In "A Rose for Emily", why did Miss Emily say she needed arsenic?

Miss Emily indicates to the pharmacist that she needs arsenic to kill the rats and pole cats at her home.  Otherwise, she would not have been allowed to purchase this poison.  Of course, in this time period, no self-respecting gentleman would have questioned a Southern lady's intentions.  The reader knows she does have a rat...Homer Baron...who is threatening to ruin her reputation by leaving her after being alone with her in her home.  Even as old as Miss Emily was at the time of Homer, a chaperone would have been expected to preserve her "ladylike" status.