The arrival of the Gooseberry Sawfly in the garden is the stuff of nightmares. Do not be deceived by these comely creatures, their discovery is enought to strike terror into the heart of any gardener.
It begins quietly with the arrival of the adults in April and early May. In the blink of an eye the pale green eggs have been laid on the underside of the leaves of currant and gooseberry bushes. Now is the time for the vigilant gardener to strike and, with nothing more agressive than a gentle rub with a finger and a thumb, disaster can be averted. Time is of the essence, as very soon the advance of the hungry “caterpillars” will begin.
First you will notice the small holes in the leaves, and then you will see more and more of the sawfly larvae as they relentless devour the leaves. As they get bigger they will move from leaf to leaf until just a skeleton is left!
The adults of the Gooseberry Sawfly are easy to overlook, only 5 -7 mm long and resembling small inocuous brown flies. If you take a closer look, you will see that they have two pairs of wings, so they are not flies (Diptera have a single pair of wings) but members of the Hymenoptera – the large order of insects which includes the bees, wasps and ants. Just to add to the confusion, not all groups of sawflies have the characteristic hymenopteran “wasp waist”, the constriction between the abdomen and thorax.
Note the two pairs of wings and the characteristic black mark (stigma) on the outer edge of the forewing.
In males the abdomen is black with a yellow tip and completely yellow below.
Adult sawflies are difficult to identify, but the larvae, leafmines or galls are often very distinctive. Although not all species are host specific, identifying the plant under attack, is a useful aid to identification.
There are well over 500 species of sawflies (Symphyta) recorded in the UK, which provides plenty of scope for biodiversity in terms of apperance and ecology. Most sawfly larvae eat the foliage of their host plants, often consuming entire leaves. In species where the larvae feed gregariously, plants can be completely defoliated. Some sawfly larvae feed inside leaves as leaf miners, while the slugworm sawfly larvae graze the upper or lower surface of leaves. The ones I find most often, apart from the Gooseberry Sawflies on my redcurrants, are the sawflies which form galls, especially the Willow Gall Sawfly (Pontania pedunculi).
Some species can produce two or three generations during the summer. I’m not sure that Gooseberry Sawflies produce more than one generation a year this far north, but I will be watching my currant bushes to look for larvae throughout the summer.
Is there a happy ending for this gardening horror story? The good news is that most heathy plants can survive even a serious attack, although they will not produce much fruit the following year. The populations of Gooseberry Sawflies seem to vary from year to year, and not every year produces a devestating plague. This may not be entirely due to the efforts of gardeners to erradicate them from their gardens, but is probably a combination of predation, parasitism and of course the weather!