After a very successful first generation, it was much more difficult to find Small Tortoiseshell nests from the second brood. Some of those that we did find had mysteriously disappeared when we returned to collect them. The reason for these disappearances has still not been identified.

Could it be predation by birds, wasps, or spiders?

There has been a huge increase in the number of spiders on the nettles and I caught a wasp in the act of eating a spider on nettle a couple of weeks ago, so invertebrates predators are a possibility!

I haven't seen any birds around the nettles at any point but it would be interesting to find out if bird predation is an important mortality factor in nymphalids.

The other possible culprit is the weather. Have all the caterpillars been washed from the nettles and drowned?

I am intent on devising an experiment to test the importance of some or all of these factors, and until then I'm keeping an open mind!!