Liability of shrub owner with hornets

We have a row of shrub that overgrew between our property and our neighbour's. The neighbour recently notified us that there is a hornet set in the shrub, in the portion over our neighbour's driveway. Our questions are the following:
1) Whose responsibility it is to take out the hornet, ours or our neighbours?
2) If someone is stung, who will be responsible for the medical expense, ours or our neighbours?
3) Is it our responsibility to trim the shrub portion that's on neighbour's property?