Entomopathogenic nematodes have potential as biological control agents of the swede midge

But field use must consider nematode selection, storage and application factors

At present, there are no effective control methods or recommendations for swede midge available to organic growers. With increased consumer demand for organically-produced fruits and vegetables in Ontario, there is a need to develop pest management strategies and operational protocols that are specifically-applicable to organic food producers. Our research project funded by the OMAF-MRA 'New Directions' program, is focused on evaluating a variety of potential organically-acceptable swede midge management tactics.

We have been investigating the potential of three entomopathogenic nematode species as biological control agents of the swede midge's soil-dwelling stages in both the field and the laboratory. Swede midge are present in the soil when mature larvae leave the host plant and burrow into the ground to form a cocoon and then pupate, emerging as the subsequent generation of adults within 2-4 weeks. With 4-5 overlapping generations of larvae and pupae occurring in the soil throughout the growing season, soil-dwelling biological control agents, such as entomopathogenic nematodes, could decrease emerging adult populations and, in turn, reduce the number of juvenile swede midge subsequently infesting host plants.

Swede midge larvae feeding around the developing vegetable head.

Figure 1. Swede midge larvae feeding around the developing vegetable head. Photo credit: Steve Marshall, University of Guelph.

Swede midge feeding damage to early-stage developing broccoli heads often results in crumpled, twisted leaves surrounding the developing vegetable.

Figure 2. Swede midge feeding damage to early-stage developing broccoli heads often results in crumpled, twisted leaves surrounding the developing vegetable.

Results from lab experiments suggest that both the larvae and pupae of swede midge are susceptible to these biological control agents (Figure 3). The nematode Heterorhabditis bacteriophora can best reduce the numbers of swede midge adults emerging from soil at higher temperatures (20-25ºC), whereas the nematodes Steinernema feltiae and Steinernema carpocapsae are relatively more effective at lower temperatures (16-20ºC).

Figure 3:  (A) Adult stage Heterorhabditis bacteriophora nematodes emerging from an infected swede midge pupa.  (B) Swede midge infected with Heterorhabditis bacteriophora nematodes exhibit a dark reddish coloration (i), whereas those infected with Steinernema species maintain their yellow color (ii), yet they appear more rigid than uninfected larvae (iii).

Figure 3. (A) Adult stage Heterorhabditis bacteriophora nematodes emerging from an infected swede midge pupa. (B) Swede midge infected with Heterorhabditis bacteriophora nematodes exhibit a dark reddish coloration (i), whereas those infected with Steinernema species maintain their yellow color (ii), yet they appear more rigid than uninfected larvae (iii).

The successful implementation of nematode-based pest management efforts in the field must consider a number of factors:

  • Formulation / storage medium (pastes, gels, granules, etc.);
  • Storage and application conditions; and
  • Nematode species and strain.

Depending on the formulation, storage times may range from 1 month (pastes) to 7 months (granules). Different nematode species also require different storage conditions, with species from the genus Heterorhabditis requiring storage temperatures in the range of 10°C to 15°C and Steinernema nematodes requiring cooler temperatures, in the range of 4°C to 8°C.

During application, nematodes are highly susceptible to desiccation and degradation by UV light. Applying nematodes under low light conditions and avoiding applications during periods of drought can significantly enhance post-application nematode survival. Avoid storing nematodes in hot vehicles. Nematode solutions should be applied immediately following mixing - nematodes may drown if left inside spray tanks without aeration or agitation for prolonged periods. Application rates of 250,000 nematodes per m2 are typically required to successfully reduce pest populations, although higher rates are sometimes needed.

The nematode species employed must have the ability to infect the pest insect in question. Steinernema carpocapsae, for example, is a species that is highly effective against surface-adapted caterpillar larvae, whereas Heterorhabditis bacteriophora is highly virulent against soil-dwelling beetle larvae. Applications must also be appropriately timed to ensure that they coincide with high numbers of the target pest occurring in the soil. While some insects are present in the soil throughout the season, others may only spend a portion of their life cycle below ground. An excellent summary of entomopathogenic nematode biology and use, including a fairly comprehensive list of susceptible insects, has been prepared by Dr. David Shapiro-Ilan (USDA - Byron, GA) and Dr. Randy Gaugler (Rutgers University - New Brunswick, New Jersey).

Native, locally-isolated strains of nematodes may offer the benefit of longer persistence in the soil, as compared to imported strains, due to their inherent tolerance of local environmental conditions. The shorter shipping & handling times and distances travelled for locally-produced strains also helps to reduce exposure to suboptimal storage conditions during transit, potentially resulting in higher quality, more virulent nematodes. However, all nematodes are vulnerable to reduced viability, if not handled properly. Prior to application, viability can be confirmed by viewing a sample of nematodes in a small volume of water under a microscope. Healthy, living nematodes should be moving actively when they are immersed in water.

Understanding and following best practices for use of entomopathogenic nematodes is important to ensure the success of these biological control agents in the field. Results and recommendations arising from our field efficacy trials with these nematodes will be shared in a future article.


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