There are two different ways cultures view the locust contrasting the Ibo tribe of Africa and the western philosophy and teachings of the bible. Locusts for the Ibo are like a good omen, they provide a visual for the believed blessings they got from their own personal deities. While the bible portrays the locust as a pest or a curse. The contrasting of the two cultures is related to the symbol of the locust because Achebe plays on the fact the bug is seen negatively by the majority of his readers and makes it the symbol of transition or progression.
I hesitate to use the word change because I see the locust more like going from season to season, but having the same year repeat the seasons over when the year comes to an end.
Achebe constructs a vision of happiness that is emanate through the tribes reaction to the locust. Locust to the tribes represent a ritualistic progression that all good yam farming seasons will have. Similar in the western culture to when a lady bug lands on you, it is supposed to bring you good luck, so does the locust to the Ibo tribe.