Andrena are a genus of solitary bees commonly called mining bees due to their nesting behaviour. In contrast to the more familiar honey bees, which nest in aerial cavities, mining bees nest directly in the ground. When the spring arrives, look at patches of bare ground in warm sunny places and you may be able to spot little mounds of dirt with a central hole, like a tiny volcano. These are mining bee nests that are excavated by the female bee. She will dig down into the compacted soil and create a small cavity called a cell. She will fill this cell with pollen, forming it into a rounded pollen ball by moistening it and combining it with nectar. When sufficiently large she will lay an egg on the ball and seal up the cell with mud. Inside the cell the egg will hatch and the tiny bee larvae will consume the pollen, developing to adulthood and waiting underground until the next season arrives.
Whilst all mining bees collect pollen, there is often tremendous variation in the types of pollens mining bees collect. Many mining bees are known as pollen specialists, as they will only collect pollen from a restricted suite of plant species from the same botanical family or even genus. A bee like the honey bee will collect from many different plant families, the equivalent of eating a healthy balanced diet with protein, carbohydrates as well as fruit and vegetables. The strategy employed by some mining bees is the equivalent of eating only lemons, or gluten-free cereals only. Whilst we might all know someone who is a little picky about the food they eat, mining bees have them beat. In Michigan, there are mining bee species that will collect only from willows (Salix), sumac (Rhus), spring beauty (Claytonia), geranium (Geranium), sunflower (Helianthus), dogwood (Cornus), trout-lily (Erythronium), waterleaf (Hydrophyllum), golden ragwort (Packera) and blueberry (Vaccinium). There are also lots of mining bees that will collect pollen from many types of flowers from lots of different botanical families, particularly in the spring time. Why is there so much variation in the strategies displayed by this group of bees?
In order to investigate this, I collected mining bees from across Michigan over the course of the 2017 season. It might not be immediately obvious, but pollen is tremendously variable. Pollen is comprised of many thousands of tiny grains made up of cells containing genetic information from the ‘male’ part of the plant. Grains from a single species have the same shape and structure, and this structure can be used to identify them in the same way that the shape and structure of a flower can be used to identify it. Female mining bees collect pollen on their hind legs and the underside of their body, and this pollen can be carefully removed and placed onto a microscope slide. When you look at pollen grains under a microscope you will see grains with sharp spikes, grains with delicate sculpturing and grains which form long, thin cylinders, to name but a few. The art of pollen identification is known as palynology and, whilst time consuming, can be just as rewarding as learning to identify wild plants. Working out which plants a bee has been visiting is a little bit like solving a problem at a crime scene using forensic evidence. Or at least I like to think so.
As well as bees that I collected myself I also looked at bees from the collection here at Michigan State University. The collection here goes back to some of the earliest days of the Agricultural College as it was known then, with some material collected as early as 1890. Collections like these are some of the only records we have of what plants and insects were doing in Michigan in the past. For both old and new specimens, I identified the pollen grains taken from the bee and the date that the specimen was collected, to build up a large dataset of pollen foraging choices (the plant species collected) and phenological choices (the time of year when the bee was foraging). Because pollen grains can be identified and traced back to the plant that they came from, I divided pollens into woody (coming from trees and shrubs) or herbaceous (coming from forbs and other low growing plants).
The proportion of pollen collected from woody plants can then be plotted against the time of year when a particular bee species is foraging (Figure 1a). I originally come from the United Kingdom, so I had a similar dataset for British mining bees to compare against the results for Michigan (Figure 1b). The results are striking – mining bees in Michigan seem to fall into three broad groups; i) spring-flying species foraging predominantly from woody plants (willow, maple, cherry, dogwood, sumac), ii) spring-flying species specialising on herbaceous ephemerals (trout-lily, geranium, waterleaf) and iii) summer-flying species specialising on herbaceous prairie plant species (black-eyed Susan, sunflower, goldenrod). In contrast the British species show no such pattern, foraging from all sorts of plants across the whole season.
Figure 1. The relationship between time of year (peak pollen foraging time for each species) and the proportion of the pollen diet collected from woody plants for mining bees from a) Michigan and b) Britain
Why is there such a difference between these two regions? One of the most striking things for me as a visitor to Michigan is the much more pronounced seasonality. In Britain things are cool and wet in the winter and warm and slightly less wet in the summer. By contrast Michigan has a harsh, cold winter and a hot, dry summer. When the cherry blooms in the spring the whole countryside seems to be white with blossom, but this ends almost as quickly as it begins. In Britain, the flowering season is more drawn out. Here in Michigan, brambles like thimbleberry and black raspberry have a short bloom period, predominantly in June. In Britain, bramble bushes flower from June until early September!
The sharper seasonality in Michigan forces mining bees to make a choice. When they emerge, female bees have around 30-40 days to mate, build a nest and collect enough pollen to produce as many offspring as possible. With short flowering periods, you have to be good at what you are choosing to do if you are going to be successful. Fully half (50%) of the 84 species of Andrena found in Michigan are pollen specialists compared to only 35% in Britain where there is less seasonal pressure to specialise. Next time you are out enjoying the spring sunshine and flowering trees, or when you visit a prairie in the height of the summer, consider the poor mining bees who are desperately trying to collect as much pollen as possible from their chosen plants before the season comes to an abrupt halt.