Final answer:
Using only insect growth regulators for flea control can be less effective because they may take several weeks to control the flea population, as they a) only target the larval stages and not adult insects.
Step-by-step explanation:
The negative effect of using only insect growth regulators (IGRs) for flea control is that they may take several weeks to affect control.
Insect growth regulators, which include substances such as juvenile hormone (JH) analogs, disrupt the normal development of insects.
While IGRs are beneficial in that they are specific to insects and their use can minimize non-target impacts and reduce the chances of developing resistance, they typically do not affect adult insects and primarily a) target the larval stages.
As a result, when using only IGRs for flea control, there can be a delay before all life stages of the flea are controlled.