The high threshold works out to an average on 1 nymph in 4 clusters, or .25 nymphs per cluster. It is best to spray at first bloom, or petal fall, but you should spray when the threshold is reached. Protect bees by choosing products with lowest bee toxicity and spraying in the evening when bees are not active. A second application of ferbam is important in this wet weather to prevent cane diseases. Get good coverage of all new growth.
Primocane growth is very variable.
If primocane growth is poor and erratic, you should suspect a problem below ground. Dig up some raspberry crowns. Use pruners to cut through the heaviest wood. Crown borer larvae are dirty white larvae with brown heads. They are big at this time of year, but hidden deep in the crown.