Monday, September 22, 2014

Pawpaw Poacher

Asimina webworm
Click to enlarge
While I was checking out a pawpaw (Asimina triloba)  grove for fruit, I noticed that most of the trees had some of the leaves hanging down, pale to dry and shriveled.  The damage seem to occur along the petiole and they were usually stuck to an adjoining green leaf with silk.  This was my introduction to a pyralid moth, the asimina webworm moth, Omphalocera munroei.

Wrapped in silk, heading back into the frass
When I began to carefully unwrap these leaves I found occasional caterpillars, small and brightly colored.  They were deep inside the leaves which were bound together by silk and filled with frass (insect poop).  As soon as I exposed one, it quickly crawled further into its protective frass-filled pouch.  When I put one on the hood of the truck it thrashed violently and crawled to escape much faster than the average caterpillar.

Adult asimina moth -  Mark Dreiling
The tiny moth lays it eggs on the underside of older pawpaw leaves.  Although studies have shown that their caterpillars grow better on fresh young leaves, the older tougher leaves make a better protective shelter.  The earliest larvae pull leaves together with their silk to create a communal home while later instars will live singly in rolled leaf edges.  Both types of shelters have been shown to help protect them from predatory wasps which lay their eggs on caterpillars for parasitic development of their young.

Green and dead leaves form the shelter
A caterpillar will chew on the petiole, causing the leaf to die and dry out.  It will then tie it onto a fresh leaf with silk, the dry leaf providing protection while it chews on the green one.  Soon there may be four dead leaves forming its cave before it heads out to another branch.  Eventually the moth can even defoliate the tree late in the season as well as bore into the fruit.

Chewed leaves and new leaf growth
While this is not good for the individual pawpaw tree, it may benefit the zebra swallowtail whose caterpillars require pawpaw to grow to adulthood.  The zebra cats also grow best eating younger leaves.  After the asimina caterpillars kill leaves, the pawpaw responds by growing new fresh leaves that the zebra cats need.  Meanwhile, the asimina stick to their mature leaf diet for their shelter properties.  In Florida this has been shown to increase the number of second summer broods of zebra swallowtails
Our asimina babies

Editor's note
We are currently raising Asimina caterpillars out of curiosity, just to see the adult moths.  Who knows why, as there are more than enough of them this year at Bull Mills.

  • Details on the zebra swallowtail-pawpaw connection are in this blog.
  • Pawpaw is also host to the pawpaw sphinx moth, pink-spotted hawk moth, and tulip tree beauty moth.

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