very low numbers of larvae?

I agree with Matthew so far. It may turn out to be a very poor season for larvae.
I spent 90 minutes today looking for larvae on Salix caprea trees in a good iris wood in Bucks. Normally, in an average season, I would expect to find about 4 larvae in that time: I found none. I didn't even see any leaves with the characteristic eating pattern, but without larvae, indicating that a predator had got there before me. This points towards a low egg lay, possibly due to a below average number of females.