Create a slug barrier

Peter Cundall first suggested a slug barrier a few years ago (he is such a wonderful cornucopia of practical gardening knowledge) but if it’s going to work, you need to do it now!

Cherries and pears (and plums, apricots and quinces) are just now coming into flower and leaf in my region so this is the signal to get the barriers down to stop the pupating glossy black sawfly (Caliroa cerasi), from emerging from the soil, flying up to the newly unfurled leaves and laying the eggs that will soon hatch and turn into pear and cherry slug. Pear and cherry slug larvae look like small black slugs that gradually get bigger as they feed on and skeletonise your leaves.

Several years ago Peter wrote an article in Organic Gardener magazine saying that one way of stopping, or at least slowing down, the infestation of pear and cherry slug, is to lay down a slug barrier made of an impenetrable mulch to stop the adult sawfly emerging from the soil. I tried it the following year and it worked and has worked each year since. So here’s how to do it.

Make sure the soil under your tree is really damp right out past the drip line, as the feeder roots are out here. Then lay newspaper over the surface of the soil. Again going out to just past the drip line. The papers should be at least 10 layers thick and need to be watered well so they are wet all the way through. There should be no gaps. If you can’t access newspaper then use cardboard, but again it is essential that it’s wet. Keep the newspaper or cardboard a few centimetres away from the trunk. Now spread thick mulch over the top of the paper/cardboard to completely cover it, but keep it a few centimetres away from the trunk to avoid causing collar rot. I used pea straw this year. Eventually the mulch will rot down, as will the paper, and add nutrients to your soil.

The paper or cardboard forms an impenetrable barrier to the sawfly, which are making their way to the surface. They can’t travel underground to find the edges, so they just die. If a few make it around the edges then they can be dealt with by sprinkling wood ash on them as soon as they appear. Any that you miss will make their way back into the soil in midsummer and reappear as adults around January, so it’s a good idea to freshen up or apply the mulch again just before that, to stop any more appearing.

If you don’t get time to mulch and end up with pear and cherry slug, then go to an earlier article I wrote here to find out the best ways of controlling them once they’ve found their way onto the tree.