On 16/05/2015 11:02, Emery Davis wrote:
On Fri, 15 May 2015 19:35:05 +0100, Jeff Layman wrote:

On 14/05/15 18:54, Emery Davis wrote:
Do I need to switch to a more

This from a 20-years old paper at Cornell Uni:
"The natural pyrethrins are contact poisons which quickly penetrate the
nerve system of the insect. A few minutes after application, the insect
cannot move or fly away. But, a "knockdown dose" does not mean a killing
dose. The natural pyrethrins are swiftly detoxified by enzymes in the
insect. Thus, some pests will recover. To delay the enzyme action so a
lethal dose is assured, organophosphates, carbamates, or synergists may
be added to the pyrethrins."


Thanks Jeff, that's useful info.

Are you using a pyrethin with a synergist (such as piperonyl butoxide)?
If not, it is quite possible that the larvae are resistant. I wonder if
a pesticide based on Bacillus thuringiensis might work.

This is a Scott's product that I use from a squeeze bottle to keep bad
aphid infestations (and they are really bad, though letting up a little
now) off the new maple growth. It's pretty "light" in that it doesn't
stop new growth or bleach out young leaves like most products do. It
turns out (closer examination, often useful!) that it does have piperonyl
butoxide, as well as Bacillus thuringiensis (serotype 3), this latter is
apparently the principle vector against caterpillars.

If on Monday I still see activity I'll try a different product.

Thanks again for the useful reply.

-E



Perhaps it would help to use a systemic insecticide while the
infestation is so damaging. That way, the bugs should ingest the toxin
and die. Once the shrub has hardened off a bit, it should be less
yummy. You can always revert to pyrethin for smaller outbreaks, so it
won't be wasted.
--
Spider.
On high ground in SE London
gardening on heavy clay