Effects of dichlorvos, maldison and pirimiphos-methyl on food consumption, egg production, egg and tissue residues, and plasma acetylcholinesterase inhibition in layer strain hens

In three experiments laying performance was studied in hens given graded levels of maldison, dichlorvos and pirimiphos-methyl either separately or combined in the feed over a four-week test period. Results conclusively demonstrated interaction between dichlorvos and maldison as measured by depressed food consumption and egg production. Combining the three insecticides at levels which when given separately had no effect, severely depressed food consumption and egg production. After four weeks on treatment, birds receiving pirimiphos-methyl at 50 �g/g of diet had residues of 0.08-0.17 �g/g in fat and 0-0.06 �g/g in muscle, and residues of 0-0.07 �g/g maldison were recovered in the fat of birds receiving it at 100 �g/g of diet. No residues of any insecticide were detected in eggs and no dichlorvos residues were detected in any tissues. Plasma acetylcholinesterase (AChE) levels were reduced by 70% with dichlorvos at 30 �g/g, by 30% with maldison at 100 �g/g and by 90% with pirimiphos-methyl at 50 �g/g. There was no indication of potentiation between insecticides as measured by plasma AChE inhibition, and effects upon food consumption and egg production appeared unrelated to plasma AChE activity. The relationship between food consumption and egg production was similar in groups receiving dichlorvos-maldison mixtures and in those receiving graded levels of untreated food, indicating that the insecticides' effect upon egg production was mediated via a reduced food intake. Maximum residue limits for pesticides in feeds should be based on a total index which takes account of interaction between the different pesticides present.