Granules are not the best product to kill chiggers. A liquid spray such as Bifen I/T
which is also a true miticide is the best chemical and formula. A wet spray over the entire area is recommended at intervals of 14-21 days until the chiggers have "cycled out" and completed their life cycle. A chigger is an immature stage of a mite. In order to kill and contol the population, all life stages have to be eliminated. This can't be done with a single treatment. It will take several treatments over a period of weeks for this to happen.
The only way to control all life forms of the mite - including chiggers is to keep a continuous chemical treatment in place to kill them as they grow and mature (cycle out).