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.

Deleo delirium: Maleficia delenda sunt!

There comes a time in the course of a home bug gardener’s life when events call upon him to take up politically incorrect tools and use them to defend and preserve all he holds dear. Or to paraphrase Cato the Elder: Passer domesticus delendus est! I guess the exclamation point is un-Latin, but then so are periods, and my Latin muse assures me that the passive periphrastic agrees in gender and number with the object that ought to be destroyed. So, no matter what you think Cato said, don’t think delenda or delendum here, but 3rd declension masculine Passer, and IMHO, they all have to go.

Why, you might ask, would someone who claims to be gardening for biodiversity urge that a bird, the alpha and omega of biodiversity, ought to be destroyed? If you are a blue-birder or have a purple martin house, then I guess you already know the bloody murder that house sparrows (HOSP) wreak. If blood and bird bodies don’t bother you, examples of HOSP attacks can be found here. Remember - don't look if you can't stand the sight of bloody birds.

This morning we pulled a dead red-breasted nuthatch from the nest box where they have been relentlessly worried by the larger, alien, invasive, and thuggish HOSP. (I’m making no pretense at being a scientist this morning.) The opening to the box is (deliberately) too small for a house sparrow and all they can do is get their head in far enough to peck. They can’t possibly use the nest box, but being bird-brained, I don’t expect they care. And that’s not all! The bloody HOSPs ate my peas! AHHH! What is a bird introduced to eat insect pests doing eating my garden plants? Bloody bad biology is what. Shades of the cane toad, the HOSP eats few insects and feeds mostly on seeds and plants!

What to do? You probably need a permit to even look at a bird with murderous intent, let alone wring its neck. Ha! Unfortunately for the HOSP, in Alberta they are considered invasive pests, and the Province offers lots of permit-free advice on how to reduce their populations. Firing off shotguns within city limits and spreading poisons aren’t among the things one can do, but trapping is encouraged. Well, more or less. According to Chris Fisher & John Acorn’s “Birds of Alberta”, HOSPs arrived here in 1898 and “People with a dislike for this introduced ‘avian mouse’ go to amazing lengths to combat the species, but House Sparrows are here to stay.” And that certainly seems to be true – it’s hard to find a block in the City where you can’t hear their constant cheaping. It's enough to make you want to squeeze them until their eyes pop out.

And then there are all the other maleficent pests scourging the garden. Formica fusca delenda est! Based on the number of people in Edmonton who whiten the ant mounds in their lawns with boric acid, I suspect the opinion that the black ant must be destroyed is widely held. Since I have no truck with lawns, though, I’ve held them no animosity. After all, ants really are extraordinary superorganisms, industrious, and often fun to watch. These particular ants appear to belong to the Formica fusca group and they can mess up my tiny patches of lawn all they want – but if they want to hang out in my garden, then they’d better be polite. This year they have been extraordinarily rude and their tunneling has already killed a purple sage and a hens-and-chicks and severely damaged a shrub cinquefoil. I tried soaking them with the hose to encourage them to move to a drier site. That does work sometimes, but not this year – a shovel and Dr Doom were required. That made me feel bad, but not so bad when I started looking around at all the collusion between aphids and ants.

Leaf miners, caterpillars, sawflies, spittlebugs, they are all bad, but there are aphids on everything! Aphididae delendae sunt!

A Gallery of Aphid Destruction

I agree with Gardening Zone 3b – where are the ladybugs and lacewings when you need them?

The natural enemies are out there, but I guess they don't like the cold wet weather any more than we do. Slowly they are coming on.


To paraphrase Betty Davis, there comes a time in every gardener's life when the only thing that helps is a glass of champagne. A couple hours with the squirt bottle of soapy water will have to come first though.