An apple producer had Panonychus ulmi (red mite) problems for years and it stopped when no acaricides where used. I am curious about this phenomenon that the producer himself could not explain to me. Does it mean that Red Mite where resistant to the acaricides anyway? Does it mean that the acaricides where destroying also the natural predator (Tetranychus urticae) of the European Red Mite? These two effects would be my hypothesis but there might be other potential phénomenons?
extra info : I got this feedback from South-West France.
I think it's quite difficult to have a clear answer to this problem. First of all, I don't think that those problems disappeared because of the resistance of P. ulmi. If they were resistant to this insecticide, they wouln't have disappeared (as being resistant to a molecule doesn't mean being dependent on it).
You hypothesis about natural enemies is justified, but many other factors may have impacted the P. ulmi related problems: other management changes that came with the elimination of acaricides, seasonal changes (this year is warmer than last year in S-W France), something that happened last year that prevented a pest outbreak, etc.
this is a well known phenomena when we stopped the use of pesticides, the natural populations of natural enemies will increase and reduce the population of P. ulmi especially the Phytoseiid mites. this usually will take two to three season to occur. in addition to that the P. ulmi overwinter as winter eggs on the bark of the twigs and need during the winter a minimum of three months of temperatures below 7 degrees Celsius to break its winter dormancy. if the winter is worm and these requirements are not achieved this will reduce the number of eggs that will hatch in the spring and reduce the initial population to start with.
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