Nursery, Greenhouse, and Landscape News

Bacillus thuringiensis Strain May Enhance Activity of Some Entomopathogenic Nematodes Against White Grubs

Although entomophagous nematodes and some strains of Bacillus thuringiensis can kill scarab grubs, their utility in the field has been somewhat hampered by unrealistic expectations by practitioners. Ambush-type nematodes like Steinernema carpocapsae that live near the soil surface are not effective because they are not able to readily locate white grubs in the soil. Cruiser-type nematodes, however, like Heterorhabditis bacteriophora or Steinernema glaseri are more able to find and kill beetle larvae. Field results with these species can be inconsistent due to either environmental conditions or to lack of appropriate bacterial symbionts in the infective juveniles. Likewise, Bacillus thuringiensis japonensis 'Buibui' (Btj) can produce disappointing results since it is much less effective against 3rd instar grubs than earlier instars. These two biological approaches could be combined to provide more effective control against 3rd instar grubs.

Using two white grubs that are important in California (Cyclocephala hirta and C. pasadenae), the mortality was greatest when 3rd instar grubs were placed in Btj treated soil 14 days before the release of infective juvenile H. bacteriophora or S. glaseri.

The researchers have speculated that Btj stresses the grub in a way that makes it less able to trigger the physiological and behavioral defense mechanisms they have against the nematodes. Interestingly, when the researchers used a more highly pathogenic nematode, S. kushidai, in the same battery of tests, the Btj was unable to increase the mortality of white grubs. This, they suggest is largely because the high pathogenicity of this strain (97% mortality) left little room for improvement. Although this approach has yet to be field tested, it could have great potential for use as a rescue treatment in high value turf.

Sources:

A. M. Koppenhoffer and H. K. Kaya. 1997. Additive and synergistic interaction between entomopathogenic nematodes and Bacillus thuringiensis for scarab grub control. Biological Control 8:131-137.

Gaugler, R. E., E. Lewis, and R. J. Stuart. 1997. Ecology in the service of biological control: the case of entomopathogenic nematodes. Oecologia 109: 483-489.


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