caterpillars on birch tree

medieval knievel · Rowan Tree Joined: 03 Sep 2007 Posts: 112

unfortunately, i don't have a photo of them - they were small, less than an inch long, dark green with small yellowish legs and a touch of yellow to the head.
there were over a dozen on some leaves - they'd stripped an area maybe a bit bigger than an outstretched hand, and sat with their tails in the air, in a kind of shallow S - do these ring any bells?
it was the numbers which we were surprised by - i reckon there were over two hundred on the few lengths of branch i snipped off.

medieval knievel · Rowan Tree Joined: 03 Sep 2007 Posts: 112

a bit of googling suggests sawfly larvae - the pics are a near perfect match.

GPI · Posted: Fri Jun 20, 2008 9:23 pm Post subject:

Yep medieval knievel, sounds like the caterpillars you mention are the larvae of the sawfly alright.
While the tree is younger and small is the time to get on top of these pests.
The larvae create mines or cut the leaves, when the larvae eventually leave the mines they drop to the ground and pupate in the soil.

To prevent the breeding of further and further generations which will become an unreachable problem when the tree grows, you must act in the early years.
You must begin spraying for the first generations of sawflies and larvae to prevent large numbers in any later generations.
The best time for the first spray application is on the the new leaves as they first develop in May.

Areas in leaves where eggs have been laid will develop a grayish shade rather than the normal green of the leaf.
Developing mines will show up as tiny almost see through spots of a lighter green color than the rest of the leaf when leaves are held up to the light.
As soon as either the transparent spots or off color areas appear, a insecticide should be applied thoroughly over the whole tree.

A suitable insecticide will contain the ingredient malathion or else you can use the more organic "Liquid derris"
A repeat spray may be necessary 10 to 14 days later if new symptoms appear.

You may have to do this spaying over a few years, but it is better to quell the sawfly population now, otherwise you will be spraying atop a tall ladder in 10 years time.
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medieval knievel · Rowan Tree Joined: 03 Sep 2007 Posts: 112

i was reading that one organic control is to spray the blighters with salt water.

GPI · Posted: Sat Jun 21, 2008 11:30 am Post subject: