A negative effect of using only IGRs for flea control is

• A. Hazardous to humans
• B. Only affects larval stage
• C. May take several weeks to affect control
• D. Works immediately

Answer: (C)

The main idea is how IGRs work: they interrupt flea development rather than killing adults outright. Because they prevent immature fleas from reaching adulthood, you don’t see an immediate drop in live fleas. Flea eggs, larvae, and pupae in the environment can continue to mature and hatch, so it takes time—often several weeks—to notice a meaningful reduction in the population. If you’re relying on IGRs alone, the reduction after starting treatment tends to be gradual as the current adults continue to lay eggs and the immature stages mature and are affected over time.

That’s why the correct answer is that control may take several weeks to become evident. The other choices don’t fit as the main drawback: these products are generally safe for people when used as directed, they do affect immature stages (not just larvae in every situation, but primarily the developing stages), and they do not work immediately.