What organic preventatives are there for stem-borer in raspberries? Is there
a cure? Are there some varieties of bramble that are more/less succeptible
than others? We live in Christchurch.
It depends what you identify as stem borer. If it is the Raspberry Crown Borer (Pennisetia marginata) spraying the larval stage with B.t. (Bacillus thuringiensis) available as Dipel will help. If it is the Raspberry Cane Borer (Oberea bimaculata) then the adult beetles and the grubs before they disappear into the plant could be sprayed with Pyrethrum.
In both cases pruning out the damage and destroying the larval stages will be good husbandry and prevention.