Bracon cephi and Bracon lissogaster
The primary natural enemies of the wheat stem sawfly in Montana are two closely related Braconid wasps, Bracon cephi and Bracon lissogaster . Both of these native parasitoids of the sawfly attack the developing larva inside the wheat or grass stem.
Photo by RKD Peterson
Until recently, both Bracon cephi and Bracon lisogaster were often misidentified as being the same species (Runyon et al. 2001). Both are ectodermic parasitoids that feed on and eventually kill their host. Adult wasps oviposit on wheat stem sawfly larvae within the plant stem, the parasitoid larva hatch and feed on the developing sawfly larva until it dies The parasitoid larva then migrates up the stem where it spins a cocoon, pupates and emerges in the spring a few weeks after wheat stem sawfly emergence (Morrill et al 1998).
Parasitoids over-winter in standing stems rather than in the stubble and can be found in stems cut but left behind by the combine (Morrill et al. 1998, Morrill 1997). One possible way to improve parasitoid populations might be to leave a greater portion of the wheat stem intact during harvest. Also, tillage practices may have a negative effect on parasitoid population through the mechanical destruction of stems.
Parasitoid Research
Parasitism levels in wheat fields have been found to be up to 98% (Morrill et al. 1998). This suggests great potential for use of these insects in a sawfly management program.
The laboratory has implemented a method for delivery of parasitoids to areas of high sawfly infestation. Stubble is harvested from areas that are known to have high parasitoid numbers. This wheat is then placed in fields where sawfly infestation is high but parasitism is low. Samples are harvested from the study sites the following year to evaluate parasitoid establishment. This method has shown to be highly successful, and we plan future replicates.
A current research project includes development of methods to mass rear the parasitoids in order to provide a source for distribution into wheat fields experiencing high levels of sawfly infestation. Large cages are placed over wheat plants. Sawflies are then introduced and allowed to infest the plants. Parasitoids are collected and released into the cages, where they lay eggs on developing sawfly larvae. At the end of the growing season, the wheat is harvested from these cages and dissected to ascertain levels of sawfly parasitism. This technique has been moderately successful, and improvements continue to be made.
Fungal Pathogens
Fungal pathogens show potential as a control agent in a sawfly management program. Experiments have been completed that show high levels of mycosis in sawfly larvae from Fusaria species. These studies show that F. graminearum, which also causes crown rot disease in wheat, is highly lethal to sawfly larvae. However, inoculating your wheat with a known wheat pathogen would not be wise. Other Fusaria species, which do not harm the crop, are being investigated as a potential management tool. Several species examined, F. acuminatum, F. equiseti, and F. avenaceum, are typically non-pathogenic in wheat but they cause lethal mycosis in sawfly larvae. Greenhouse and field trials are underway to evaluate their potential for broad scale applications.
|
