June 15, 2010
By Jen Llewellyn
OMAFRA nursery crops specialist

When I started working with entomopathogenic nematodes to better understand their efficacy on nursery crops, we were working with various species of Steinernema and Heterohabditis megedis (H.m.).  We found H.m. to be quite effective for the management of black vine weevil larvae (Otiorrhynchus sulcatus) in container production, but a comparatively low level of suppression in the field. Since then, Heterohabditis bacteriophora (H.b.) was introduced into the North American market. We have found this species to be even more effective at black vine weevil management than H.m.

There are several suppliers of H.m. in Ontario. Most of the H.m. comes from biocontrol rearing facilities in Europe and must be ordered at least one week in advance. Viability and quality of any live biocontrol organism can be an issue, especially with long distance transport. Biocontrol suppliers conduct regular checks on viability in order to help ensure efficacy of their products. Are you checking viability before nematode applications? All you need is a microscope….or a glass of warm water.

When it comes to entomopathogenic nematodes, checking viability can be quite simple. For those who have compound or dissecting microscopes, you could easily take a small sample of stock solution and view the liquid under the microscope to do some viability counts. Dead larvae are very straight and just lie immobile, suspended in the water. Live larvae tend to have a bend in them and will move and wiggle along in the fluid, though not necessarily continuously. Those who do not have a microscope can take a sub-sample of the stock solution and add it to a clear glass of room temperature water. Now (this is very cool), after about 10 to 15 minutes, hold the glass of nematodes and water up to the light, et voila! You will see the DEAD nematodes suspended and floating aimlessly throughout the water column. The LIVE nematodes will actually sink to the bottom of the water column. If you hold a 10-x hand lens up to the glass, you will be able to see the outlines of the nematodes. It should be noted that live nematodes from a clay media base will not be as active as those from an aqueous food base, but that the former may still be quite effective at controlling insect pests. If you are concerned about viability, you should contact your biocontrol supplier immediately.

Many researchers believe that nematodes reared on live insect hosts (in vivo) will retain more of their parasitic qualities and be more effective as a biocontrol agent in your production facility. Not all stages of nematodes are infective to insect hosts. In the case of Heterohabditis nematodes, only the juvenile stages are infective and it is the last juvenile instar that is most active and mobile. Recent research has indicated that nematodes, just like other biological organisms, can actually produce pheromone cues to signal certain behaviors in their population. It has been demonstrated that when populations are high, nematodes will send out a pheromone to stimulate dispersal (and discourage hunting). This is significant when it comes to choosing application rates of entomopathogenic nematodes. You will notice that the “curative” rate is usually four to five times higher than the “treatment” rate. We can’t help but think, if I apply it at the curative rate or higher, it will work even better! Now you know: stick to the rates on the label and be conservative.  

Some biocontrol researchers have found that Hypoaspis predatory mites will swarm black vine weevil larvae and some growers may be trying Hypoaspis in synergy with nematodes in hopes of increasing the efficacy of the nematode treatment. Interestingly, some researchers have found Hypoaspis predatory mites to be effective at reducing larval populations of black vine weevil in field production crops. The predatory mites will actually swarm the larval pest and feed on it by inserting their proboscis and sucking out the insect’s fluids, leading to mortality. Hypoaspis are not winter-hardy in our climate and need to be introduced during the growing season each year.
Jennifer Llewellyn may be reached at 519-824-4120, ext. 52671, or by email jennifer.llewellyn@ontario.ca. Read her Nursery-Landscape Report: http://apps.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp