An in vitro test to determine the acaricidal effect of organophosphorous
insecticides (OP) is described. The effect of
parathion,
phoxim and
phosmet against the pig
mange mite Sarcoptes scabiei var. suis was evaluated. The test is based on the migration ability of mites on the surface of
agar gels containing the
acaricide. The mite activity is expressed as a migration index (MI) and compared with the OP concentration in the
agar. Good dose-response data were obtained for all three OPs tested, although the instability of
phosmet required special precautions concerning the analysis of the
agar. The test was found to be accurate, sensitive, easy to carry out and applicable for routine determinations. However, the test requires that the actual concentrations of the OPs in the gel batches are determined. For the three OPs used analytical methods were developed. While the lower threshold for acaricidal effect in vitro was approximately 1-2 micrograms g-1 for all three OPs tested, a significant difference in the higher concentration range was seen between the dose-response curve for
parathion and the curves for
phoxim and
phosmet. While the latter curves decreased only slightly at concentrations above 3-6 micrograms g-1 (corresponding to MI values around 5-10), the curve for
parathion was linear down to an MI value of 1, corresponding to a
parathion concentration of approximately 30 micrograms g-1. This discrepancy was ascribed to different rates of uptake through the cuticula due to differences in the lipophilicity of the OPs.