Best time for field herping in Midwest?

It really depends on the target species but if you want to see the most species you need to track the breeding seasons. For salamanders this means checking ponds and woodland in the autumn until mid spring. For frogs and toads, start hanging out around ponds in February. The later breeders may not show up until June! I know that sounds a bit vague but if you have target species in mind then I should be able to give you a more specific answer. Welcome to the forum by the way!

It also depends on your latitude, the further north you are the later in the season you will see amphibians. Where John is, he will see activity a lot sooner than us in the northeast. Example, gray treefrogs are breeding or have bred in Texas, they don't breed till June here in Massachusetts.