On 14/05/15 18:54, Emery Davis wrote:
Anyone have an idea about how long a pyrethrin-based bug spray would take
to work on viburnum beetle larvae? My viburnum opulus has been
skeletonised the last several years, this year I saw the caterpillar-like
beasties at work and sprayed. Two days later there were still lots
alive, I sprayed again. A day later they're still not dead but may have
stopped eating. Do I need to switch to a more noxious product?

This is a rooted cutting, now quite large, but my friend who gave it to
me is unaffected by the bugs!

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."

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.

--

Jeff