Caterpillar on buds

Best thing you can do is pick them off and spray the leaves with a product called BT, it is sometimes branded as caterpillar killer and it washes off easily enough. If you anti spraying when in flower like a lot of users then you just have to pick em off everyday. If you picked it off and did not kill it, it might come back to the plant to much on some more leaves. they have preferences for leaves and not all plats are tasty to them. The bud rot might not have been from it though. bud rot normally manifests itself when moisture levers are to high and the airflow is lacking. Growing outdoors in CanadaI almost always run into bud rot on outdoor plants that are not autos as they just grow so dam thick and the rainy season with cold days end of the month fuck it all up. Good Luck!