Wednesday, November 11, 2015

Monarch Parasitoid

Monarch parasitoid from WOLF School
Ready to pupate at WOLF school
In front of the 5th grade WOLF School a few weeks ago we found a few Monarch caterpillars in their final instar, crawling on the late season butterfly milkweed while preparing to pupate.  The students came out and quickly also found aphids, small and large milkweed bugs and a milkweed beetle.  They checked the plants daily, bringing in the chrysalis to follow the developments.

While several of the Monarchs emerged, one chrysalis shriveled and tiny black insects emerged, a mere 3 mm in length.  The students kept the container covered with plastic wrap until we could collect them.  By microscopic photographs we were able to identify the parasites and here begins the tale of the Chalcidid wasps.

These are not the wasps I knew as a child.  For one thing they are tiny, 2.5 to 9 mm long, (0.1 to 0.34 inches for those allergic to the metric system).  They don't build nests or dig holes for their young but instead place their eggs on the larvae of lepidoptera or diptera (true flies), or less commonly other insect species.  Their larvae develop within the victim, their birth announcing the death of their host.

Ventral view of parasitoid wasp
Less than 10% of Monarch eggs produce a living adult butterfly.  When the caterpillar leaves its egg, it turns and eats the remainder of the egg, and sometimes eats a nearby Monarch egg.  Spiders, assassin bugs, lacewing larvae, birds and others attack the young larvae.  Parasites ranging from viruses and bacteria to nematodes and mites threaten their health by growing in them.  And then there are the parasitoids.  These are flies and wasps that lay their eggs on or in the larva where their larvae digest the caterpillar emerging from their victim as adults or even a pupa.

Chalcididae wasps are common parasitoids of other species.  However in the case of Monarchs they may also directly attack the pupa.  According to Monarchprogram.org, "Tiny wasps from the family Chalcididae unsuccessfully penetrate the pupa casing therefore leaving a small hole.  The pupa begins to turn dark and dies."

  Pteromalid Wasp -  Marci Hess
There is good evidence that Chalcidid wasp are Monarch parasitoids,  In our case, we know that there were multiple tiny wasps which emerged from the chrysalis.  Wasp identification is difficult under the best of circumstances, and my photographs fall far below this, failing to show the wing vein patterns and other details.  We will just have to be satisfied with the identification as a Chalcidid wasp.

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