The Pygmy Shrew: A little mammal that is causing big problems in Canadian overwintering colonies

Typically when critter infestations come up into beekeeping conversation these common mammals come to mind: bears, skunks, mice, opossums and raccoons. Just like their size, pygmy shrews often fall under the radar. However, Fletcher Colpitts, Chief Apiary Inspector of New Brunswick, Canada, is working to make information about the pygmy shrew more available. He recently posted an info sheet about the pygmy shrew that every beekeeper should read:

Pygmy Shrew. Photo from the mammal society:

Pygmy Shrew. Photo from the mammal society:

The pygmy shrew is the smallest mammal native to North America. It can fit through a hole in a honey bee hive as little as 1 cm, and surprisingly only weighs an average of 3 grams. Although they are tiny, pygmy shrews are also extremely fast, and consequently have a high heart rate of 800 bpm. In order to support their high respiratory rate they must eat on a constant basis (at least every 15 or 30 minutes) during the day and night. If they go more than an hour without eating they are at risk of dying of starvation.

They are generalist insectivores, but in northern climates (eastern Canada) they have learned to seek out honey bees for nourishment. Shrews become a problem in the early spring when bees are still tightly clustered due to low temperatures. They feed on colonies by grabbing a bee from the outside of the cluster where it’s colder. Bees on the outside of the cluster are sluggish and unable to defend themselves against the shrew invader. The shrew will then carry its prize away from the cluster and move to the bottom of the frame or sometimes near the top under the inner cover. There it will remove the head and tunnel into the thorax using its pointed snout to consume the contents.

So, how do you identify if a shrew has invaded your colony?  And how do you tell shrew damage apart from mouse damage? To answer these questions I contacted Fletcher Colpitts, and I was fortunate enough to interview him over the phone. He told me that pygmy shrews can be identified by the mess they leave and their feces which differ from mice. The shrew will leave a “trash” pile as Fletcher called it (appropriately named) of heads, wings and legs. At first glance, shrew feces looks very similar to mouse feces, however it is much different if inspected more closely. Pygmy shrew feces are elongated with irregular diameters (rough looking).

Pygmy Shrew feces. Photo by NB Provincial Apiarist Chris Maund

Pygmy Shrew feces. Photo by NB Provincial Apiarist Chris Maund

It is also important to note that shrews will never nest in hive boxes and will never be found during the summer months as mice sometimes are. As temperatures rise honey bees begin to be able to defend themselves so shrews will leave. The primary food source in the hive for mice is pollen and honey; however shrews will only target the honey bees themselves. On occasion, shrews will die in the hive, which is the best evidence you will find.

Occasionally, shrews will become immediately apparent as Fletcher Colpitts discovered.  While watching his colonies very intently, he noticed a shrew (not so much a form, more like a gray flash).  These little creatures move so fast, much faster than a mouse, that it is very difficult to spot them, but definitely possible.  Further confirmation was observed in the form of a headless bee walking out of the entrance of the hive.

Last year was one of the worst years for shrew damage for a major blueberry producer in Prince Edward Island, Canada. Fletcher Colpitts inspected their colonies for a winter loss insurance claim last year. Over the winter of 2012-2013 this particular producer lost approximately 700 out of 1000 colonies (70%)! Shrew damage contributed to a large portion of this loss, although not entirely (there were some other management problems). Fletcher said that on average 2 hives out of every pallet of pollinators displayed evidence of shrew predation.

It’s glaringly evident that pygmy shrews pose a large risk to beekeepers (at least in Canada), but there is still good news to be had. It’s very easy to build a modified entrance system to block them from entering your hives. The system has a screen with holes 3/8th’s of an inch. This allows bees to enter without losing pollen from their baskets while also being small enough that pygmy shrews cannot get in. Fletcher has had a 100% success rate with his system and has since kept shrews out of his hives for 30 years.

Shrew Entrance System by Fletcher Colpitts

Shrew Entrance System by Fletcher Colpitts

After researching this topic for nearly a week, there is one thought at the front of my mind: Is shrew damage possible in the northern United States? Some species of pygmy shrew are in the northern US (ME, NH, upper state NY and the Appalachian mountains). It’s just a matter if whether they have learned to seek out honey bees or if perhaps beekeepers have previously mistaken shrew damage for mouse damage. So I ask anyone who may be reading this: Have you seen evidence of shrews in your hives? If so, please share!

Written By: Rachel Bozarth

Rachel Bozarth has written 18 post in this blog.

I work for the University of Maryland as a research assistant analyzing honey bee alcohol samples from the Bee Informed Partnership and the APHIS National Honey bee Survey. I specialize in Nosema spore counts, but also enjoy field work in the USDA BRL bee yards. I have my bachelor’s degree in Environmental Science from Wesley College and I wish to continue my education in Entomology and beekeeping. Before coming to BIP, I worked on a variety of projects at UMD including scouting corn fields for brown marmorated stink bug, testing the effectiveness of SHB traps and assisting with horticulture research at the UMD Wye Research Center. I love the learning environment my job provides and in the future I hope to start a bee yard of my own.


15 Responses to “The Pygmy Shrew: A little mammal that is causing big problems in Canadian overwintering colonies”

  1. Jerry Shue

    Regarding Rachael Bozarth’s post about pygmy shrews and honey bees, this last spring in SE Utah (Grand County) I found five colonies (up to seven miles apart) dead or near dead with a bottom board covered with dead bees, roughly half of which had no heads, but abundant heads were among the dead bees. The bees that were headless had hollow thoraxes. Your comment ” it will remove the head and tunnel into the thorax using its pointed snout to consume the contents” is a perfect description for what I was seeing. There was not a lot of dismembered debris or wings, just heads and bodies. Phorid flies and vespids seem to be ruled out by the scale of the carnage. We probably don’t have the pygmy shrews here, but several other species of shrew are common. The website link I added above (if it works) shows decapitated bees. I have not had much luck finding an explanation for what I was seeing. I would be interested in receiving any feedback.

  2. dave

    Live in Southern New Hampshire. Late winter early spring 2014 found positive evidence of shrew predation. Believe we lost a number of colonies to shrews. First time we have encountered this pest.

  3. TOisGood

    This article mentions 3/8″ inch holes, but the link talks about 1/4″ till first pollen then 1/2″ holes.

  4. Rachel

    Hello! Sorry about the discrepancy on screen sizes. 3/8″ was the measurement that Fletcher gave me over our phone interview. I believe that either 3/8″ or 1/2″ is fine as long as you don’t use 1/4″ because it will cause pollen to fall off your bees.

  5. Lewis Struthers

    We have pygmy shrews here in west central m
    Minnesota. I can see that they are active in my hives in winter. I often kill them when I unpack in the spring but they are very fast

  6. Richard J Ronconi

    I have had tremendous losses this winte rdue to pygmy shrews. I am in Upstate New York. Dead headless bees, empty thoraxes, bee parts deep on bottom board. These critters are a menace. They reduce cluster so much that the small number of bees left die from sudden drops in temperature in late winter/ early spring. I think that explains numbers of dead bees mixed with headless ones on bottom board. Entrance reducers, mouse guards, maybe less than 1/2 inch holes should help. Upper entrances must be attended to also. It seems shrews will climb up the side of the hive. This is a relatively new problem for us in this area. l