By Danae Frier

This spring we did some surveys of pollinators of some Saskatchewan fruit crops, including sour cherry (Prunus cerasus). We ventured up to Imperial, SK to visit Hill Berry Acres to survey for pollinators and determine pollination levels. Sour cherry is known to be mostly wind pollinated, but fruit set can be much improved by insect pollinators. When we first visited these crops, there didn't seem to be much insect activity, aside from the clouds of mosquitoes! But on closer look we were able to identify a number of small bees, and TONS of flies visiting the flowers. Although flies do not collect pollen the way that bees do, they will still venture deep inside flowers in search of nectar and the hairs on their body can get covered by pollen. As they move from flower to flower, they can transfer pollen from one to another and act as pollinators.

Soldier Fly
A soldier fly (Odontomyia pubescens) collecting nectar from a sour cherry (Prunus cerasus) flower.
Photo courtesy of Dr. Cory Sheffield.

But were these flies actually moving pollen around and serving as sour cherry pollinators? We wanted to find out, so we collected some soldier flies (genus OdontomyiaOdontomyia pubescens) and brought them back to the lab to see if we could find out whether or not they were carrying pollen on their bodies. I rinsed them in alcohol and filtered the resulting mixture through a vacuum filter onto a piece of filter paper. Then I dabbed the filter paper with a small piece of fuchsin gel and mounted it onto a slide where we could look at it under a microscope. The fuchsin dye in the gel absorbs into any pollen grains that might be present, making the tiny grains easy to spot. Sure enough, I found pollen! An interesting factoid: you can tell the pollen from different types of plants apart by their shape; sour cherry pollen has a distinct shape as compared to, for example, dandelion pollen. This means that not only can we tell the flies are carrying pollen, but that it is definitely from sour cherry.

Sour Cherry Stigma
An extreme close up of a stigma of a sour cherry flower, covered in tiny pollen grains (appearing purple due to the fuchsin dye we used to make them more visible).
Photo courtesy of Danae Frier.

Pollen
The pollen grains we isolated from the bodies of soldier flies (Odontomyia pubescens), amidst the other debris that came off them. Again, the pollen grains appear purple due to the fuchsin dye. 
Photo courtesy of Danae Frier.

Our observation of the density of soldier flies on this crop, and our confirmation that these flies are moving pollen around on their bodies, suggests that flies, rather than bees, may be the primary insect pollinator for this delicious Saskatchewan crop! Sour cherry is done flowering for the year but this opens an interesting line of research in the coming years to determine exactly how important this insect is to optimizing sour cherry fruit yield.

Soldier Fly
The soldier fly (Odontomyia pubescens), potentially a very important pollinator of sour cherry in Saskatchewan.
Photo courtesy of Danae Frier.