what product should be used reduce apple production on fruit trees, when should they be sprayed?

You can always use the insecticide Sevin as a thinner. The most optimal timing for thinning is around 10mm fruit. Remember that Sevin is an insecticide too, so watch for bees in your orchards if you plan to use this material. There are other plant growth regulator materials that are used by commercial fruit growers. I am not sure if you have just a few trees or orchards... We have lots of thinning guides at MSU if you want further information that I can share.

Nikki Rothwell