Why Do Bumble Bees Bump Against / Fly At Windows?

Question:
I keep seeing large bumble bees bump against windows of our home.  Why do they fly into windows? Will they be hurt?

The short answer is:
Queens are frequently attracted to shade zones as they seek out nest sites.  From the outside, the window appears to lead in to a shaded area, and bumble bees are attracted to investigate.  In the process, they fly into the window, but I have never witnessed these events causing lasting harm.

Why do bees fly against the window?

Every year I am asked this question!  It frequently happens early in the season, typically when new bumble bee queens have emerged and are looking for a suitable nest site.  

Nest densities can be high in gardens¹.  As the queens explore the area for a suitable nest site, some bumble bee species are attracted to shade, in the hope of finding just the right cavity, hole or crevice that will provide shelter.

Looking from the outside in, what the bees see is a dark area worth exploring.  Unfortunately, this is when bees will fly into windows.

Does it hurt the bee when they fly into windows? 
Since I have not heard of bees knocking themselves out as a result, and no reports of dead bees by windows, I assume no lasting damage is caused!

Read more about bumble bee nests.

“Bumble bees keep getting trapped in my house”

If you often have the windows open, then you may get one or two bees flying into the house.   To prevent this, a net curtain or suitable temporary screen made from a piece of netting, should allow you to leave the window open whilst air can circulate. 

Please do not use screens impregnated with insect killers or deterrents, or you may find you will have the ghastly sight of beautiful butterflies and bees adhered to the sticky substance used to coat the screen! 

If bees sometimes get into your home, my advice is to keep calm – they are not out to sting you on purpose!  However, if you are afraid of being stung, by all means wear protective clothing. 

Gently place a cup or glass over the bee, and slide a piece of card beneath the glass, and take the bee outside.

If you are allergic to bee stings, then it would be better to allow some-one else to remove the bee for you.

Bumble bees bumping against windows inside a greenhouse

During the warm weather, if you have the greenhouse door open, you may well get bees flying in, attracted by your tomato plants. 

Similarly, butterflies, hoverflies and other bees species may end up inside the greenhouse too.

I personally find that prevention is better than cure.  These days I use a piece of netting over the open door to prevent bees and butterflies getting inside, and accidently being trapped in a spider web, or simply using their energy reserves as they struggle to find a way out.

But what if it's too late, and the bees or butterflies are already trapped?  You could try to guide them out, but this can be tricky and result in injury for the insect.

Alternatively, my recommendation is to position pots of bee-friendly plants just outside the open door, to encourage the bees out of the greenhouse if necessary.  Choose something the bees will like, tall if possible, but favourites such as lavender or other herbs may help.

It may seem that the flowers would attract bees into the greenhouse in the first place.  However, I tend to find that I can leave the bees to find their own way out when I have plants by the door.

Reference

1. Osborne, J. L., Martin, A. P., Shortall, C. R., Todd, A. D., Goulson, D., Knight, M. E., Hale, R. J. and Sanderson, R. A. 2008. Quantifying and Comparing Bumblebee Nest Densities in gardens and countryside habitats. Journal of Applied Ecology. 45 (3), pp. 784-792. https://doi.org/10.1111/j.1365-2664.2007.01359.x

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