Did you put mouse guards on your hives? Depending on the local mouse populations, which vary by location and winter season, bee colonies could have suffered considerable comb damage. Perhaps though, the bees endured the cold without being visited by foraging mice, which might be just good fortune, hardly a management plan.
Comb pests change with the seasons, leaving wax moths in the summer heat, and turning to mice in the winter cold. What about the transition in the fall? When I monitored varroa mite populations in about 100 top-bar hives with screen floors and sticky boards, I could tell the very first mouse incursions into the hives. As the mice chewed the combs, bits of wax fell on the sticky boards. These wax pieces were larger than the much finer wax bits resulting from bees chewing comb during day-to-day brood nest activities (see Figure 1).
Even when the days remain warm with plenty of bee flight, though the nights turn cool enough for the bees to cluster, leaving peripheral comb exposed, mice gain the advantage. They can enter the hive for a quick taste of pollen and honey (see Figure 2). The bees repair the small amount of comb damage the next day as they reoccupy the unguarded comb, erasing any signs of the mouse’s intrusion. Inadvertently, the bees hide early mouse forays into the hive.
Under the screen, like a silent witness, however, my sticky boards recorded the mouse’s night reconnaissance by recording pieces of the chewed comb. Furthermore, the location of the wax on the sticky board indicated the mouse chewed the comb just above, showing me the vulnerable places on the comb. As the cold weather becomes more prolonged, the mice will keep venturing into the hives, causing more damage.
Overall from the mouse and bee behavior, the first mouse invasions were stealthy and cryptic. Hardly detectable. For bee management, the top-bar-hive lesson with screen floors is clear: Install mouse guards on the hives early, just before the nights turn cold.
Beekeepers have made various styles of mouse guards, mainly from wood and hardware cloth (a grid of wires similar to the screen used with a screen floor hive, but with larger meshes). To keep mice out of my top-bar hives, I place strips of hardware cloth over the entrances. I attach the wire to the hive with small sheet metal screws. With a small ratchet wrench, I can tighten the screws quickly. The meshes are half an inch wide, probably a little larger than some other beekeepers would use. For me this mesh size keeps out the mice. The larger mesh size helps the bees to drag out the dead bees and keeps the lower entrance holes from becoming blocked with them.
In other locations a slightly smaller mesh size might be a better match to local mouse populations. I confirmed the half-inch mesh size was small enough after several winters using game cameras to photograph mice trying enter my top-bar hives (see the last picture, Figure 18). For my mouse populations, the quarter-inch size would be needlessly small, that is, too small. How could the quarter-inch size mesh be too small? Of course, a mouse guard must exclude mice, but its other function, sometimes less appreciated, is important too.
A mouse guard must not hinder, or make dead bee removal difficult, and certainly not accumulate dead bees and block the entrance. Dead bees easily become caught on the hardware cloth wires when the undertaker bees drag them through the meshes. Obviously, a larger mesh (opening) makes their job easier. Therefore, the mesh size of a mouse guard results from a tradeoff (and the mesh sizes of available hardware cloth). The mesh size is the smallest size needed to block the smallest mouse (by its skull) of the smallest local mouse species that invades hives, versus the largest mesh size allowing the easiest dead bee removal. For example, at bee meetings away from my home area, I have seen mouse guards with quarter-inch mesh. Naturally, I wondered, was this smaller size really needed?
For my frame hive operation, I have homemade mouse guards shown in Figures 3 and 4. Figures 5 and 6 give the main points for constructing the mouse guards. For each operation, top-bar hive and frame hive, I have marked storage containers holding each style of mouse guard. In the fall, I load these containers on the bee truck. Now all the mouse guards are with me in the apiaries, nothing to forget. (The ratchet wrench for attaching the top-bar-hive mouse guards stays on the bee truck.)
Recognizing with confidence the symptoms of mouse activity in the hive is an important beekeeping skill. That determination could come in the winter cold when the hive cannot be opened. It could occur during a warm winter day on a brief winter inspection, looking in an entrance slot, seeing a mouse had evaded the guard meant to keep it out.
Mice typically eat pollen from the pollen band of brood combs. That damage in the upper part of the combs can leave them weak, oftentimes worthless, especially top-bar combs suspended from only the top bar. As mice gnaw combs, pieces of wax fall on the hive floor or they may appear on the alighting board. The size of the chewed wax pieces is important to distinguish the mouse problem from other situations when something chews the comb.
Bees opening honey cells (capping-cutter bees), robber bees, and mice all produce roughly different-sized pieces of wax when they gnaw comb. Some size overlap of the wax pieces occurs. By the current conditions, one can figure out what produced the pieces of wax. In early spring (sometimes fall too) capping-cutter bees chew open caps and move honey closer to the cluster. They produce very fine bits of wax, on the order of what fell between the brood combs in Figure 1. In mass robbing when bees become intensely excited to rob honey from comb, particularly new comb (with no fibrous cocoons), they gnaw away the upper cell walls. The size of the pieces is much larger than those made by the capping-cutter bees, but generally smaller than the pieces made by a mouse gnawing combs (see Figure 7).
Dead, decapitated bees are another symptom of a mouse invasion (at least under the foraging conditions in my area). It is well known that mice eat pollen, a protein source with presumably a sweet taste to them (potentially from the nectar mixed with the pollen. That seems straightforward and reasonable. Less well known however, are mice eating dead bees, but for some cryptic reason just the heads. On numerous occasions, I have found mice nesting in a hive and dead bees covered the floor, most all of them headless (see Figure 8). The conditions when mice eat dead bees are …