This is probably a newbee question but when do you know if your bees need pollen substitute? The obvious answer is when they are in short supply of pollen, but how can you tell?
One must look at the local environment to see what forage and diversity would be available to the bees. Remember, we keep bees in places we want them. And this might not be what nature or the bees would choose if left to their own. All insects thrive in certain locations based on climate, food sources, etc.
Traditionally years ago, old-time beekeepers would pull up a frame of pollen at any time of the year, and say that "this isn't needed in there" or something else in thinking that the hive may be pollen bound by such a find.
And if you were to ask (I did) in bee groups, how many fed you would see something like this....
Feed syrup in spring...25%
Feed pollen in spring...10%
Feed syrup in fall...50%
Feed pollen in fall 0%
But we now know that symptoms closely related to what was once called "Dwindling disease" was closely related to lack of fall pollen amounts, and also associated with poor quality pollen.
Many have written about pollen recently, but most of this is just a rehash of "lost" information that was collected years ago. DeGroot based nutritional values for bees in 1953. And Australia documented "dwindling disease" twenty years ago, after gaining knowledge of what happens to bees that are weakened by lack of pollen amounts, or by low quality pollen.
Much of this can be read at this site. Keep in mind that the plants and information is based on Australia. But the finding of what happens to bees, and how the bees life is cut in half by going into winter with low internal stores of protein.
Every beekeeper should read this site....http://www.honeybee.com.au/Library/pollen/nutrition.html
It was one of the first things I looked at when CCD was diagnosed, and had presented this to members of the CCD group, as well as discussed with other research types in the industry. You would of thought by some reactions, that this information was never seen before. It may not be the cause of CCD, but poor nutrition may certainly play a part. And not one modern bee book, discusses the problems with protein deficiency.
If you have great pollen stores, then feeding may not be needed. But knowing the pollen seasonality of your area, what stores are in your hive, and the pitfalls of what the bees will face if not enough is available, is something ALL beekeepers should be versed on. Many can tell you if they have a light hive when it comes to honey, but have no clue about pollen stores.
You should have at least several frames of stored pollen going into winter for bees to utilize in early brood production