Sunday, 1 May 2011
Let’s hear it for Billy Bloodworm
As waders pile through northwards as spring progresses, and we search for those that drop down to muddy pools, we might ponder what they are eating. Now I have always been a fan of the humble chironomid, the ‘non-biting midge’, such that my CB ‘handle’ on the Isles of Scilly (back in the days when a CB was the height of fashion) was ‘Billy Bloodworm’ (bloodworm = chironomid larva). These little beasts play a key role in many a food chain, notably forming an important component of the diet of many birds. Pochard have been found to dive selectively over areas with high chironomid larvae densities. In one study, they formed 80% of the spring diet of Pied and Yellow Wagtails. Furthermore, the pupa, emerging sub-adult and adult insects are vital for breeding duck, forming a major food item for ducklings. And of course those passage waders; we know they stop and feed at wetlands where high densities of larvae are available at appropriate water depths (54-84% of diet in one study of 4 calidrid species). Nothing much new here perhaps, but how many sites deliberately try to maximise this food at the appropriate times?
Many other things eat chironomids as well, for example, bigger invertebrates and fish. But chironomids can rapidly colonise new wetlands and reach high densities by exploiting the nutrient flush resulting from flooded decomposing vegetation, before being pegged back by their predators. So managing early successional wetlands (‘scrapes’) to maximise their biomass is possible; any reserve manager worth their salt should be thinking a lot about the requirements of these little critters. Chironomids have a large range of foods, however, algae and detritus are commonly consumed. Temporary pools frequently have high production levels of chironomids. Flooded vegetation within pools acts as both a substrate for the larvae and food source, first as a substratum for algae, then as a source of bacteria as it decomposes. So, a careful rotational management regime of flooding, drying, vegetating, grazing (dunging), rotovating and re-flooding should all be considered to get the best out of Billy Bloodworm.
So how often should ‘scrapes’ be allowed to dry out? Chironomids pass through four stages: egg, larva, pupa and adult midge. The last two stages are generally very short. 33% of studied species have one generation a year (uni-voltine), 44% have two (bi-voltine) and 18% have several. Uni-voltine species mostly emerge in spring/summer and pass the winter as mature larvae. Bi-voltine species usually emerge in spring and autumn. So, flooding for 12-18 months, before drying might be ideal. With several scrapes being flooded and dried in different rotations, maximum production might be achieved. Drying scrapes in spring by 2-4 cm a week, to reveal mud and shallow water of 2-5 cm should make enough chironomids available to feed a succession of those passage waders. Let’s hear it for Billy Bloodworm.