As if it wasn’t enough to have sawfly larvae eating rose leaves, another type have been spotted eating the leaves of a hardy Geranium (possibly Geranium ‘Johnson’s Blue’). In June this year, it was noticed that the leaves had been eaten away so that only a skeletal structure of veins remained. The lady who grows the plants hadn’t seen a pest getting at them, but kneeling down for an extended peer at the underside of the leaves revealed tiny, greenish caterpillar-like larvae, slowly but deliberately making a meal of the foliage – the larvae of the Geranium sawfly.
Sawfly larva damage on Geranium leaf - a tiny larva is visible in the middle of the image.
As the leaves were already pretty ragged, we decided to cut off the remaining foliage, give the plant a feed and see if this would deter the sawflies or send them somewhere else. The idea would be to try and break the life cycle, remove the larvae before they pupate in the ground near or under the food plant, then hatch out into flies, lay eggs and start the whole process over again.
An active Geranium sawfly larva
The foliage was duly removed and fresh new leaves grew back within a couple of weeks, with flowers following a week or so later. All looked well, but then the leaves started being nibbled again in the same way.
Looking into it more closely and reading up on the gooseberry sawfly, I learned that the larvae will probably have started eating the leaves when they were still in the base of the plant, almost at soil level, so cutting the foliage off won’t help a lot because it wouldn’t get rid of them. The larvae themselves are so small that picking them off by hand isn’t really practical and, by the time they’ve grown big enough to be easily seen, the damage has been done and much of the plant is gone.
Having a nap, maybe?
What to do? Put up with a tattered plant, grow something else in that spot, or spray with an organic pesticide such as pyrethrum. My own choice would be to grow something else. Artemisia 'Powis Castle' could fit the bill.
Trying to find a name for this pesky sawfly larva wasn’t easy. No one seems to want to talk about it very much and what little I did find suggests that not a great deal of research has yet been done. It is named in a list published by the Natural History Museum as being of the tribe Hartigiini. The website BioInfo lists it as Protoemphytus carpini, and it was mentioned in R.B. Benson’s 1958 key to sawflies. For me, just for now, it is enough to know that it is a sawfly whose larvae eat Geraniums.