Bee starvation usually happens when a bee colony finds no nectar while foraging to feed itself and its brood.
Bee starvation is very common among beekeepers this time of year.
Especially this year! I cannot stress enough to check on your bees
weekly during this dearth. If you can, check more than weekly. Brad
recommends getting into your hive every 10-14 days during the growing
season (March through October). Even though you see bee flight and
activity around your hive does not mean it is all fine and well inside
the hive. I've lost a couple of mine to starvation just these past two
weeks, and I'm currently on round 5 of feeding them. Make your bees a
priority! Sugar water is cheap insurance to ensure your bees thrive
during this dearth. Especially since the weather has not been good, the
bees haven't been out to forage, and can easily starve.
You can mix a sugar-water ratio of 1:1 right now (1 cup of sugar to 1
cup of water). I would recommend feeding your bees until they don't take
the feed anymore, or feed them up until the blackberries and until you
know they have a nectar source coming in. Worker bees use sugar
water/nectar to not only feed themselves, but to convert those resources
to royal jelly in which they feed the queen and larvae brood.
If you examine your bees from the outside and notice that they are
lethargic and non-active, they are probably very hungry. They might even
discard their larvae and you might find piles of dead bees on the
bottom board. Continually check your bees in case they need resources
they are relying on you to provide them. Some of these resources include
boosting colonies with extra brood frames, needing queens, or sugar