The silkworm is in the larval stage. After the larvae matures, it spins silk to make a cocoon. After spinning the silk, the silkworm larvae turns into a pupa that cannot eat or move. It seems to be "dead", but in fact it is not dead. The pupa will emerge into a silk moth (adult) after a while. Therefore, the "death" of silkworms in the poem refers to the end of the development period of silkworm larvae. The "death" of silkworms refers to the pupal stage of silkworm development.
So the answer is: larvae