The phrase "immature stages" refers to the early or young life periods of a living thing before it fully grows or develops into an adult or mature form. Full definition
All pets in a household should be treated with an insect growth regulator to prevent the development of immature stages of the flea. (medi-vet.com)
They randomly chose homes about 2 kilometers apart for the presence of adult mosquitoes and developing immature stages in water - filled containers. (sciencedaily.com)
This prevents immature stages from accumulating in the environment and breaks the life cycle. (fleascience.com)