If your trees have an active infestation, the biological control agent may not work fast enough for effective control.
"It is very important to understand that Nosema locustae does not work rapidly. The spores must be applied against the small grasshoppers (by 3rd to 4th instar) in and near the hatching areas for maximum efficacy. This disease can be an effective control, but it will act slowly. It will have little or no impact on later instars or adult grasshoppers that move into your yard or garden."
This article from Australia has some other suggestions for organic control: