Pear Slug Sawfly Larva

2011 December 10

Sandy found what looked like a very small (8 mm long) slug on July 15, 2011. It was eating a leaf on one of our little plum trees. Its most obvious un-sluglike feature was the fact that it was out and about in broad daylight.

Once I got some pictures, though, it was evident that this was not a slug. For one thing, it had no eyestalks. Instead, it had a distinct head capsule, and from the side view it obviously had legs.

A search of “sluglike caterpillar” eventually turned up a prime candidate. It’s a nearly-ready-to-molt Pear Slug, Caliroa cerasi. They are a type of sawfly, which are related to ants, bees, and wasps. The larvae vary from green to black, starting out as green just after molting, and gradually turning black before their next molt (or just before pupating). They infest pears, cherries, apples, roses, and their relatives, many of which are common fruit and ornamental plants. They skeletonize the leaves by chewing out the green portions, which is pretty unsightly. They really are as slimy as they look, and the skin is a bit sticky (you can see one of its droppings stuck to its side in the next picture).

Pear slugs have two generations per year in most places, and from the timing, this one was near the end of the first generation. It would have dropped to the soil to pupate by the end of July, and then emerged as an adult in August. We saw the second-generation pear slugs infesting a tree in the middle of September. The second generation is the most numerous one, and if they are going to cause any significant tree damage, that’s when they are going to do it. However, since the second generation is late in the season, and about the time the trees are getting ready to drop their leaves anyway, they usually don’t have any serious effect on tree health unless they are extremely numerous.

Their slime coating makes them distasteful to predators, and as yet another non-native species, they don’t have significant predators in North America. They are easily controlled by most pesticides, but it generally isn’t a good idea to spray pesticides on fruit trees in your own yard, so mechanically controlling them is better. Small numbers in the first generation can be picked off by hand, and second-generation infestations can be knocked back by spraying down the tree vigorously with a garden-hose spray nozzle, set for a hard, fast spray.

These are a case of convergent evolution, becoming very similar to slugs. Like slugs, they’ve adopted the crawl-slowly-while-abrading-away-leaves lifestyle, and use the “distasteful slime” defense against being eaten. This form is obviously well-suited to that lifestyle.

5 Responses
  1. December 10, 2011

    So you’re an insect, munching on plant juices all day. What to do with that digested juice? Well, if you’re not concerned about the environment, you simply excrete it away through your digestive tract, blithely contributing to the ecological devastation that comes from consuming too much. If you’re the type of insect that is eco-conscious, concerned about Global Warming and shops at Whole Foods, you recycle that plant juice into a slimy cover that protects you from the elements and predators.

    Reduce, reuse, recycle slime!


  2. December 12, 2011

    Yep. Waste not, want not.

    Of course, I don’t know whether using any and all available materials to avoid getting eaten counts as being “eco-conscious”. I think “survival-minded” is more like it.

  3. December 13, 2011

    I found these (or something remarkably similar) on our new sweet cherry trees last fall. They came back this spring and caused a lot of leaf damage. Interestingly, the fall/late summer generation didn’t happen this year (I wonder why?) I’d never seen them before and it took me a while to figure out what Order of insect I was looking at!

  4. December 14, 2011

    TGIQ: Thanks for the data point, it sounds like they are just moving into the northern areas – I’d never seen anything like them before here, either, and I am practically on the Canadian border (and pretty close to the same latitude as your location, with probably a similar climate).

    Although if I had seen them, I might have just assumed that they were actual slugs. Maybe I should add a new category – “I can’t believe that’s an arthropod”

  5. janice permalink
    August 30, 2015

    Neem spray works well for sawfly. Apply as soon as you see the beginning of leaf damage early in the season.

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