A synthetic pyrethroid, Bifenthrin, which is the ingredient in Bug Clear and others, is the alternative. As with Derris it is applied when the first pink fruits are seen. It is purely contact with no residual effect. Bug Clear bottle says you can eat crops next day....
.. infection may have come from local blackberries I would guess that most of yours now come from your own plants.
Non-chemical advice is to hoe around the base of the plants in the Spring to expose the pupae to the elements and birds.
Useful info, Salmo, though I have some 'buts' -
I must look for Bifenthrin. But as someone else said, no matter when you spray there are flowers/fruit at different stages. However, I have always tried to apply derris to unopened flowers only, this being the advice I received. Most significantly, I do not know at what stage the fruit is attacked.
I suppose my infection came from blackberries but there is an aspect which indicates that treating the ground will be no use: I planted these canes late one year on a plot which had never had raspberries or blackberries (which are in a distinct area of ground 10 yards distant) and next year, the first crop was infected. So exactly how does infection occur? Where does the adult beetle lay its eggs and how far do they travel to do so?
For those who have not seen the problem, it is a small grub/maggot which is usually squirming in the core of the berry. I went right off them years ago when on holiday in the Norfolk Broads. I picked a bag full of blackberries, left them for one day and found a mass of maggots in the bottom of the bag. Anyone who picks raspberries or blackberries and cooks or eats them immediately without looking may well be eating plenty of grubs without knowing. That is why I don't eat other peoples' blackberry pies.