in mind, though, that the New Jersey research indicates HB may be marginal in its effectiveness against grubs other than Japanese beetles. If you are able to optimize the timing of application and follow with adequate water, you certainly increase the likelihood of success.

2.  What is the best time to apply nematodes to control white grubs?

The challenge in using nematodes to control white grubs is that the nematodes need to be able to penetrate through the grub, so the grub needs to be large enough for the nematode to get in. White grubs have "sieve plates" on their spiracles, which provide a level of protection. And grubs often can sense nematodes and spend some time "grooming", trying to prevent nematodes from entering through the mouth. And if you delay application until grubs are already in the third, and largest, developmental stage, nematodes may not be effective.

So ordinarily you want to make the application when most of the larvae have hatched out of the eggs and are first instars or very early second instars. For some of you in southeastern Massachusetts, that may be the first or second week of August. For those of you further north, you may need to give the grubs a chance to finish hatching, and your timing may be a week or two later.

Don't forget that you must follow the application with plenty of water - immediately after the application. Aim for at least a quarter inch, but a half inch is even better. The EPNs need the water to help move through the thatch to the soil. Also nematodes are very sensitive to high temperatures and sunlight, so do not apply between 9:30 AM and 3:00 PM.

Submitted by: Dr. Pat Vittum

DISCLAIMER - As always, it is the responsibility of the applicator to verify the registration status of any pesticide BEFORE applying it. Different states have different regulations as well. The author and the University of Massachusetts are not liable for any consequences of any pesticide "recommendations". Mention of any trade name is not to be considered endorsement of a product.

Links followed by the symbol are external links that open in a new window.