Morning Cloak Caterpillar aggregation

Gathering of Mourning Cloak ”Cats”
Location: Marsh area, N. Ohio
June 27, 2011 10:16 am
Dear Bugman: I spotted this cluster of Mourning Cloak caterpillars, while on a nature trail in a northern Ohio marsh.
I have never seen so many of these cats gathered all together before. At first I thought they we some kind of tent caterpillar. They were feeding on willow leaves.
I noticed an interesting behavior. While some ”cats” were eating, others kept their heads up and made jerking, bobbing motions with them, all in unison. It was like synchronized head bobbing. Is this some kind of defense or early warning system behavior?
Signature: Chrstine O.

Morning Cloak Caterpillars

Hi Christine,
We love your photograph.  Caterpillars that form aggregations by remaining in close proximity to one another while feeding and resting generally derive benefits from the safety in numbers notion.  While we cannot comment specifically on the head bobbing you witnessed, your hypothesis is consistent with a defense explanation.  Morning Cloak Caterpillars will remain in a communal state until they are ready to pupate, though often the chrysalides are also found in “colonies”, though the metamorphosis generally occurs away from the food plant.  It has been our observation that this year we have seen more Mourning Cloak Butterflies in Los Angeles than in any year in recent memory.  It seems from the mail that we have received that there may be a spike in the population of Mourning Cloak butterflies and caterpillars this year.  Often population advances and declines are cyclical.  Perhaps the unseasonal rains in both the Pacific Southwest and in the Mid West this year are responsible for the population surge.

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