In these warm weather conditions bees will start looking into other neighboring hives to find substance and robbing occurs. Here is where things get tricky. If other beekeepers in your area have harvested the honey in the fall, it is very likely that those bees will be looking for something more nutritious then the sugar water that the beekeepers normally "trade" for their bee's honey.
Sugar water feeding is not what the bees need for the winter. Sugar is alkaline and the bees gut is designed for an acid PH. A bee's gut is designed with the amazing ability to "slough off" its lining if there is any type of dangerous foreign intrusion and the wrong PH disrupts this delicate balance. It is an important part of the bees immune system.
So we leave it to nature and don't want to introduce sugar water into the hive. I know there is a lot of pressure to feed the bees sugar, but now you know better. The second part of the typical "feeding concoction" is or course water. The bees need to evaporate a lot of that water and the moisture develops in the hive just when the temperatures are falling. Cold damp bees going into winter are not going to be happy bees.
So although we hear the mantra "feed, feed, feed during the fall, so the bees have plenty of stores for winter." In actuality the bees also need empty comb cells to crawl into during the winter months to create a cluster and to keep warm. The empty cells becomes warm insulated sleeping bags in the winter. Feeding will cause the bees to store up the feed in their little winter beds, so they will have no place to rest their sleepy heads.
So instead of thinking "feed, feed, feed" think Insulate for the winter. The bees use the honey stores to keep warm by consuming their honey and converting the correct PH "sugars" into energy, the energy fuels their wing muscles which shiver and generate warmth. I know, shiver sounds cold, but that's what the behavior is called. A well insulated hive enables the bees to use less fuel i.e. honey to keep warm, so the honey that they have collected and stored lasts through the winter.
Also feeding in the fall can attract robbers bees which can be detrimental to colony. Robber bees can take down a hive within a day or two. Read more about robbing Bees Robbing a Hive
And see all of our Winterizing Your Hive articles
Winter Feeding Your Hive
Winterizing and Insulating Your Hive
Overwintering Your Hive with Insulation