Jul 23 2013

These Are Not Pine Cones

Published by at 7:30 am under Insects, Fish, Frogs,Trees

Bagworm moth caterpillars on cedar tree (photo by Stephen Tirone)

When Steve Tirone learned the identity of these pine cone look-alikes he had his work cut out for him.

Early this month he sent me this photo (below) and asked:  “Any idea what this is? I had a few on my house last year. They are near my cedar tree.”

Bagworm moth caterpillar (photo by Steve Tirone)

I emailed Monica Miller who replied: bagworm moth caterpillar.

Bagworms adorn themselves with disguising vegetation that eventually becomes their pupating bag.  Until then they chow down on their favorite foods.

Steve looked closely at his cedar tree and discovered it was covered in caterpillars that were eating it alive.  No wonder it looked sick!  He also learned that pesticides don’t work on these bag-covered bugs.  The only way save his tree was to pull off each one by hand, as in…

[I picked them off] for 2 hours today, along with chopping out large parts of the tree.  Ugly work, ugly results, and there are still tons left. Every time I go by the tree I say “How did I miss that one, and that one?”  Easy, because they look just like pine cones!

If any remain on the tree, what will happen next?

Monica did not specify the bagworm species — there are over 1,000 of them worldwide — but I’ll tell you the story of the evergreen bagworm moth (Thyridopteryx ephemeraeformis (Haworth)) because it’s a particular pest of cedars (Thuja occidentalis also called arborvitae).

From May to August the caterpillars eat and mature through seven instars.  In August the mature caterpillars prepare to pupate by hanging their bags from host plants by a strong silken thread.  Then they turn around inside the bag to face downward.

Four weeks later, in September and early October, the males emerge.  They’re about an inch long and are easy to overlook because they’re small and black, about an inch long.  They eat nothing.

Bagworm moth (photo by Mark Dreiling, Retired, Bugwood.org)

The females never emerge.  They’re wingless, legless and have no functioning mouth parts.  They’re just a bag of eggs and pheromones waiting for a male to land on their bag and mate with them.

As soon as the females have mated they shut off their pheromones and lay 500-1,000 eggs inside the pupal sack inside the bag.  They live a couple of weeks, crawl out of the bag to die or become mummified inside.  The males are long gone, having died within a day or two of their emergence.

Over the winter the bag hangs on the tree with 500 to 1,000 potential caterpillars waiting to hatch next spring.

If you pull these bags off your cedars before April you’ll save your trees a lot of trouble.

moth_bagworms_in_cedar_1860096_rsz_bugwoodTwo bagworm moths overwintering on a cedar (photo by Lacy L. Hyche, Auburn Univ, Bugwood.org)

For more information see this fact sheet from Penn State.


(first two photos by Stephen Tirone.  Moth photo (5462023) by Lacy L. Hyche, Auburn University via bugwood.org.  Bags photo (UGA1860096) by Mark Dreiling via bugwood.org)

6 responses so far

6 Responses to “These Are Not Pine Cones”

  1. George Bercikon 23 Jul 2013 at 10:38 am

    Insidious devils they ! It never ceases to amaze me how Nature seems to fill every conceivable niche in some of the most diabolical ways. What’s a poor Cedar to do?

  2. Barbara Simonon 24 Jul 2013 at 1:03 am

    Once you pull them off the tree, is crushing the bags underfoot sufficient? Do you just throw them away? What is the recommended course of action?

  3. Kate St. Johnon 24 Jul 2013 at 5:39 am

    Barbara, the literature says to either crush them or drop them into a bucket of soapy water. The literature also says to cut the silken band that holds the bag on the tree. Otherwise it will girdle the branch tip.

    Interestingly, bagworms are more of a problem in urban/suburban areas because their natural predators aren’t available.

  4. Libby Strizzion 24 Jul 2013 at 8:20 am

    ugh !!

  5. Joon 03 Jan 2015 at 4:50 pm

    What are the bag worms natural predators? We found some on blueberry bushes, do they eat those?

  6. Kate St. Johnon 03 Jan 2015 at 8:08 pm

    Jo, there are so many moths/butterflies that make odd cocoons that it’s hard to say what kind you have and whether they will eat the the blueberry bushes. If you feel like experimenting, you could pull one off and put it in a jar and wait to see what happens. If you’re worried about the blueberries, ask a local expert to identify the bugs.

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