Some pest problems are seasonal. Clothes moths tend to do their damage in the spring, when their larvae hatch out in search of food. Rats and mice tend to enter houses for warmth in early winter. In some cases, however, unusual weather can cause problems at unexpected times. A particularly wet summer can bring rats into contact with people in big cities as water levels rise and the rats seek higher ground. If the sewers of London are flooded, the rats are forced to the surface.

Unfortuna...