Cattle grubs, first-instar larvae of Hypoderma bovis (L.), were removed from nine slaughtered heifers. Two of these heifers had previously been treated with coumaphos, three with Ruelene, and two with trichlorfon. The remaining two heifers were not treated with insecticides and served as sources of control grubs. The grubs, after removal from the hosts and fixation in formalin, were incubated with one of the two cholinesterase substrates, acetylthiocholine iodide or 5-bromoindoxyl acetate. Cholinesterase was completely inhibited in the central nervous systems of the grubs whose hosts were treated with Ruelene. This inhibition was also virtually complete in grubs from heifers treated with trichlorfon and was usually complete in those from the heifers treated with coumaphos. This is presented as histochemical evidence that the three systemic insecticides inhibit the cholinergic enzymes in the grubs, thus causing them to die while still in the body of the host.

Larvae of Hypoderma bovis (L.) and H. lineatum (De Vill.) when incubated with the substrate acetylthiocholine iodide showed the presence of cholinesterase throughout the central nervous system and sense organs, in gonads, oenocytes, Malpighian tubes, anlagen of adult structures, part of the posterior midintestine, and at the junctions of the muscles of the body wall. Incubation with the substrate 5-bromoindoxyl acetate showed all the aforementioned and in addition demonstrated the presence of aliesterase (organophosphorus-sensitive esterase) in the cell cytoplasm of all the midintestinal epithelium, the ring gland, tracheal epithelium, salivary duct epithelium, and in the perineurium of the nerve sheath. Aromesterase (organophosphorus-resistant esterase) was particularly abundant in the epithelium of the midintestine. Lipase was prominent in the lipid droplets of the fat body cells and in similar droplets in the haemocytes.When the larvae or their tissues, prior to incubation with the substrates, were treated with any one of the three organophosphorus insecticides coumaphos (Co-Ral), trichlorfon (Neguvon, Dipterex), or Ruelene, the cholinesterase was inhibited and so was much of the aliesterase, but not the aromesterase or lipase. Fixation of the larval tissues in formalin did not result in loss of enzyme activity, but on the other hand was not necessary for good histochemical demonstration of the enzymes.It is concluded that these insecticides by their anticholinesterase action kill the first-instar cattle grubs before the latter can migrate to the lumbar region of the bovine host.