In the previous article, we encountered a colony, strong in the spring, whose population had decreased, until calamity finally struck in the summer.
Slime began oozing from the hive entrance. Cryptically, small hive beetles had been destroying its unprotected combs, which most likely had resulted from long-term queenlessness. While not readily noticeable from the entrance, mainly because bees were readily present there, including some forager traffic, beetle larvae had forced a smaller colony population from large areas of comb. I have observed that behavior before in my single-comb observation hives. Pollen bands provided large amounts of protein for beetle larvae to support their rapid population growth. (Beetle larvae also consume brood, but that was waning here.)
Last time with thermal images, we witnessed masses of beetle larvae producing their own heat, giving them perhaps a way to endure cool nights. As far as we know, what governs beetle larvae congregations shown in the previous article, and again in Figure 1 for reference, is not understood very well.
In the usual scenario, the beekeeper is not present when the hive actively leaks slime as we saw in the previous article. The affected hive could be in an out-apiary, maybe not inspected often in the summer. Even after the beetle larvae have vacated the hive, a beekeeper should confidently recognize the symptoms of comb destruction without opening the suspect hive for confirmation.
More beekeepers around the world will be confronted by dead colonies in the summer, where small hive beetles have converted resources (brood, pollen, and some honey) into thousands of progeny beetles. As a destructive invasive species, small hive beetle is moving into countries far from its native range (sub-Saharan Africa): USA, Australia, Canada, Mexico, El Salvador, Nicaragua, Cuba, Jamaica, Brazil, Italy, and the Philippines (see Al Toufailia et al., 2017 for references). Idrissou et al. (2019) showed that the international beeswax trade facilitates some of the small hive beetle movement around the world. They also cited South Korea, Sicily, and Mauritius as new ranges for the beetles.
Besides the lack of bee flight, the remains of dried-up slime could be present, although from the limited number of cases that I have seen, the material there is not readily obvious. On the other hand, once I observed a dry powdery orange substance, presumably from where slime production was limited when the beetle larvae exited the hive. On the outside of the hive, larval trails may remain in dried slime. Figure 2 (left) shows these larval trails on one side of a top-bar hive. Figure 2 (right) shows a closer view of the compact nature of the trails. While the slime was wet, numerous beetle larvae crawled over the front of the hive, and proceeded around the corner, abruptly halting for some unknown reason on the side. My top-bar hives have a generous overhang from the metal roof to protect the wooden hive. The side of the hive slopes inward, away from rain exposure. Both design components might have helped to preserve the details of the trails.
While out patrolling my apiaries one night, I found small hive beetle larvae leaving a colony (see Figure 3). Most larvae went no farther than those shown at the entrances, at least they hesitated there. I have observed this paused exodus only once. Together with the mass of larvae at the back corner in the frame hive (Figure 1), and the abrupt directional change (Figure 2), I wonder if these beetle larvae exhibited social behaviors for mass movement, perhaps a beetle version of that seen in social tent-building lepidopteran (moth and butterfly) caterpillars (Fitzgerald and Costa, 1999).
As mentioned in the previous article, all my hives, top bars or frames, are on elevated hive stands; I can quickly capture migrating beetle larvae by attaching a bucket under the entrance(s). These buckets are the standard five-gallon ones containing about eight inches of dark loose moist soil, favorable for beetle larvae pupation. In an exhaustive analysis, Ellis (2003) found that soil moisture was particularly important for beetle larval survival. Therefore when using soil as “bait” in the bucket I am careful not to let it dry out. That concern becomes more important when leaving the buckets on the hives for a month or so to sample mature larval production from colonies not displaying any overt damage from them.
Spiewok and Neumann (2006) reported a low level of small hive beetle reproduction (colonies with <50 larvae) in the eastern United States (Maryland and Florida) from colonies without obvious small hive beetle damage to combs, particularly from those colonies with queen problems and a layer of detritus on the hive floor (Figure 4). A layer of detritus functions as a protective refuge for beetle larvae. Before the invasion of small hive beetles, greater wax moth larvae of variable sizes could frequently be found under this detritus layer on the floor.
In Virginia, during hot, humid summers when rainfall is bleak, colonies of moderate strength can have several hundred beetle larvae under a detritus layer. Because of small hive beetles, I have been recommending two areas of the hive for greater inspection scrutiny: 1) The bottom board, look for larvae and adult beetles under the detritus layer, if present. As I work through the frames of the brood super over the bottom board, the working space between the frames gives a transect view from the front to the rear of the hive, revealing any detritus for greater inspection after the frames are done, which is, for standard hives with the frames perpendicular to the entrance, called the cold way. For hives with frames parallel to the entrance, mostly for top-bar hives with the entrances on the end of the hive, the warm way, just watch for detritus as the working space moves to the rear of the hive, often the location of detritus. 2) The pollen bands, which can be the sites where adult beetles, along with their microbes, infect the pollen and change the cells, particularly swelling the cell rims, causing the bees to cede those areas to beetle reproduction. In my experience, once a small area of a pollen band has become infected with an “aggregation” of adults presumably enlarging its edges, the bees cannot reverse the spread, although the colony appears otherwise normal and well populated elsewhere.
Finding a dead colony where small hive beetles have demolished the combs, the bees and beetles having left the hive, the ground should be treated to prevent a large burst of beetles from potentially occupying the remaining colonies. While university websites tell of various ground treatments, logistically I do not keep that material or the delivery systems on the bee truck. I do have another way to quickly disrupt the beetles’ development. First the relevant background biology.
After the wandering beetle larvae tumble off the edge of the alighting board, crawl a ways and dig into the soil, they form their pupation chambers, which are hollowed-out places where they pupate. Figures 5 and 6 show beetles developing in their pupation chambers. Beetle larvae vary in their direction and distance from the hive entrance (the origin) to the location where they dig across the ground (the x-y plane). The wandering larvae vary in how far down they dig before stopping to form their pupation chambers (the depth dimension, the z-axis). The underground locations of the pupation chambers form a spatial distribution (which would be all their (x, y, z) coordinates). Imagine you have the hive on the ground because its height above is not important here. Now in front and around the entrance imagine you can see all and only the pupation chambers (no dirt, roots or rocks). How are the pupation chambers “scattered” or distributed?
Within one foot from the hive entrance, Pettis and Shimanuki (2000) found 83% of pupating beetles. About three feet from the entrance, they found 17% of the beetles, and none at six feet. The hive location was in south-central Florida on sandy soil. It is reasonable to assume the distribution was radially symmetric away from the entrance, but I have found that hard and loose regions (volumes) in the soil might deplete or bunch, respectively, the pupation chambers. Pettis and Shimanuki (2000) also found pupating beetles about eight inches (20 cm) deep into the soil with nearly 80% of them within the first four inches (10 cm).
Insects moving through soil are fascinating. I think the champion is the tiny Coffin fly, Conicera tibialis (Diptera: Phoridae). In 2011, The Journal of Forensic Sciences had a case from Spain originating in an old graveyard where …
