A new species of willow grub, Nematus Oligospilus, has arrived in Tasmania, and will become a feature of our trout fishery for years to come. This will mean fatter trout, as the grub is an abundant feast, and we found the largest fish have been drawn into major feeding lanes on our willow infested rivers. They appear at this stage to be in isolated locations.
Willow grub varieties have always been present (to my knowledge), yet never seriously being a feature on the trout’s food menu. This new species are a leaf consuming grub that are very juicy and grow to around 20mm. I have seen the trout eat saw fly adults, the grub and the pupae cocoon that was falling from the tree.
The pupae grow in the leaf, until hatching into the grub. The grub becomes the saw fly. They appear to be dependent on the Salix variety of willow, which includes the crack willow, a pest plant in Tasmania.
The trout will become selective while the grub is present during the summer months, so good imitations, along with good fly presentation can undo some of these excellent wild fish.