Bitter cherry sawfly

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Last year, my young bittercherry (Prunus emarginata) went into full flower for the first time, was well pollinated, and started to set lots of fruit. All but about 3 succumbed to a sawfly, which made the tiny fruits turn brown and fall off. I poked around on the net, and found only one sawfly mentioned that attacks wild cherry fruits, Hoplocampa lacteipennis. Each larva eats the forming seeds of several baby fruits, then drops out and overwinters in the soil.

bitter cherry flowers dead fruits.JPG

This year the same thing has happened--nearly 100% of the fruits have turned brown. But when I cut open a couple of the brown fruits, not expecting to find anything interesting because it was a bit late in the year to catch the culprits, out popped tiny black shed 'skins' (pupal cases), along with the sawfly frass! They were too small to be the sawflies, and they were obviously wasp-waisted hymenoptera (sawflies have thick waists, like flies). Hooray! I've got sawfly parasitoids!

damaged cherry wasp pupal cases

I found a few still-green fruits, and opened them up in case I could turn up something at an earlier phase of the operation. I found two more little black pupal cases, a few active thrips, some partly eaten empties, a dead and slightly mangled sawfly larva, and then I finally hit the jackpot--a fresh sawfly larva with an extra little larva attached to it. So a parasitoid is indeed at work, doing its best to rescue my future bittercherry crops for the birds.

sawfly larva with wasp larva

My current hypothesis is that the sawfly larvae are probably either Hoplocampa lacteipennis or a close relative. The little black pupal cases must be from parasitoid wasps which eat the sawflies, maybe skin and all (since I found no sawfly remains other than one head part), and thrips and possibly other culprits eat any left over cherry pulp and perhaps any left-over sawfly parts, turning the fruit into a leathery shell filled with frass, a pupal case and pretty much nothing else. About half of the fruits I opened, both brown and green, had a wasp skin, so I have some hope that the birds will get at least a few mature cherries to eat next year.

Which all leads to more questions, of course:

For a related story, it's worth looking up the european apple sawfly, H. testudinea. It looks quite similar to my sawflies (though much bigger, and more pigmented), and it seems to have a similar lifestyle except for choice of fruit. There's also an interesting hypothesis about how parasitoid wasps find the sawflies in Chemical ecology of the European apple sawfly, Hoplocampa testudinea. The fruits normally emit a group of chemicals called terpenoids. When attacked by the sawfly, a few of these terpenoids are emitted in much larger amounts. It seems that at least some wasps can home in on these compounds to find their prey. So perhaps the fruits themselves yell for help!

A few general references: