Many years ago I read a short story about two old women spraying their garden for pests during an alien invasion. They talked about how they didn't think the invaders were dangerous. Off in the distance giant aliens stride toward them spraying the Earth as if for pests. It was in a book of short stores but I don't remember the book title or author.
It's Pattern, published 1954, a short story by Fredric Brown.
The aliens (and their spaceships) are gigantic but lack density: if an alien stomps on a house, it does not destroy it; people inside the house just see a shadow, as if a cloud covered the sun for a moment. So, after the initial horrified reactions, humans start ignoring the aliens.
The story, which is really short, ends with the situation described in the question: while outside the giant aliens can be seen creating some sort of clouds, one person asks another (who was working in the garden) whether she was giving some fertilizer to her plants, to which the answer is something like "Not fertilizer. Poison." (thus suggesting that the aliens are going to exterminate the humans using those venomous clouds).
Many thanks to user ImaginaryEvents for finding the original title. I only knew its Italian translation: in Italy, this story was published in 1961 in Il secondo libro della fantascienza (= the second science-fiction book), a collection of short science-fiction stories, as Questione di scala (= a matter of scale).