When talking about willow grubs the conversation always seems to emanate
around New Zealand and their famous little grub that is found along most
streams in that country. But this is different; we are now talking about
large willow grubs up to around 2cm long and a couple of millimetres thick.
The willow grub is actually the larva of the Willow Sawfly Nematus oligospilus. Our scientists think it may have come over from New Zealand and the suggestion is that it might have been blown over or come over in shipping containers.
This willow grub is normally found in temperate zones in Europe but in recent years it has spread all over the Southern hemisphere. It has been found in South America, South Africa and turned up in New Zealand back in 1997 where it is now widespread. It first showed up in Canberra in 2004 by accident they say, or maybe it is like the Calicivirus for rabbits, I mean whoops; slap my wrist for thinking of such a thing!
Over the past four years it has spread rapidly and is now in the Snowy Mountains, along the upper Murray, South Australia, Tasmania and big time around Eildon in Central North East Victoria. Believe me it will not be long before it is common wherever willows are.
The lifecycle of N. oligospilus starts with the eggs, which are laid on the leaves and the young larva are a pale yellow/green in colour. They are only small, around 2 to 3mm long but grow rapidly to around the 2cm mark. As they grow, their colour changes to almost chartreuse and slightly translucent. The head of the grub is a dirty yellow with tan slashes on each side of its head.
They are short lived; the eggs develop at around six days depending on temperature (the ideal is about 23˚c) and the grub stage lasts for about two weeks before pupation. The cocoon containing the pupa is found in the bark crevices on Willow trees or in the leaf matter around the base of the trees. From what I have been able to glean, it takes around nine days for the grub to pupate. It is also known that the Sawfly larva hibernates in its cocoon over winter to emerge again in Spring.
The adult Sawfly is only small, around 5 or 6mm and is orange with a chartreuse mid section on its body. It is a strong flyer and has been known to migrate well over 50 to 60 kilometres from its emergence zone.
The grub in New Zealand appears around October and I see no reason to suggest any difference over here, because as soon as those leaves are out so will be the grubs. Our temperate climate is ideal for this grub and in a warm, dry season it has been noted that the Sawfly can have around four generations each season.