Novel apicultural situations, at least in my apiaries, can occur most any time. Here I report on an apparent cause for pupal remnants at the hive entrance fairly early in the spring, due to only the adults of small hive beetle (Aethina tumida). First, I review the known causes of this symptom. I include detailed technical photographs of all the occurrences.
Figure 1 shows the frame entrance of Hive 30 on Sunday morning, May 30, 2021 after a night of the cold snap on Memorial Day weekend. Before the unseasonable temperature drop, our spring nectar flow was past its peak and decreasing, but definitely not done (see Figure 2).
Pupal brood presenting at a hive entrance during the spring has two main causes, long known to beekeepers. A normal fast-growing colony caught in a spring cold period sometimes causes chilled brood. In the warming temperatures, the colony could cover the brood, which could have extended to the lower edges of the comb. To maintain the core brood nest temperature, particularly in the early morning cold, the bees contract their cluster. The cluster contraction exposes the margins of the brood comb to the cold. With the return of warmer weather, the bees reoccupy the exposed comb, finding dead brood.
If eggs and larvae were along the comb edges, the beekeeper would probably not notice the loss because bees consume small brood as part of its removal. Sealed (pupal) brood requires more labor to remove. Obviously this brood must be uncapped. The bees partly consume the pupae while removing them from the cells. Then the bees fly the remaining structural parts of the pupae from the hive. Some of the pupae become abandoned on the alighting board. With fortunate timing, beekeepers may see the discarded pupal brood on the alighting board. Colony 30, strong with bees, stacked with supers, suffered slight brood loss from the cold, which was a brief period.
The second cause for pupal brood remnants on the alighting board is starvation. A colony near death consumes its pupal brood. The structural parts of the pupae, the remnants not consumed, fall on the hive floor. The bees removing them leave some on the alighting board.
To distinguish between the two causes, a preliminary procedure is to heft the frame hive, provided it does not have much equipment (supers) stacked on it, which it should not before the main spring nectar flow (because the extra wood weight becomes confused with the honey weight). When hefting a frame hive (tilting it from the rear back corner by grasping it and lifting slightly), three outcomes could occur. If the “weight” (actually the torque) feels like mostly wood, little if any honey (weight) is in the hive, an indicator of starvation. If the weight feels heavy, more than the wood of the hive, the bees were not starving because they had honey. Chilled brood remains as the culprit (provided a cold snap occurred recently). Lastly, the weight may not lead to any obvious conclusion, and the beekeeper needs to inspect the colony. (Honey weighs 12 lbs. per gallon and water weighs 8 lbs. per gallon. Honey is quite heavy, a fact well known to anyone carrying 60 lb. buckets or stacking supers in a bee truck.)
A version of chilled brood along the comb edges occurs from making nucs with large areas of sealed brood and a smaller bee population that cannot cover and keep it warm when the weather turns cold. This past spring, crazy busy with numerous bee projects, one job was eking out six top-bar hive nucs to house some unexpected queens. It was hard for me to accept a cold snap would occur almost into June (in Piedmont Virginia) over Memorial Day weekend 2021. The nucs kept their brood covered (see Figures 3, 4, and 5), except for one.
I figured that nuc would be problematic. It was at the end of the row and the farthest away from where I initially divided up the brood combs and bees coming from other apiaries. (I make up nucs in several ways. This time all donor brood combs and bees rode together in one hive body back to the nuc apiary. The bees had to wait until nearly dark when I quickly divided them into the nucs. The next day the bees learned their new hive locations, usually with not too much initial drift — bees going into the wrong hive — which here could produce needlessly strong and destructively weak initial nuc populations.)
Figure 6 shows the problem nuc, immediately after removing the adjacent empty comb and shooting past the upper honey band because our interest is the lower edges of the brood comb. The brood caps down there have no bees covering them all around the lower margin of the comb. Figure 7 shows the view from the floor of the nuc.
With our interest on warmth, a better way to observe conditions inside the weak nuc is by viewing thermal images. On the same day after the bees settled down, Figure 8 shows the first thermal image of the bee distribution in the weak nuc.
Now we “see” the cold (as blue) invading the lower margins of the brood comb. Also important, notice the sharp contrast in the falloff in the brood nest warmth to the surrounding cold at the bottom and the top of the comb (at least from this position). At the bottom the falloff is sharper, the width of the red and yellow transitions is narrow, before coming to blue because the heat rises away from the brood lower down. At the top, the warmth smears wider into the honey band beginning with red and going across in yellow. That region was in the direction of the rising warmth. (Then in top-bar hives the warmth spreads laterally, helping the colony to grow horizontally comb-to-comb, as verified by thermal images.)
Lowering the iPhone with the thermal camera attached, I took a series of thermal images with the temperature indicator on. At the hive floor, I “aimed” for the same lower edge of the comb, trying to match the same region shown in visible light from Figure 7. (With the camera held down in the nuc box, I cannot look through a viewfinder or focus, or check other settings on the camera.) Figure 9 shows the second thermal image as if we were in the nuc box seeing the comb from the hive floor. In this design of a top-bar hive nuc, the dead brood will be initially discarded here. Then the bees will need to drag them up to the entrance. The pupae cannot accumulate on the nuc box’s small tilted alighting board.
Aside from its lack of use in this nuc design, it is best not to rely on seeing pupae on the alighting board as an indicator of a difficulty with the hive. A colony is starving badly by the time …