Any group of over a dozen people sets off this fear unless they are all well known to you. The self-control roll is at -1 for over 25 people, -2 for a crowd of 100 or more, -3 for 1,000, -4 for 10,000, and so on.
You are afraid of all “bugs.” Large or poisonous ones give -3 to self-control rolls. Very large ones, or large numbers, give -6. Avoid hills of giant ants.