Plant Doctor Archive

Slugs on plum tree

WE planted two plum trees last winter in our new garden. They took a while to come into growth in the spring but then seemed to be doing very well. However, the leaves now look like skeletons - all the green flesh has been stripped away and on some there's a horrible slimy bug that looks like a leech. What is it?

YOUR plum trees have been attacked by cherry slug (also called pear slug). It's the larvae of a species of fly and feeds on the upper surface of the leaves. When fully mature they drop off on to the ground, burrow into the soil and pupate, eventually turning into the adult fly which climbs out of the soil, flies off to mate, lays eggs and starts the cycle over again. Usually two generations occur each season, with the second causing the most damage in late summer.

Apples, crab apples, quinces, pears, cherries, plums and hawthorns can all be attacked. If you do nothing, your plum trees will lose their leaves in autumn and in spring produce healthy new growth, but they will probably be reinfected next summer.

With older, larger trees the damage is not always as noticeable and it can be hard to spray them effectively. With your young trees you could help break the pest cycle by dealing to them with a contact spray such as Nature's Way Insect Spray, Mavrik, pyrethrum, neem oil or dusting with Derris Dust. Make sure to apply it thoroughly so the spray comes in contact with as many of the cherry slugs as possible. One good thing is their feeding on the upper surface of the leaf makes them easy targets.

In spring, cultivate under the affected trees by hoeing or forking to kill off pupae which have overwintered in the soil. You may still get future infestations, however, as the adult flies can travel long distances.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.