1,367. Fasting Because of Swarms, Locusts

Hilchos Taaniyos 2:9

When there is an invasion of animals that swarm, like snakes, scorpions or dangerous birds, we do not fast or sound trumpets because of them but we do call out to God. This also applies to things like hornets, mosquitoes, etc.

Hilchos Taaniyos 2:10

Regarding locusts, even if only one wave is seen in all of Israel, the whole country should fast and sound trumpets. We do this upon the appearance of even a few of the species called govai; for the species called chagav, we don’t fast or sound trumpets but we do cry out to God.