What do the supermarkets do about food residues?
Supermarkets also monitor residues in food that they sell, but rarely tell anyone much about what they find. 
Marks and Spencer
Marks and Spencer has eliminated a number of pesticides and are continuing to phase out more. They make some information available, both about their policies and on their residue tests.
Co-op
The Co Op publishes its pesticides policy, and has banned and restricted the most dangerous pesticides. They also publish residue results.
Waitrose
Waitrose sources its non-organic  fruit and vegetables from crops grown using Integrated Crop Management (ICM) systems, which aim to achieve minimal use of chemicals. Some further information is available in their latest corporate social responsibility report.
Sainsburys
Sainsburys publish their pesticide policy and some residue results.
Tesco
Tesco, though its Nature’s Choice programme, monitors the use of pesticides, but does not restrict the type of pesticides used nor does it publish residue tests.