PESTICIDE BIOCHEMISTRY AND PHYSIOLOGY 18, 169-173 (1982) Neurophysiological Effects of Insecticides on the Labellar Taste Receptors of Lucilia cuprina Wied C. VIRGONA, G. HOLAN, E. SHIPP," T. H. SPURLINC AND G. QUINT CSIRO, Division of Applied Organic Chemiste. GPO Box 4331, Melbourne, Victoria, 3001, Australia; and *School of Zoology, University of New South Wales, Kensington, Neti, South Wales, Australia Received January 8, 1982; accepted May 13, 1982 The effect of insecticide concentration on the nerve impulses at the labellar receptors of the blowfly Lucilia cuprina Wied was tested and found to be significant for the DDT type of insecticide, but not for the DDT-pyrethroid type. The experiments were performed at three different times of the day, but time of day had no effect on the multiplicity indices calculated from nerve spikes induced by the compounds. No effects on the nerve response were elicited by the synergistic action of sesamex. INTRODUCTION In previous reports on the effects of chemical compounds on labellar taste re- ceptor nerve impulses, we demonstrated (1) a structure-activity relationship for DDT-type diary1 insecticides. We also dem- onstrated that pyrethroid and DDT-pyre- throid insecticides each produced differ- ent nerve responses in the labellar chemo- receptor (2, 3). The insecticidal activity was concentration dependent, suggesting that the nerve impulses at the labellar taste receptor might also be concentration dependent, especially if they initiated the physiological changes that kill the insect. In our earlier work the test compounds were applied at a concentration of 1 x 10e3 M, and other workers (4, 5) have also used fixed concentrations without investigating the concentration effect. We have now tested our compounds at concentrations of 1 X lo-*, 1 x 10d4, and 1 x 1OV M. There is also evidence (6-9) that time of day at which an insecticide is applied is a significant factor in determining the toxi- cological response of the insect. We tested 2,2-bis(4-ethoxyphenyl)-3,3-dimethyloxe- tane (Table 1, I), R( -)-3’-phenoxybenzyl I-(4-ethoxyphenyl)-2,2-dichlorocyclopro- pane carboxylate (Table 1, II), and olive oil (the carrier solvent) at three different times of day. We also investigated the effect on the im- pulses at the labellar receptor (5) of the mixed function oxidase inhibitor sesamex (10) with the compounds DDT, l,l-bis(C ethoxyphenyl)-2-nitropropane, R( -)-3’- phenoxybenzyl-l-(4-ethoxyphenyl)-2,2- dichlorocyclopropanecarboxylate, and R( -)-1’-(3-phenoxyphenyl)-prop-2’-ynyl- R(-)-1-(4-ethoxyphenyl)-2,2,3,3-tetra- fluorocyclobutane carboxylate (Table 1). All compounds were tested against insects pretreated with sesamex (5). In separate tests the compounds were combined with sesamex before application to the labellar hairs. Results from these tests were com- pared to those carried out with each com- pound alone at the same concentration. MATERIALS AND METHODS Some details of our experimental method were previously reported (I- 3). The labella cut from a 3- to 5-day-old female fly (9) was fitted over a silver reference electrode and a glass microelectrode filled with 1 x 10-l M KC1 (10) was placed over a selected labellar hair. This electrode was connected by a silver wire to a Grass P16 amplifier. The signal was conducted into a Neurolog 169 004%3575/82/050169-05$02.00/O Copyright 0 1982 by Academic Press. Inc. All rights of reproduction in any form reserved.