There are no toxicants registered to poison shrews, nor are there any fumigants registered for use against shrews. It would be impractical to use fumigants because of the porous nature of typical shrew burrows.
It is difficult to envision a situation where it is necessary to use pesticides, although certain seed-eating species were treated with toxicants decades ago in tree plantations. Be certain that, indeed, shrews—and not rodents—are the species of concern.
Resources: (online)
Shrews
