We are eager in spring to see buds opening and flowers blooming after a dark, mostly rainy winter.
Sometimes in spring when we venture out on a nicer day, we are met with the surprise of our currant leaves being reduced to veins, a mere skeleton of themselves.
The imported currant worm (Nematus ribesii) is a sawfly larva that feeds on and skeletonizes currant and gooseberry leaves. It is neither a “saw” nor a “fly,” but a member of the order Hymenoptera (ants, bees and wasps).
The adult wasp has a thick, broadly joined waist and looks very much like a fly, but with two pair of wings. Flies (Diptera) have one pair of wings.
The female has a saw-like ovipositor (a tubular organ through which a female insect deposits eggs), hence the name “sawfly.” She uses this to insert eggs into plant tissue, usually along the veins beneath the leaf. It does not have a stinger.
We rarely notice the adult of this species as it is small, about one-third of an inch in length, and is black and yellow. Adults are usually found on flowers.
The larvae look like a butterfly or moth caterpillars with a recognizable “head” and soft body, but there are noticeable differences to the specialist.
The larvae emerge from the newly opened currant leaves. They grow rapidly and can completely defoliate the plant almost overnight if action is not taken.
Larvae are whitish at first and then become green with black spots as they mature to about one-half inch. If a group of feeding larvae feels threatened, they hold on to the leaf edges with their “true” legs and wave their rear ends in the air. They exude a brown liquid from their anal glands to repel predators and parasites.
Monitor your bushes daily in April as they begin to leaf out by examining the underside of the leaves. Mature larvae drop to the soil to pupate. There may be a second generation in our area about June or July.
Most sawflies feed on forest trees; there are about 240 species known to exist in our area. Even so, healthy plants can tolerate sawfly damage, so be sure to give them proper care, including watering and fertilizing.
Another food tree-damaging sawfly is the pear “slug.” Its larva has a slug shape and a black slimy covering. It feeds on pear, cherry and plum leaves. The adult form of the pear slug looks like a small black bee.
Tilling the soil under the base of your currant bushes or trees in fall and spring will damage some cocoons and bring others to the surface for ground beetles, centipedes and other beneficial insects and animals to feed upon.
Wasps, yellow jackets, hornets, lady beetle larvae and lacewing larvae are often found on trees helping to control numbers as are parasitoid wasps. Like sawflies, parasitoid wasps do not have a stinger and are typically small. (Most range from the size of a fleck of pepper to one-half inch in length.)
You can hand-pick currant worms from bushes and use the “jet” setting on your hose nozzle to wash pear slugs from trees. These methods are protective of our helpful predators. Always use nonchemical methods of control first.