Introduction:

The balsam fir sawfly (BFS) is an increasing problem in balsam fir stands in eastern Canada. Defoliation by this insect results in significant growth loss and can lead to tree mortality. At present, nothing is registered for control of BFS in Canada.

Three products were assessed as possible biological control agents for BFS in the summer of 1999. The botanical insecticide neem, the bacterium Bacillus thuringiensis subsp. israelensis (Bti), and a naturally-occurring virus pathogen were used.

The Products:

Neem is derived from the seeds of the subtropical neem tree (Azadirachta indica) and is registered for use in many countries, including the USA. Neem can act as a stomach poison, antifeedant, and insect growth inhibitor, and is effective against a wide range of insects. Because its contact toxicity is relatively low, it has little impact on beneficial insects and is a suitable alternative to chemical insecticides.

Bacillus thuringiensis israelensis (Bti) is a naturally-occurring bacterial pathogen of larval Diptera. This biological control agent is available and registered for mosquito control in Canada. While no published information exists on the efficacy of Bti against larval sawflies, preliminary studies indicate it may have some impact in the field.

A nuclear polyhedrosis virus (NaNPV) was isolated from a population of BFS that was in decline. It appears that NaNPV is responsible for the localized collapse of BFS populations under natural conditions. However, virus epizootics do not naturally occur until the insect population is very dense and considerable damage has already been done. We are investigating the possibility of initiating an epizootic with this virus.

Results:

All products were applied quite late due to the early development of the insect during the summer of 1999. It is expected that targeting earlier larval instars will result in better efficacy. Therefore, the efficacy observed in these trials is probably lower than could be expected under similar operational conditions.

Neem was the most efficacious of the options tested, although it is clear that the product will have to be formulated to improve its adhesion to foliage in rain before it can be a useful option in forestry biological control operations. The low rate of application (20 g a.i./ha) provided good foliage protection and reduction of larval survival, even when used against late instars. It is possible that even lower rates could be used successfully against the balsam fir sawfly, reducing the product cost per hectare considerably.

The Bti appeared to provide some control for larval balsam fir sawfly. However, the results are not clear and the level of control does not appear to suggest that Bti is a viable option for balsam fir sawfly. Earlier instar larvae are generally more susceptible to pathogenic agents, so trials against these larvae may show increased efficacy.

The virus, while slow to act against the late instar larvae, did reduce the larval numbers in the spray block. While foliage protection was not the goal of the virus application, the level of defoliation occurring in the virus block was somewhat surprising. The fate of the virus will be monitored throughout the 2000 season, to assess establishment and rate of spread in the insect population and to determine whether we have been successful in initiating a collapse of the insect population.

What Next?

• Pursue Canadian registration of neem against sawflies
• Repeat efficacy trials against earlier instar larvae
  
• Closely monitor insect population for disease incidence
  
• Pursue registration of the virus against balsam fir sawfly