Chalcid Wasp Parasites in Monarch Pupae

In recent weeks, we've had the opportunity to collect a large number of Monarch (Danaus plexippus) larvae and pupae in wild-grown conditions at Metamorphosis Butterfly Farm in nearby Plant City. A large area of Giant Milkweed (Calotropis gigantea) grown there serves as a breeding ground for an enormous population of Monarchs, and collecting caterpillars there is as easy as picking berries off a bush. However, some of the pupa we've collected have come with some unwelcome hitchhikers - Chalcid Wasp parasites.

Chalcid wasps are a very large family, containing more than 22,000 species. The species attacking our monarchs is likely to be Pteromalus puparum. It's harmless to humans, but not so for monarchs. The wasp lays its eggs in newly-formed monarch pupae that haven't had a chance to harden yet, or even through the skin of a caterpillar preparing to pupate. The eggs hatch and the wasp larvae eat the chrysalis from the inside out. When they're full-grown, they eat a small hole through the monarch pupal case and escape to mate and continue the cycle. Several hundred Chalcid wasps can emerge from a single pupa.

Note the tiny holes in this monarch pupa, used by the wasps to escape.

Pteromalus puparum was introduced to the U.S. along with the Cabbage White (Pieris rapae) butterfly, when crops such as cabbage and other crucifers were first brought here for agriculture. Their primary pupal host is P. rapae, but they will readily use other pupae including monarchs when P. rapae is not as available. A study by the University of Minnesota MonarchLab in 2009 noted:

...there was a 6-7% parasitism rate by P. puparum.... monarchs exposed in or near agricultural fields containing host plants of other known P. puparum hosts were more likely to be parasitized than those exposed in other locations... P. puparum was more likely to attack monarchs late in the summer (August and September). Source

This research meshes perfectly with our experience. Plant City and the surrounding area is heavily agricultural, with large crops of collards and mustard grown locally in the winter months. In August and September monarch populations are high, while populations of Cabbage Whites are minimal since their host plants are not in season.

There are other parasites that can attach monarch chrysalides, including tachnid flies, and of course there are bacterial, fungal, and viral issues as well. This is why we generally keep our pupa separated in individual closed boxes in our lab. This allows us to minimize the spread of parasites and disease, maintaining a healthier population overall.