Joan Childs – Head of Volunteer Service
When I returned to live in North Yorkshire recently, I unearthed some old records of the rare hoverfly Parasyrphus nigritarsis from the Farndale area, and decided to look for this species along the River Rye, Helmsley – close to where I now worked, because it looked to be a similar habitat. This section of the riverbank is designated as a Site of Importance for Nature Conservation (local designation for important wildlife sites) from Helmsley Bridge down to West Ness.
The larvae of this hoverfly feed on the eggs and larvae of Chrysomelid beetles on alder, sallows i.e. willows, and dock (Rumex spp.). Back in May, I found some of the dock beetles by the River Rye, and started to turn the dock leaves over to look for their egg clusters on the underside. While I was doing this I noticed that a hoverfly was engaged in exactly the same search pattern as me, hovering around the leaves, and disappearing underneath them, one by one! This behaviour, and the broad, dark-orange stripes on the abdomen, suggested that I had found the hoverfly in question.
Catching one of the three I saw confirmed that it was, and that it was a female. Several of the clusters of dark orange beetle eggs were overlaid by the paler yellow eggs of the hoverfly.
In June, I returned to the site and found that larvae had emerged from the P nigritarsis eggs and were feeding on the beetle eggs and larvae. Over the next few weeks the larvae were observed feeding and growing, eventually becoming harder and harder to find as they dispersed following the beetle larvae and then going into diapause (suspended development) in leaf curls and in the leaf litter, where they will spend the winter before pupating and emerging as adults. The hoverfly larvae will face a number of potential dangers over the winter, including a rise in water levels depositing mud across the lower dock leaves, and possible negative effects from agricultural land management.
Having found a technique for locating the P nigritarsis, I checked many clusters of beetle eggs on dock in a number of other sites, but did not find any further hoverflies. This suggests that this is a genuinely scarce species, not just overlooked. Other sites where the beetle occurred on dock but the hoverfly was not present, showed extensive beetle feeding damage to the leaves – which did not exist at the site by the River Rye, showing that P nigritarsis can significantly affect the beetle population.