Colony Post-Mortem

We found out the other weekend that our bees didn't survive the last cold snap.  Actually, on opening up the hive, it looked like they had been gone for a while.  The bees we saw going in and out a couple weeks ago were probably robbers.  Not sure why a few dead bees kept showing up on the landing strip, though.  Anyway, we wanted to post an autopsy here since we like to think of this not so much as a dead-out, but as a learning experience!

Here's the story of this hive (most of this from memory, so some details may be lost in the rendering):

We got it in early May last year: a 3-lb package with a real cutie of a varroa-sensitive queen (aka VSH for varroa-sensitive hygiene).  The timing was unfortunately on the downhill side of the spring fruit tree blooms, and then we got a freeze a week later. So, we fed them 1:1 sugar syrup from the start, and they kept taking it, so we actually had the feeder on there for most of the summer and through the fall (switching to 2:1 sugar water in late August).  They eventually filled most of two 10-frame deep boxes with brood, pollen, and honey, with most of the honey in the top deep box and most of the brood and pollen in the bottom deep box. We had an entrance reducer on for the first couple weeks, then opened up the entire landing strip to allow for major summer traffic.

In late September, we noticed a few yellow jackets hanging out on the landing strip, but they weren't going inside the hive.  The next weekend, the yellow jackets were still there, so we put the entrance reducer back on, but didn't open up the hive, other than to replace the feeder.  In early October, we did an inspection on a warm day, only to find that most of the bottom box was empty, both of brood and honey.  The top box was still full of pollen and (mostly) honey.  This was the first inspection during which we didn't see any eggs (although we didn't check every frame), and the population seemed to be lower than expected (probably 6-7 frames-worth covered in bees).  It was also the first time we saw varroa mites on a few of the bees and on the removable debris board.  The bees didn't seem overly agitated, so we figured the queen must have been slowing down on her egg-laying for the fall and hanging out on frames we didn't pull.  The bees continued to take the syrup until late October, when the temperature was mostly under 50 °F.  At that point, we took off the feeder and added a moisture quilt.  Through mid December, there were bees near the top of the top box.  The cluster seemed small, but present.  In mid-January, we added a candy board, and there were still bees near the top middle of the top box (although the ones we could see may have been dead already at this point).  We didn't notice much activity through early March, other than a few dead bees showing up on the landing strip and a few bees going in and out.  Given how small the cluster looked in December, we were kind of surprised that they would have lasted this long.  Last weekend, after another cold snap, we figured we better decide whether there were live bees in there or not, and discovered that there were not.  Time for an autopsy:

Starting with the screened bottom board: lots of debris and a few hundred dead bees.  Some chewed comb, a couple mouse turds, a few rows of cappings from the honey.  Many half-bees, probably victims of the yellow jackets.

In the bottom box, the frames close to the middle looked like this: missing comb close to the entrance, and the comb near that edge filled with a hard substance--almost propolis-like, but lighter in color. 

Here's the whole frame.  Just a little bit of honey in the upper left.  Otherwise, completely empty.  Frames to the outside looked similarly empty, but without the missing/propolized comb part.

In the top box, the middle two frames had the most bees and the least honey.  The rest of the frames are still full; the top box weighs probably 60 pounds.

Zooming in on the bees that were left shows several of them head-down in the comb, which usually means starvation.  (However, there's honey, like, six inches away.)  The rest are just kind of frozen in place.

A few frames showed a trail of chewed comb like this, which from what we can tell, means mouse damage.  We don't think the mouse killed the bees or chased them away.  It probably just got into the hive because there weren't enough bees to keep it out, and left because there weren't enough bees to keep it warm.  Or maybe it ate too much honey and got a tooth ache.  Or maybe it wasn't a varroa-sensitive mouse.

So, what ultimately befell our poor hive?  We think we didn't do enough to protect it at the end of the summer, and the yellow jackets and robber bees cleaned out the bottom box, in the process killing the queen.  When we put the entrance reducer back in, the remaining bees were able to hold off the attackers for a few days, then it turned cold.  That put a stop to the attacking and robbing, but with no new bees forthcoming, the numbers would eventually dwindle to the point that they couldn't stay warm enough to move to new honey.  That's our hypothesis.  What are the other possibilities?  From reading a number of articles and blogs, there seem to be three candidates:

The hive absconded due to pressure from yellow jackets, robbers, or mice:  It would be hard to rule this one out conclusively, especially since we never found the queen.  But normally before a hive absconds, it cleans out all the honey and finishes raising the brood; we still had a ton of honey left.  Plus, there were enough bees left to form some semblance of a cluster through at least mid-December.  But other than those two things, this one fits, too.

Colony Collapse Disorder: It's hard to conclusively rule this out, too, since there was a box full of honey and very few bees to defend it for (apparently) most of the winter.  So by some accounts, it should have been robbed out if it hadn't failed from CCD.  But usually a CCD hive is left with a small number of bees, including the queen, and plenty of brood.  We only had one small section of one frame with brood (maybe a dozen cells), and no queen.

Varroa mites decreased numbers in the hive, which meant not enough bees to stay warm: We think this is unlikely since we collected a number of the dead bees off the bottom board and did an alcohol wash (á la Randy Oliver), and got zero mites per hundred bees.  We also saw no deformed wing virus, which is transmitted by varroa, and goes hand-in-hand with the mites.  There were a few mites on the debris board, but following Anna's mite counting protocol, we ended up with ~145 total mites.  We scraped the debris board clean in late September, so those mites were collected over at least two months.  During that time, the colony's numbers were probably dropping, but since a typical lower limit for mite treatment is 50 mites per day, all of those mites could have collected in three days time and we would still be ok without treating.  We paid a little extra for the VSH, so looks like we didn't get swindled!

If there's a silver lining to this ordeal, it's that we found out we were what the French call 'sans bees' early enough to order another package for this year, and the incoming package will have plenty of honey for a good head start on their stores.  But it would be nice if we could take a hive through the winter at some point!  So, what can we do better next year?  Mainly play better defense, we think:

1. Take better notes so that if we have to do an autopsy again, we'll have a more precise timeline!
2. Trap yellow jacket queens when they first emerge in the spring so they don't put so much pressure on our bees in the fall. Although we have to admit, we're a little ambivalent about trying to decrease the population of an otherwise beneficial insect.
3. Put the entrance reducer back in earlier.  Like early July, probably, as soon as the main nectar flow is over.
4. If we notice robbing/yellow jacket activity even with the reduced entrance, put on a robber screen.
5. If we discover the colony is queenless in September, but still has a lot of bees, order another mated queen asap and hope she can get 'em through the winter.
6. Do the alcohol wash multiple times throughout the year to establish a baseline for the varroa count.  Then we can deal with it in the fall if necessary.

What do you think happened to our bees?  If you have another theory, let us know if the comments section below!  How did your bees do this winter?  Let us know that, too!