I pay a monthly pest control service to spray the interior and exterior for fleas and other pests but this year it has not worked. The fleas reappear after a day or two. I did a search for this question but there were not many answers about seven-dust and other natural flea killers. This house is very old and there are lots of crevices where the fleas can hide. We cannot afford to move until at least the spring of next year (we live in South Mississippi). Help!