Still no Large Blues on Collard. Today was another perfect day of flight weather – clear blue skies and very slight breezes.
So why aren’t they here yet?
It’s a bit of a mystery. Since their introduction, their flight period would traditionally begin around mid-June. Over the years, this flight period has become earlier and earlier. 2012 saw Large Blues beginning to emerge as early as the 1st of June.
This adjusted flight time is very much what everyone has been accustomed to.
Last year was a bit of an upset to this pattern. The Blues didn’t first emerge until the 14th of June, this was predominantly down to a run of seriously negligible weather; extreme wet followed by drought conditions meant the majority of the thyme across the site was in a real bad way, and the general trend of increasing LB numbers was reversed, falling to equal the totals of 2010.
This year’s weather has been comparably perfect. We’ve had a mild winter which has brought out many of our flowers and insects out earlier than we’re used to. We’ve also had clear skies and ideal flight weather since the 6th of June, plenty of time for them to emerge.
The interesting thing about the delay in their emergence on Collard is that it seems to be intrinsically linked to the flowering of the thyme.The thyme across the site is clearly still in its early stages, as anyone who’s visited the site will have seen for themselves. It is clearly a good thing that the LBs didn’t emerge before their larval food-plant is at its appropriate stage.
Strictly speaking we’re not sure exactly of the mechanism which means the phenology of the LB and thyme are so tied together, but it’s extremely unlikely that the pupa of the LB can ‘tell’ (for example from scent) that the thyme is in flower. It seems more likely that the thyme flowering and pupa emerging are driven by the same external factors.
Specifically, I’d speculate that a possible external factor is ground temperature.
If ground temperature is a key factor in thyme flowering and LB emergence, it might make sense that everything is a bit behind at Collard. England saw record amounts of rainfall in January, and throughout the year Collard has seen its fair share of rainfall. It’s possible that the clay-heavy soils of Collard have retained a huge amount of this water, keeping the ground temperature lower than comparable sites.
It’s also very possible that we may be seeing some kind of knock-on from last year’s bad year. With the LBs complex life cycle it is tricky to pinpoint exact impacts.
This is mostly speculation, and we won’t have a full picture of what might be going on at Collard until after they’re out, we’ve monitored their numbers and fully assessed climatic effects. Until then this is pretty much my line of thinking though!
Today’s report turned out to be more of an essay, I’ll try to get back to form tomorrow!
Thanks for reading! We’d be glad to hear anyone else’s thoughts on the matter in the comments.
Jono, The Large Blue Ranger