On 2009-10-02 11:37:11 +0100, K said:

I have a persistent problem with red spider in the greenhouse. I know
that people do use biological controls successfully (Sacha's husband
has for years and was one of the pioneers), and I've had success with
the red spider predator in previous years.

However, in my but of W Yorkshire, we have had three rotten summers on
the trot. The last two years the red spider predator seemed to have no
effect, and I came to the conclusion it wasn't very happy unless the
weather was warm.

This year, the early part of summer was shaping up a bit better, so I
bought the predator. And then the weather went downhill and once again
the predator seemed to fail. I gave it about 6 weeks (when successful,
there has been about 3 weeks of apparently no effect, then after that
I've suddenly realised there don't seem to be any red spider around any
more) and since then I've operated on a combination of spraying with
soapy solution rather than water and removing the worst affected
leaves, and this has kept it to a level where the plants have been
cropping reasonably well.

Has anyone else noticed this? Is it a known problem that biological
controls work better in the warm south of the country? Or am I
imagining it?

Kay, I asked Ray about this and he'd had identical experiences many
years ago when using a compost which contained something that killed
off vine weevil but unfortunately, it also killed off the predators.
He can't remember the name atm but I'll post it if and when he does!
For quite a long time the makers denied this but in the end, so many
people remarked upon it that they had to admit it should not be used
where biological control predators were being used. So he wonders if
you've been using something similar. The other possibility is the
residue of old pesticides in the greenhouse killing off the predator.
If the effects of pesticides have now passed, the predator will be
fine.
OTOH and to cover all bases, Phytoseiulus persimilis which feeds on the
eggs and on the active spider mite needs good light levels and daytime
temps of around 21C so as to breed faster than the mite. HTH in one way
or another.
--
Sacha
www.hillhousenursery.com
Shrubs & perennials. Tender & exotics.
South Devon