The image of migrant workers freezing, locking up their bosses and marching for better pay and conditions is not typically Swedish.

Yet these are among the catalog of incidents that have occurred among Asian workers spending the season picking wild berries in the forests of central and northern Sweden.

They are occurring, union officials and aid workers say, mainly because collective bargaining rules, implemented recently, are proving difficult to enforce, and because of what appears to be exploitation by employment agencies based in Asia. The government is monitoring the situation and says it may adapt its rules.

In some cases, humanitarian agencies, local authorities and churches have stepped in to help stranded and precarious workers.