Come visit us sometime

"Come visit us sometime," Fred happily told his wifes' cousin and her husband, "We'd be glad to have you over a weekend." "Oh, thats wonderful!", the mother of the brood gushed, "As long as there is space, of course, we don't want to impose." "Oh, you won't be imposing. We've got a large place," Fred continued, "Beautiful house, hardly any rats, only leaks when there's rain." "Eh? You have rats?", the woman was a little doubtful now.
"Well, not so many anymore after the snakes moved in."
"Snakes too?"
"Yes, they eat rats you know, 'though there's not a lot of snakes left, due to the venomous tarantulas."
"I think ..."
"But don't worry about the spiders, the wasps have been killing the spiders"
"Wait, perhaps ..."
"Of course, the toxic smoke from the factory has pretty much kept the wasps in check ...".
Several hours later, Freds wife was expressing her puzzlement, "I can't understand why she reacted so oddly to my invitation."
"Me neither," said Fred