A year without field work?

This summer, it seems, there will be no field work for me. The reason is the difference between a thesis that have to be printed in September and the absence of named thesis. So this summer is for writing. It might seem as an odd topic given the name of the blog. However I can take this opportunity to write a bit about the organisms I have studied in the field the preceding summers.

The past four summers I have spent most of the time in field studying pollinating insects and pollination. These are mostly Bumblebees, Solitary bees and Hoverflies. They behave very differently both on the flowers and flying between them, and in other aspects also for that matter. Recent studies, for example, suggests that bumblebees follow linear elements in the landscape when they search for food but can go straight to the hive when they are full of nectar and/or pollen and have to return. This means they probably use different cues to navigate, depending on whether they forage or fly to and from the nest. Maybe a combination of land mark orientation and sun. They also have three ocelli, eyes that are supposed to be used for light intensity measures, and maybe also polarised light. Bumblebees can work and navigate very early and late at day when there is little light.

Bombus pascourum foraging on Centaurea scabiosa

Solitary bees are not social, as the name suggests, but live in aggregated groups where each bee has its own hole to lay eggs in. They, as the bumblebees, has to go back and forth to their nests to fill up food storage for their larvae. The eggs of solitary bees however is left alone. The adult only supply the egg with enough food and then start to build a new chamber for the next egg. Solitary bees do not fly that far and have to have both food and nest habitats within close distance. They probably use different cues, maybe more land mark orientation all the time.

Hoverflies on the other hand do not have a nest to return to. They lay eggs in all sorts of substances from muddy waters to sap-flows of plants. The eggs are left to their own and the adults do not have to go back to care for them. This makes the hoverflies move in a different manner. It is almost only for themselves they need food. Usually they fly around in flower-rich habitats and eat mainly from open flowers.

During spring, one can observe another pattern. There can be a very high activity of hoverflies in some areas, for example one can find aggregations of the species Episyrphus balteatus near shores. This has to do with another fascinating movement in E. balteatus and a couple of other hoverflies, they migrate. They departed from south or central Europe early in spring to end up in the cost line or shores of lakes in i.e. Sweden. They then lay their eggs in colonies of grass living aphids, this is why one can find them in high densities in e.g. reed. Later in spring and during summer one can find them in all sorts of habitats where they forage on all sorts of plants and lay eggs on aphids living on other grasses than reed. Among others the cereals, which also make them to important natural enemies. After a couple of generations they migrate back in the autumn to spend a couple of generations in warmer places to come back in spring.

Scaeva pyrastri foraging on Turnip rape seed

Hoverflies are generally very apt flyers and can, as their name suggest, hover in the air. This can bee seen both over flowers but also in midst air. Sometimes it is individuals of for example Eristalis sp. or Syrphus sp. that, kind of, protect a small territory. I am not sure if it is males or females or both. They can anyhow be very aggressive, especially the Eristalis sp., and I have seen them attack even bees if they come to close. They are extremely fast and can one moment hover almost still in the same spot and then suddenly disappear.

All the three groups of pollinators, bumblebees, solitary bees and hoverflies are good flyers and can probably move around in the landscape more than we have thought them to do. This is a interesting field to look into. But just not this summer.

/Georg