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Are there any effective biological pest control measures for greenhouses and polytunnels?

I was wondering if it is ok to release a few praying mantids into a polytunnel in the summer months to keep pests at bay.  Does anyone know if this can be done or if there are other beneficial bugs that can be used in this sort of environment?

Parasitic wasps are the usual one Rob. I think there are lots of suppliers.
They are tiny things, and lay their eggs in the pest insects.
I believe it's got to warm up a fair bit before you use them though.

we put every ladybird we can find in there in summer, also plant up some herbs and let them flower, brings in hoverflies, you can buy ladybird larvae and parasitic wasps , lots of places on the internet, biological control, I think it's called

found the web site wwwjust-green.com
nature's natural defence  :)