Pine Sawfly

2009 March 7

These were pretty numerous in early June of 2007, both inside and outside the house.

I’m pretty sure that this is a conifer sawfly, in the family Diprionidae. Sawflies are in the same order as bees, wasps, and ants (Hymenoptera), and have a lot of wasp-like characteristics, except that they don’t have the narrow waist and don’t have stingers.

Their wings are clear and membranous, like the wings of the other hymenoptera. I tried to get a good picture of the veins, but this is as close as I was able to come, so it’s probably useless for identification purposes:

I’ve been having a bit of difficulty identifying this one, but oddly, not because there’s no information. There’s quite a lot of information about conifer sawflies, because many of them are pest species – they can defoliate pine trees, sometimes killing them. No, the problem is that most of the information about them, and practically all of the pictures online, are of the larvae, which look a lot like moth or butterfly caterpillars, but longer and with extra prolegs[1]. There are hardly any decent pictures of the adults. This is because, when you see them defoliating your trees, it is the larvae that you see, but the adults are just another small fly-like insect that nobody pays attention to.

Of the few pictures I found, it looks kind of like the blackheaded pine sawflies on this page, except that those are chubbier, have slightly different back markings, and their abdomens are a kind of lurid green. So, while this one is probably related to those (which would probably put it in the genus Neodiprion), I doubt it is the same species. There are a couple of species of conifer sawflies that have been accidentally introduced from Europe, too, and it might very well be one of those.

I saw several pictures of male sawflies, and they have big, feathery antennae, so this one is probably a female. Her neck is kind of amusing, it is very narrow and stalk-like.

I mentioned above that they don’t have stingers. That’s because they still have ovipositors for laying eggs, and since stingers are modified ovipositors, they obviously can’t have both. Sawflies get their name from the shape of their ovipositor: they look like saws. They are used like saws, too: the females use them to cut slots in things like pine needles so that they can lay their eggs in the slots.

The Hymenoptera in general have an interesting way of selecting sex: fertilized eggs come out one sex, and unfertilized eggs come out the opposite sex. For example, in honeybees the fertilized eggs hatch out as females and the unfertilized eggs hatch out as males. Apparently, some (but not all) species of sawflies are the other way around – it is the unfertilized eggs that come out as females. The significance of this is that those sawflies can reproduce parthenogenically, laying eggs that hatch out as females, which lay more eggs, and so on. Usually there are still some males around so that they can reproduce sexually from time to time[2], although sometimes there are populations where the males have died out and the remaining females continue on by cloning themselves indefinitely. I don’t really know what the breeding status was for this one. We saw bunches of them that looked just like her, but I don’t remember seeing any of the feathery-antenna males, so the local population may be all female.

[1] Sawfly larvae get mistaken for caterpillars all the time. If you count the prolegs, moth and butterfly caterpillars have no more than five pairs of prolegs, while sawfly larvae have anywhere from 6 to 20 pairs (except for the ones that bore into plant stems or fruit, which have no prolegs at all). One of the things with pine sawflies is that the larvae tend to feed together in groups, and when disturbed they all rear up in unison and wave their heads at you.

[2] It is believed that the advantage of sexual reproduction is that it lets individuals swap and recombine genetic information in their offspring, which gives them enough variation that they will have better resistance to diseases and parasites. For example, if a female lays eggs that hatch out as females that are absolutely identical to her, then any disease or parasite that can kill her will also kill all of her offspring. However, if she mates with a male that is resistant to some things that would kill her, and maybe she is resistant to some things that would kill him, then their offspring will not be identical – some will have resistance to one thing, and some will have resistance to others. So, when the disease comes through, at least some of her offspring will survive. So, while sexual reproduction does tend to slow down the reproduction rate, and the parthenogenic females could easily out-breed the sexually-reproducing females, there is the whole issue that it is better to have only 25 offspring with 10 of them surviving the next disease outbreak, than 100 offspring with none of them surviving the disease outbreak.

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