On 2009-10-02 16:07:15 +0100, K said:

Sacha writes


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.

I haven't changed my compost between .3 years ago when I got good
control and the last three years. I use New Horizon.

And it does have anything added to it to control vine weevil -
something Green, IIRC. It's one of those names that just will not stay
in my head!

The other possibility is the residue of old pesticides in the
greenhouse killing off the predator.

Possible, but I use pesticide in the autumn, then not through winter or
spring because I'm keeping the cacti dry.

However, the pesticides get onto e.g. plastic pots and that holds a
residue for rather a long time. I have at the back of my mind some
experiment or other that Ray carried out - or found out about - I'll
try to remember to ask him about it.

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.

I think this is the most likely. Our air temperatures have been in the
teens most of the summer, and it takes a bit of sun to get the
greenhouse higher than this, which we haven't had. How good is 'good
light levels'? - do I need some serious glass cleaning? ;-)

In sunny Leeds - perish the thought. ;-) i.e. Probably!

I still have a few predators - the odd cocoon type thing here and
there, so slow breeding seems the most likely.

Of course, once the predators have eaten all the nasties, they, too,
will die off so you may have to start again next year.
--
Sacha