If residues of grain are left in a hold following discharge of the cargo. there is a high chance of infestation in due course, even though the cargo was not itself infested. Should infestation occur, removal of all residues is essential, to be followed by complete fumigation of cargo spaces, and possibly, depending upon the severity of the outbreak, fumigation of the remainder of the vessel. Some fumigation products are safe for use by relatively inexperienced crew (eg: types of smoke-emitting pellets), but others are more dangerous and toxic to humans. and should be used only by experienced practitioners under the supervision of health authorities. If the insecticide itself leaves residues these may taint a later cargo, so great care must be exercised in its use. Certain cargoes may be loaded with the remains of previous fumigation agents still present. an example being grain from storage silos containing partly wasted self-destroying pellets. Other grain cargoes may be loaded and a vessel’s hatchcovers closed and sealed before fumigation takes place. Naturally, the cost and time of such an operation should be adequately covered in the text of charterparty clauses. Apart from insect infestation of cargo spaces and of living accommodation, (eg: by cockroaches) ships are liable to invasion by rats. Consequently. all ships are subject to six-monthly inspections wherever they trade. and if evidence be found of the presence of rats aboard, remedial treatment will be performed. When effectively cleared, the vessel will be issued with a Deratisation Certificate valid for six months. If remaining clear of the problem at the end of that period, the vessel will be issued with a Deratisation Exemption Certificate valid for a further six months.