Currant Worm seems to be a very local pest; we have removed vigorous plants twice over the years because of them. The attacks were immediate; first-year.

However:

3 blocks away, I have kept an eye on parking strip black and red currant bushes for over 25 years and have never seen any leaf or fruit damage. This parking strip is largely untended, unwatered, and weedy.

1/2 mile away a large gooseberry bush near a sidewalk always looks completely healthy, year after year. That garden is tended.

Has anyone tried organic Neem/spray oil, insecticide soap or something like lime sulfur (If you can get your hands on it) to nuke these critters? I have three currents and one jostaberry. So far I haven't seem these worms but will keep a look out and think about something early in the season to help prevent the plant from ever having them. 

In Washington there are about 20 native Ribes (currant, gooseberry) species so we have a lot of associated pests (and predators of those pests). For years I grew Ribes unmolested by the sawfly (Pristophora) or imported currant worm (Nematus) as I was in the city of Seattle and fairly isolated, until about 3 years later. Gooseberry, Red Currant and Jostaberry all may be discovered in our region. It was pointed out to me several years ago by our local excellence in Entomology and insect observation, Sharon Coleman, of WSU Extension, that you can simply look out for the first damaged leaves (a single netted leaf) typically toward the center and 12-18" from the ground- around mid-April. I am sure there is a bio-fix model that could be used here to determine the arrival of the Hymenopteran. Consider Late April in the shade, and Early April in the sun. After many years of observation, the damage occurs when leaf expansion is about 30 mm. Tiny white sausage eggs are laid on the underside (often on 2 or 3 leaves)-these are harder to spot obviously-and in a few days the leaf is consumed by the tiny hatching worms. Look for 'netted' leaves as the tiny critters consume the parenchyma cells before the leaf veins. These mid-bush leaves are easy to spot and easy to remove as a single leaf. In my experience the 'netted' leaves arrive over a period of 4 or 5 days, so without laying down and looking at the undersides of your leaves for eggs, you can revisit your bushes (I have 80) daily for a few days to be quick and handy at simple manual control. 

Been there. The !#$% imported currant worm attacked my white currants. I diligently squished them for a couple of seasons, tried Bti for a season, then gave up and removed the bush.

Next to the white currant, I had a Crandall black currant that the worms eventually discovered. Clearly not their favorite meal, but it would do in a pinch.