Green’s Dictionary of Slang

(US) a louse or other insect.

1920		(con. WWI) W. Carter Devil Dog 34: It was here we first made the acquaintance of man’s most clinging companions, ‘cooties,’ which were...named ‘Arithmetic Bugs,’ by the boys, because they added to our troubles, subtracted from our pleasures, divided our attention and multiplied like —ll.
1944		P. Kendall Dict. Service Sl. n.p.: arithmetic bugs ... mosquitos, insects. They add up to the misery, subtract from the pleasure, divide the attention and multiply like Hades.
1961		Bosworth Craws 104: They [i.e. cockroaches] were real arithmetic bugs!...They could multiply — very rapidly. They could add — to our trouble. And they knew how to subtract from our comfort.
2005		Coyote Bob’s Nature Jokes at www.ci.boulder.co.us 🌐 Which insects are known as ‘arithmetic bugs?’ Answer: Mosquitoes. They add to misery, subtract from pleasure, divide your attention, and multiply quickly.