Farmer Fran sprays her sugarcane field with a highly effective pesticide called 'Bayou Bug Blast'. For five years the pesticide works without a hitch, effectively killing almost all of the bayou weevils trying to eat Farmer Fran's crop. However, during the 6th year, some of the crop takes a hit. During the 7th year, she loses half of her crop, and during the 8th year, she loses almost the entire cane field to bayou weevils, despite using twice the usual dose of Bayou Bug Blast. What happened?