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Wild Bee Pollinators: The Genus Andrena

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By Sara Bushmann

In May and June of 2010 and 2011, I collected wild bees from 24 blueberry fields in Hancock County in the towns of Orland, Penobscot, Sedgwick and Blue Hill. By “wild” bees I mean any bee that is not a honeybee or a commercial bumblebee. Most of the wild bees are native, occurring naturally in Maine, but a few are not native and have been brought here by humans at some point in time. The wild bees are generally small- the size of a honeybee or less. The wild bees are mostly solitary, living individually or maybe in aggregations, but never forming the large colonies seen with honeybees. Naturally occurring bumblebees are wild, and they live in fairly large, social colonies, but I did not collect bumblebees. In the spring the only bumblebees that are out in the blueberry fields are the queens that are foraging for nectar and pollen before starting to lay eggs. If I collect a queen, I prevent the founding of an entire large colony of bumblebees. I did not want to do this, so I avoided collecting bumblebees whenever possible. 

I used two methods of capture. The first method involved placing small, colored cups filled with soapy water in the fields for 24-48 hours. This method catches anything attracted to the cup, including plenty of insects that are not bees (and unfortunately slugs like the cups as well). Many of the bees I’ve caught have come from these cup traps, but just because I catch a bee in a blueberry field doesn’t mean that that bee pollinates blueberry flowers. So, I also caught individual bees found foraging on blueberry flowers. From these bees I have made a list of possible wild bee blueberry pollinators.
A field assistant shows how to hand-catch bees on blueberry flowers

A field assistant shows how to hand-catch bees on blueberry flowers; taken in Addison ME, 2012.

This list includes 223 individual bees from 34 species caught on blueberry flowers. More than half of these 223 bees come from only three species:

Andrena carlini
Andrena vicina
Andrena carolina
 
The Andrena are ground-nesting bees. The adults dig a main burrow and create lateral burrows for the cells. They coat the walls of the cells with a waterproof lining to protect the eggs and larvae. In each cell they place a food mass (mostly pollen, but some nectar drops might be included) and one egg. The pollen is the food for the growing larva and sustains the bee until it reaches adulthood. Almost all Andrena overwinter in their cells as adults.  Most Andrena make individual nests and don’t share entrances with other adults.  They may place their nests near others. I have seen this in one blueberry field that had a southern-facing, sandy slope. Early in May when the blueberry flowers were just opening but no leaves were out, I came upon an aggregation of Andrena nests on this slope. 
 
Below is a bee at the entrance to a nest. It is not one of the three species listed above, but it is a similar size. The bee is about 3/4 an inch long and the hole not much more than 1/2 inch in diameter. This nest was located in a small, sandy bare patch in the middle of the blueberry field. 
 
These three species of Andrena might be quite important for blueberry pollination. In general, in 2011, in the fields where I caught the greater number of the Andrenid bees, the fruit set also was higher.
bee at the entrance to a nest

Bee at the entrance to a nest.

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