They are predators on small insects, other mites and their eggs. A complete life cycle requires from 7 to 10 weeks, but from April to cold fall weather there are always some of the bothersome larvae around. Plenty of moisture probably increases chigger survival. After a rainfall, chiggers and a lot of other little beasts climb up onto vegetation to avoid having to swim. Hence, they're well situated to climb onto your ankles.
Frequent rains make the grass and weeds grow rapidly.
Even if hot, dry weather chases the chiggers back down off the vegetation, good plant growth provides a shaded, moist microclimate near the soil for many weeks to come. Then, on any overcast or humid day the chiggers will be back onto the plants and hungrier than ever. Chiggers do not suck blood, and they do not burrow into the skin. Rather, they attach their mouth parts to the skin surface, often in a hair follicle or a pore, and secrete saliva that digests skin cells. According to some of the literature on the subject, chiggers usually do not produce much itching if they are not on the body for more than an hour.