Final answer:
IGR treatments prevent fleas from molting, which disrupts their development and prevents reproduction, effectively controlling the flea population.
Step-by-step explanation:
An insect growth regulator (IGR) treatment affects fleas by interfering with their development. Specifically, IGRs act on the larval stages of insects, preventing them from reaching maturity. Generally, IGRs prevent fleas from molting, which is a critical process insects must go through as they grow and develop into adults. By interfering with molting, IGRs effectively prevent fleas from reproducing since only adult fleas can reproduce. In addition to preventing reproduction, the inability to molt to the next life stage means that larval fleas cannot survive to adulthood, thus limiting the flea population.