Sawfly Sunday: Slugs on the Cherry

In Alberta in the summer it seems a new sawfly is always just awaiting a few moments of spare time to claim one’s attention. I use ‘sawfly’ in the general sense – this week’s offering is in the larval stage. We don’t call caterpillars butterflies, but for some reason ‘sawfly’ usually seems to do for both adults and larvae. Not always though, there are such things as currantworms and then there is the Pear Slug Caliroa cerasi L.

 The University of Alberta’s Strickland Entomological Museum has an excellent write-up on the “Pear slug (official), Cherry slug, Cherry sawfly”, but no explanation for the ‘official’ common name. Considering that Linnaeus named the species ‘cerasi’, one would assume it was collected from the sour cherry Prunus cerasus (also named by Linnaeus), it being unlikely that the cherry was named for the slug (‘cerasus’ is Latin for cherry). As for the generic name, I am at a loss. ‘Cal-‘ in Greek means beautiful and ‘cali-‘ in Latin a wine cup, neither of which seems reasonable for a cherry slug unless Linnaeus had too much wine before coining the name. Come to think of it, ‘slug’ is used for a drink in English and the word seems to derive from the  Scandinavian  ‘slogga’. Perhaps Linnaeus was having a pun or two after a bit too much cherry wine, but ‘roa’ has triumphed over my etymological skills and imagination. Or perhaps 'Caliroa' refers to beautiful roses, another host. 

 The adult of the Cherry Slug is a small black sawfly that may or may not have been digitally captured in the Home Bug Garden – we have pictures of several nameless small, black sawflies. Like the sour cherry, the Cherry Slug is a relatively recent introduction to North America and like some other invaders, e.g. the Creeping Jenny Sawfly, the males seem to have been left behind. Most evolutionary biologists, at least those of the vertebrate ilk, are more than a bit uncomfortable with asexual reproduction (parthenogenesis). It just doesn’t seem right to them and they have invented numerous theories to prove that it should not exist, or if it exists it should die out, or if it doesn’t die out then it should be limited to out-of-the-way places no sexual species would waste their time on. Unfortunately for their theories, parthenogenesis is common in many invertebrates. In fact, all of the Hymenoptera, the order that includes the sawflies, are at least partially parthenogenetic – males are produced without sex and usually have only half a set of maternal chromosomes in their cells.

 The Cherry Slug can cause economic losses in pear and cherry, and is capable of feeding on numerous other members of the Rosaceae (including saskatoons). The second generation seems to do the most damage, but in Alberta only one generation a year is thought to occur. I guess that is why I don’t mind the Cherry Slug – there aren’t many of them and birds, especially those bloody House Sparrows, do more damage to my cherry crop than a few slug sawflies. There’s one advantage of a short summer: fewer pests to worry about.