Bees need a thinner syrup in the spring to build combs. 1:1 sugar
syrup is about right. I believe you heard right about thinning down
the HFCS. I have heard 20 to 30 percent water to HFCS to make the
syrup to the right consistency for spring feeding.
I would not worry about pesticides in corn syrup, but you generally
have to use a HFCS from the bee suppliers to get the correct type of
HFCS for bee feed.
Dale