Definition
Insects grow inside a rigid outer shell called an exoskeleton, and unlike skin, that shell doesn't stretch. So as a young insect gets bigger, it has to periodically shed its old exoskeleton and emerge in a new, larger one underneath — a process called molting. An instar is simply the name for each distinct stage of growth between those molts. A newly hatched insect is in its first instar; after its first molt it becomes a second instar, and so on, until it reaches adulthood (in species with a pupal stage) or its final adult molt (in species without one).
You'll often see instars referenced by number when a pest control professional or entomologist is describing a pest's life cycle, such as a "third instar cockroach nymph" or a "fifth instar bed bug." This matters for real-world pest control for two reasons: identification and timing. Different instars can look noticeably different in size and sometimes color, which affects how a technician identifies what they're looking at and how old an infestation might be. Instars also matter for treatment timing, since some pest control products — particularly insect growth regulators — work by disrupting the molting process itself, so they're most effective when applied at a stage when the target pest still has molts ahead of it.
- Bed bugs pass through five nymphal instars before becoming reproductive adults, each requiring a blood meal to molt to the next stage.
- Mosquito larvae go through four aquatic instars before pupating, which is why treating standing water at the right time interrupts the cycle before adults can fly.
- Termite workers develop through several instars, and colony development stage is one factor technicians weigh when choosing a treatment approach.