Gypsy Moth Caterpillar and Adult

2011 October 1

Gee, thanks, Etienne Trouvelot. Thanks a lot.

Sandy found a couple of these caterpillars eating her new rosebushes on July 13, 2011. She was greatly displeased with them – the bush they were on was only about a foot tall, and just two of the caterpillars were coming close to completely stripping the plant.

The most obvious distinctive features are the double row of colored dots running down its back: eight blue dots, followed by twelve red-orange dots.

Then, looking closer, the head is also distinctive.

Specifically, there is a hairy tubercle on either side of the face, kind of like ears.

This is definitely a gypsy moth caterpillar, Lymantria dispar. You’ve probably heard of them. They eat leaves from most of the common types of tree, so I went ahead and fed them the ever-popular apple leaves for a few weeks until they pupated. They made these scruffy-looking, slightly hairy pupae (the top one is still full, the bottom one is a shed shell after a moth emerged from it).

And this was the first one to emerge:

The white wings and huge abdomen pretty clearly mark this as a female (the males are less massive and have brown wings). She was too heavy to fly, and didn’t even try.

Another indication that this is a female is the relatively thin antennae.

While she does have some featheryness to her antennae, the males (which fly around sniffing for females) have much more pronounced feathering so they can sniff her out.

So anyway, I’m not very well pleased to see these caterpillars appearing here. They are a well-known invasive species that regularly gets so numerous that they defoliate entire forests. And, unlike most invasive pests, in this case we know exactly who to blame for their introduction:

Etienne Leopold Trouvelot, a name that will live in infamy.

To his credit, he did realize that what he had done could be a problem, and he did let people know that he had accidentally released a potentially serious pest. And the magnitude of the gypsy moth problem didn’t become obvious until years later, just before he died. Still, his reason for importing the gypsy moth was a classic blunder. He thought that they were related to silk moths, and that he could cross them with silk moths to improve their disease resistance. Of course, they are not very closely related to silk moths (they just look similar as adults). They are actually a type of tussock moth, which aren’t even the same family as silk moths. And as we can see from the pictures of the pupae above, they don’t even make proper silk cocoons. So, Trouvelot’s whole cross-breeding idea was deeply, fundamentally flawed, and doomed to fail from the very start.

The sad thing is, shortly after his accidental release of gypsy moths, he lost interest in entomology and became a rather renowned astronomical illustrator instead. If only he had switched hobbies a few years earlier, we could have been spared all this.

While they were introduced to the US in 1868, they only relatively recently reached Michigan, and have only been in the Upper Peninsula since about 2000. I think they just got established in Houghton County in the last three or four years, and these are the first ones I can remember seeing on our property.

Since the females can’t fly, the dispersal is mostly either by the caterpillars, or by people moving things that happen to have egg masses on them. When freshly-hatched, the caterpillars can “balloon” for up to a mile or so by extruding a strand of silk and being carried by the wind (something that is also done by baby spiders). But, for really fast motion from one forested area to another, it is hard to beat the speed of an egg mass on a piece of firewood that somebody is hauling up to their vacation retreat. Which is why we hear the constant refrain “Don’t Move Firewood!” from all the people who are trying their best to prevent the rapid spread of invasive pests.

There are some non-pesticide methods of control, but unfortunately some of them aren’t very selective. For example, Tachinid flies parasitize and kill gypsy moth caterpillars, but unfortunately they also attack a lot of the native large moths, like the large and beautiful Cecropia and Luna moths. So, outbreaks of Gypsy moths lead to large populations of Tachinid flies. Which then move on and pretty much wipe out the other kinds of moths after the Gypsy moth population crashes.

It actually turns out that the predator that keeps the Gypsy moth population low most of the time is the deer mouse. They rummage around in the leaf litter, and eat the pupae (which are large and fat). Apparently, population explosions of Gypsy moths often follow crashes in the deer mouse population.

The downside of the mice (and you knew there was going to be a downside, didn’t you?) is that the mice are carriers of several serious diseases, like Hantavirus (which is spread by direct contact with the rodents or their waste products), and Lyme disease (which is transferred from them to us by Deer Ticks).

Hum. Gypsy moths, or Lyme disease? Decisions, decisions.

6 Responses
  1. October 1, 2011

    Tim, did you ever see the movie, Cane Toads?

  2. October 1, 2011

    Not only have I seen it, I have it on videotape! A great invasive-species documentary. I think it might be up on YouTube in 10-minute segments, too.

  3. Barb permalink
    October 1, 2011

    My dad would get rid of the tents with caterpillars in them by nailing a coffee can to the end of a pole, putting a roll of toilet paper in it, soak it with kerosene, light it and stick it up into the trees to burn the buggers. Great fun for us to watch him 40 years ago and beg for a turn to burn them. He never allowed us to do that, probably afraid we would burn the woods for him and he was also a lawyer and volunteer fireman.

  4. October 1, 2011

    I’ve got it in VHS, too. Stories like this one always make me think of that movie.

  5. October 5, 2011

    I saw such furry caterpillars in my garden and I thought they would produce large beautiful butterflies and it turned out they are future moths…What a mistake!

  6. November 9, 2011

    I am an undergrad biology student at the University of Wisconsin – La Crosse and am in the process of making an informative webpage about gypsy moths to be published on for a class. I am currently doing research for this, which has brought me to your website where I noticed your photographs. Because these photos are very informative and detailed, I want to request your permission to use some of these photographs for tools of illustration on this webpage, which is intended only for educational purposes. If this is acceptable, all pictures that I use will be properly accredited to you. Please let me know if this is possible or if you have any additional questions.
    Thank you,

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