Currant Borers

Currants are a refreshing berry used to make jams, jellies, pies, and wine, but currant borers can really put a dent in your crop.

In California, the invasive currant clearwing moth (Synanthedon tipuliformis) makes its appearance each April, laying eggs in bark crevices, at old pruning cuts, and in narrow branch crotches. While individual moths only live one week, overlapping generations can extend egg-laying through August. These pests bore into the canes of currants, and sometimes blackberries.

Currant clearwing moth (Whitney Cranshaw) CC BY 3.0 us

Currant borer description

Currant clearwing moths look more like wasps than moths. They have a long, narrow black body with yellow stripes. Males have four stripes; females have three. Wings are triangular and mostly clear, and the front and back wings are hooked together. Legs and antenna are long, and moths have a slightly paddle-shaped rear end. The wingspan is 1/2 to 3/4 of an inch wide. Larva are white to yellow, with brown head and legs, and a dark dorsal line may be visible. Larva look much like the American plum borer. Pupal cases are reddish brown and 3/4 of an inch long. Eggs are pale pink to red and very tiny.

Currant borer larva (Gyorgy Csoka) CC BY 3.0 us

Currant borer pupa (Gyorgy Csoka) CC BY 3.0

Currant borer damage

When the eggs hatch, larva chew entry holes in canes, burrowing into the cambial tissue and sapwood. Here, they will feed on sap and vascular tissue for the rest of the summer, and then overwinter within the canes. In March, they will resume feeding, going through several instars, before entering a pupal stage. All this boring and feeding can girdle branches, causing twig and branch dieback. It also restricts the growth of infested plants, resulting in fewer blooms (ergo, less fruit). You may be able to see sawdust-like frass mixed with sap oozing out of entry holes, and longitudinal scars on infested twigs and branches.

Currant borer controls

Once a branch is infested, it should be removed and destroyed, or thrown in the trash. Severely infested plants should be removed completely. Once borers are inside a plant, there isn’t much you can do. Some people try shoving wires into entry holes, trying to stab the larva, but this is only marginally effective. Woodpeckers enjoy catching adult moths on the wing and braconid wasps parasitize these pests. There are also some beneficial nematodes that are available commercially. I’m not sure which species of nematode will work against currant borers, but you basically inject the live nematodes into the borers’ holes and spray the branches with nematodes in solution. Nematodes are fiddly and I have not had good luck with them, but they may work for you. Rather than losing valuable plants, it is far easier to prevent borer infestations int the first place.

Healthy plants are better able to protect themselves against infestation, and rapid growth tends to make stems susceptible. Caring for your currant plants in a way that keeps them healthy and growing at a steady pace, rather than overloading them with fertilizer for faster growth, is the best way to prevent infestations. Also, avoid water stress with regular irrigation, especially during the peak of summer. Whitewashing the trunks of mature tree-trained currants with a 50:50 mix of water and latex interior paint can help reduce sunburn damage, which is more susceptible to infestation.

Pheromone traps can be used to monitor for this pest. Just remember that the down-side of using pheromone traps is that they end up attracting pests from a greater distance. According to the U.S. Dept. of Agriculture, insecticides should be applied 7 to 10 days after the first male moth is captured in a pheromone trap. If you decide to use an insecticide, please, Please, PLEASE, follow the directions on the label exactly. Insecticides are poisons.

Finally, regularly inspecting your currants and blackberries for signs of infestation can give you a unique opportunity at borer control. Just before clearwing borers emerge, they have a habit of pushing themselves halfway out of their tunnel and just sitting that way for a bit - catching their breath, I assume. So, look for areas of damaged bark and frass. If you see a borer half-way out of a twig, cane, or branch, help it out the rest of the way and then feed it to your chickens or stomp on it.