What stage does the parasite of Alfalfa Butterfly, medicaginus cotesia, attack?

The parasite of the Alfalfa Butterfly, Cotesia medicaginis, specifically targets the early instar stage. During this phase, larvae are more vulnerable and have not yet developed the size or defenses that come with later instars. This is a strategic choice for the parasite, as the early instar stage is when the caterpillars are still feeding and actively growing, making them prime candidates for parasitic attack.

The early instar stage presents a window of opportunity for the parasitic wasp to lay eggs or inject larvae into the host, ensuring that the offspring of the parasite have a ready supply of nutrients as they grow within the caterpillar. By the time the caterpillar reaches the late instar stage or the adult stage, it becomes more difficult for a parasite to take hold or be successful, as these later stages have developed physical resistance or are closer to pupation.