An Exterminator and Two Missionaries

 A long-time friend recently shared this story with me.
 
When an old-time exterminator came to my apartment for the monthly pest control service, two Mormon missionaries were already inside.  The exterminator said to the missionaries; "You know our work is somewhat similar?"  The young men replied; " What?  How is that possible?"  The old-timer said; "Well,  when you enter a home, you might say: "Let us pray," whereas,  I say,  "Let us spray... "