Monostegia abdominalis feeds on plants in the Primrose Family (Primulaceae) including loosestrifes (Lysimachia) and pimpernel (Anagallis arvensis) and was first described from Europe by Johann Fabricius (probably Linneaus’ most famous bug student). Actually, I should say plants that I was taught belong to the primrose family – apparently there are molecules that suggest these two genera would be better placed in the Myrsinaceae along with cyclamen – but in any case Linneaus described both plant species so we have a very tight origin of names here.
In Canada, our probable alien invader sawfly was first recorded on the introduced and naturalized garden plant Creeping Jenny (Lysimachia nummularia) around Ottawa and Montreal in 1965. In 1970, Peter W. Price – the famed insect ecologist who seems to have spent some time with the Forestry Research Lab in Quebec during his migrations from the UK to the USA – published a very nice study (Canadian Entomologist 102: 491-495) of our – lets call it the Creepy Loosestrife Sawfly – devastating a once unusually successful colony of the native Lysimachia terrestris around the frequently flooded margins of Caousacouta Lake in Quebec.
Now one might think that a loosestrife with the species name terrestris was more terrestrial than most, but it seems to be like most loosestrifes – they like boggy ground around water. Like most loosestrifes that I am familiar with, it has yellow flowers, so the common name Yellow Loosestrife is a bit useless. Swamp Candles is another common name and seems more evocative – especially if we want to build up some outrage that it is being eaten alive by an alien insect. However, the unusual success of Swamp Candles at Caousacouta Lake was due to artificial flooding by the local hydroelectric company creating an excess of suitable habitat. Without the flooding, the plant might well have been too uncommon to notice it being defoliated – and perhaps would not have been colonized by our Creepy Loosestrife Sawfly to begin with.
Peter Price’s paper has an excellent overview of the life history of the sawfly and its interactions with the native loosestrife. The sawfly had, more or less, two generations a year – but some larvae in the first generation hedge their bets and decide to wait until the next year to emerge from their underground pupation chambers. The second generation started in August in Quebec, but I suspect there is only time for one generation here in Alberta. Males are extremely uncommon and Peter suggested that our Creepy sawfly is thelytokous – that is females do not mate, but reproduce parthenogenetically. The females live only a few weeks but mature 30-70 eggs that they have available for sawing into the upper surface of the leaves of the loosestrife. The larvae will drop off the leaves if disturbed (I can vouch for that) and consume a tremendous amount of plant material (I can also vouch for that) – so much so that the loosestrife is mostly defoliated and many plants die. Or at least that is what was happening 40-odd years ago – I wonder what is happening at Caousacouta Lake now? Did the sawflies eat all the loosestrife and go locally extinct? Was the hydroelectric company forced to stop flooding the lake – what would that do to the dynamics? Has the loosestrife evolved resistance to the sawfly? One wonders if this isn’t an interesting historical entomology study waiting to happen.
Setting aside my minimal angst at the decimation of my Creeping Jenny (it was a free gift from Adrian, did far too well for the first couple of years, and now I don’t seem to need to worry about it becoming weedy), I do have a bit of worry about the sawfly’s ability to colonize native species. Dave Smith – the North American sawfly guru at the USDA with about 250 publications on sawflies - has a pdf plate on the web showing the adult, pupa and late instar larva and damage to the native fringed loosestrife. The fringed loosestrife (Lysimachia ciliata) is one of two loosestrife species that live around the lake at our place in the country (the other is Tufted Loosestrife Lysimachia thyrsiflora). They aren’t very common, so perhaps they are relatively safe – unless someone starts a loosestrife farm nearby and the overflow of Creepy sawflies eats them out. Also, I wonder if the sawfly isn’t the reason my expensive Variegated Golden Alexanders (Lysimachia punctata ‘Alexander’) disappeared from my bog garden. Hmm, just checked my notes and I’ll be buggered if it doesn’t say “cocked it Winter 2007-08, after sawfly defoliation 2007”. Bloody sawflies!