Willow Grubs

Willow Grubs
Mick Hall
Please note that this is an abridged version of an article that is to appear in the March issue of The Flyfisher. The only reason that I am putting part of that document on this site is that the Willow Grub has created so much attention by our local fly flickers and their struggle to find a reasonable pattern, I felt it imperative that I do so. Note that this article was written during the 2008 infestation. Flies tied and designed by Mick Hall.

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.

It is not only the crack willow that is under attack; the fact is most willows are now susceptible to this little bug. There are plenty of people out there who will be over the top with the appearance of this willow leaf eating grub. It is their hope that we will see the end of this introduced species and their bull dozers are busy along most of our streams. The willow grub just may be a lot cheaper way of dealing with the willow problem; it has been suggested that willow trees may handle defoliation only three or four times before it could kill them.

But I can tell you this, the willow grub is here big time and it is here in huge numbers. So much so that the trout feeding on these grubs, which were falling from the overhanging, trees 24/7, were really pigging out and putting on a lot of condition.

Further on in the article........
This also worked but Warryn had found a block of chartreuse foam which was actually a door jamb from a two-dollar shop. Again both worked well because they floated and the way these flies floated we felt was a key link in attracting Old Speckles. But we were still getting rejections.

At around the same time I got hold of some Rainy’s Cross-Link Chartreuse Sheet foam 2mm thick and a couple of packets of Rainy’s Parachute Cross cut foam 1/16” Posts. www.rainysflies.com

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.

After a full week fishing with Warryn and Paolo we came to a number of conclusions being that these flies had to float to get the best reaction from our trout. How they floated was another debating point between Warryn and me. I felt that floating on the surface was adequate whilst Warryn thought that they needed to sit lower in the surface film. The number one conclusion was that the flies needed to be tied with a detached body and this was very much a mutual feeling.

Hook size also became a relevant issue; to aid floatant I used a size 18 Mustad C49s, whilst Warryn used a larger size 14 to give a little extra weight to the fly and hold it a little deeper in the surface film. Warryn calls it ‘neutral buoyancy’. Sadly Warryn passed away last August 2008 but his contribution to the Willow Grub saga was immense.

The final and most important conclusion was that the foam versions were far superior, not only by attracting the trout’s attention but with floatation and without doubt, the cross-link foam was tougher and lasted a lot longer.

The New Zealand experience with their common small willow grub was that as the grub hit the water, on many occasions it tended to curl up. With this new grub we found this to be only partially the case; some did curl and some did not. With the sheet foam I was able to cut out a curved body, which meant that it would hold its shape, whilst with the post material I was able to tie a straight body but if I rolled it up, it would partially uncurl on the water.