Yes worms are becoming resistant to warmers due to over worming. Conventional wisdom these days is do a fecal count and worm accordingly. Some argue that that the fecal count is not always accurate as tape worm don't always show, and egg counts give low reading when really they are high. So to use a wormer specifically for tape worm once a year. Everybody has a philosophy and way of handling worming schedules, a lot depends on what part of the country your in if you get a freeze that kills them in the ground, or like me in Florida where nothing ever dies. Personally I get fecals done twice a year and worm accordingly.