Michael Jaross of the [mtbakerbees] discussion group posted this reply to Charlie’s post on inspections and robbing. Much of the response talks about what is blooming now and gives a clue to how you can tell that a dearth is in effect. I don't think Charlie will mind me posting that he texted me this evening that he'd installed feeders in all his hives, but not yet filled them with syrup. I've edited my reply a bit, but here's the gist of it:
No need to fill the feeders unless you see uncapped nectar begin to disappear from the outer frames. If they're still heavy, you're probably good. I don't know when the late spring nectar dearth is in the south sound, (Olympia), but up here it starts about third week in May and in late years has continued well into June. I've already fed 14 gallons of 2:1 to eight hives and their nucs. I'm projecting that I will probably pass 30 gallons before it's over. Spring dearth is just an inconvenient fact of beekeeping life in the Pacific Northwest and many other places in the world.
There may have already been a nectar dearth in your area and that may have caused the robbing issues. Even though some hives may appear to still have good stores of nectar, the bees sense when forage is coming to an end and will begin to look for a free lunch in the neighborhood; i.e. robbing.
So, I don't wait until any particular hive gets low on nectar. It's often the nucs that I notice getting low on food first because I'm checking them for queens and new eggs. Young nucs have only minimal foraging ability so run out of food very quickly. Once I've observed that, I “drizzle” all the nucs ASAP and then install and fill feeders in the production hives. I never leave a feeder empty because bees accumulate in the feeder tubes and when you eventually fill the feeder with syrup, you end up spending a lot of time fishing out bees, some alive, but mostly dead.
At this time of year, most hives have lots of brood and lots of bees, all needing food 24/7. A late-spring hive goes through much more nectar than we, as beekeepers, ever see. Which means that a hive can die practically overnight from starvation even if uncapped nectar was seen recently, especially if there's a day or two of rain when the bees can't forage.
If you take a survey in your neighborhood and don't see very much in bloom, it's probably wise to fill the feeders. Up here, the Hawthorn trees beginning to drop their blooms is a sign that the May/June Dearth is imminent if not already started.