It was for this reason that we invited people to record the vegetation under their trees in 2011 when recording the amount of damage that the moths had done. The final analysis is close to completion, but a look at the preliminary results suggests that:
The presence of mown lawn under horse chestnut trees, which was taken as a measure that leaf litter would not have been present over the winter*, is related to a reduced amount of damage recorded for any particular location and time.
(* Of course, some trees with other types under the tree canopy could have had their leaf litter cleared away, but our observations suggest that this would be a minority. If we had been able to take that into account that would have strengthened our results. So, why did we not directly ask people whether there had been leaf litter under the tree the previous winter, or whether it had been cleared away? The answer is that we wanted this project to be open to anyone - so they could answer all the questions even if passing a tree they had never seen before.)