The ecology of small mammals in arable fields drilled with winter wheat and the increase in their dieldrin and mercury residues

A study was made of the dieldrin and mercury residues in small mammals inhabiting arable fields before and after they were drilled with dressed wheat. Results showed that Apodemus , which inhabited both field edge and open surface, fed on the dressed grain immediately. Mean whole body loads increased 68 times from 2.46 to 166.8 μg dieldrin (0.17 to 10.18 w/w p.p.m.) and 11 times from 0.62 to 6.57 μg mercury (0.04 to 0.39 w/w p.p.m.). Maximum body load recorded was 298 μg dieldrin and 13.8 μg mercury. Concentrations increased during the two weeks after drilling and in some specimens reached a level to cause mortality. A marked decrease in stored lipid with onset of cold weather may exacerbate the effect of dieldrin residues. Clethrionomys inhabits only the field edges and did not show these large increases except for occasional individuals. The uneven distribution of Apodemus over the field surface suggested the possibility that two populations existed, one foraging onto the field from the edge and one inhabiting the field surface continuously. There was an increase in body weight and in the ratio of males to females with increasing distance from the hedge into the field. The results indicate that even though there is a ban on the use of dieldrin in spring its continued use as a dressing for winter wheat would appear to present a similar danger for mammal‐feeding predators such as the barn‐owl and kestrel.