Anchorage Daily News

Q. I have currant bushes that are infested yearly with worms/caterpillars that defoliate them. Even when we are aggressive about picking them off, they win. We have resorted to using a powder insecticide but would like to use something less harmful to the environment. Do you have any suggestions?

A. The likely source of your problem is the imported currantworm. Currants and gooseberries love cool summers and cold winters. They would be the perfect fruit crop if it wasn't for these little devils.

The larvae of imported currantworm feed voraciously en masse and seem to defoliate a currant or gooseberry bush overnight. I once grabbed hold of a currant leaf and felt a writhing mass of currantworms beneath. My resulting shriek tickled my companions.

The adult imported currantworm is a sawfly, and herein lies the problem in trying to control it with one of the most common garden biologicals, Bacillus thuringiensis, also known as Bt. There is a strain of Bacillus thuringiensis effective against caterpillars and one that controls fly larvae. Sawflies are neither caterpillars or flies and cannot be controlled with Bt.

When the eggs first hatch, the larvae are so tiny, it's almost impossible to see them. Insecticidal soap and neem oil will kill them while they're small. Neem is a botanical extract used as an insecticide. You can also find it as an ingredient in toothpaste. The biological pesticide pyrethrin and its synthetic counterparts have greater efficacy in controlling the larger larvae.

Early detection is critical for imported currantworm control. Now is the time to check all your currants and gooseberries daily.

Apparently, the imported currantworm has preference for red currant over black. Last year, my red currant bush was completely defoliated. The black current wasn't touched, and it was growing so close to the red that the larvae could have wiggled right over.

I still suggest monitoring black currants, because even though the currantworms don't prefer red currant, black can still be a host.

There seems to be only one generation of imported currantworm in Southcentral Alaska per year, but the adults can emerge from their pupae to lay eggs over a period of a couple weeks.

Last time I checked my currant bushes, they were fine except for a leaf roller that had webbed together and was feeding on the leaves of both red and black currants.

But the damage the little nibblers were doing was nothing compared to the manic munching of imported currantworms. Once bushes are defoliated, there is no need for applying insecticides, organic or otherwise.

The UAF Cooperative Extension Service is working with the U.S. Forest Service to see if nematodes and the beneficial fungus Beavaria bassiana can be used to control sawflies as they emerge from the soil. Wouldn't it be great if these organisms could also be used to reduce the numbers of imported currantworms?

Julie Riley is horticulture agent with the University of Alaska Fairbanks Cooperative Extension Service in Anchorage.

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