Premature death of a Monarch caterpillar
by Stan Staszewski
(Plano, Illinois, USA)
It seems that the only time a Monarch caterpillar dies while hanging and waiting to form its chrysalis is when I notice 2 or 3 small (size of a wooden match head) oval-shaped, dark-colored organisms in the bottom of the jar which held the caterpillar during its feeding and growth period. Any idea what these are? Where do they originate? Do these cause the death of the caterpillar? Do they spread a fatal disease? Is it merely a coincidence that they occupy the same jar as the caterpillar?
It sounds like the tachinid fly pupae that you are seeing. Take a look at this information on the tachinid fly with photos to confirm this. The fly lays it egg and eventually the caterpillar dies usually in the chrysalis. I would dispose of the caterpillar or chrysalis (put in zip lock baggie and freeze or just throw away) if you can verify that this is what is happening. Otherwise you are going to have new tachinid flies laying eggs on more caterpillars in your garden. This is not a virus so is not spread from monarch to monarch. Hope this helps.