Instructions
Research the larvae in an insect field guide to compare them to mature larvae of the same species. Mature larvae typically grow large and plump before entering the pupal stage. If your larvae are motionless, but are smaller and thinner than mature larvae they are most likely deceased.
Research the larvae to determine when they typically pupate. Many insects become dormant in the colder months in preparation for emerging as transformed adults in the spring.
Evaluate the larvae's environment, if observing captive larvae. Larvae need moisture, ventilation, food and adequate space to thrive before their pupal stage. If larvae are motionless in the presence of these necessities, they are probably dormant. If only one or a few larvae are motionless, quarantine them in a separate ventilated container with food and water to prevent contaminating healthy larvae in case the motionless ones are actually dead.
Examine the larvae closely, under a microscope if necessary, to detect any subtle movements. Larvae may move slightly and briefly if gently touched.
How Can I Say Whether the Larvae Are Dormant or Dead?
Insect larvae may become dormant if placed in a cold area such as a refrigerator, or as they mature and prepare for entering the pupa stage. When larvae are dormant, they stop moving and eating, and may become darker or paler in color. For these reasons, it can be difficult to determine whether a larva is alive or dead. Careful observation can help you make the distinction.