The coffee leaf-miners Leucoptera meyricki Ghesq. and L. caffeina Wshbn. are serious pests of arabica coffee in Kenya. At the present time sprays containing parathion, methyl-parathion or diazinon are used extensively for their control.

To facilitate preliminary investigations into various aspects of insecticidal control of the larval stages of these pests, a laboratory bioassay technique was developed. Larvae were reared on detached coffee leaves placed on moist filter paper in plastic boxes. When the larvae were in their third instar, the leaves were sprayed in a Potter tower and then kept in a moist air stream for three days before the mines were dissected to enable mortality counts to be made.

Of 22 new insecticides screened, only Sumithion (O, O–dimethy1 O–(3–methy1–4–nitrophenyl) phosphorothioate) and fenthion gave mortalities higher than those achieved with diazinon. Parathion gave somewhat higher mortalities than methyl-parathion, and emulsifiable formulations of these insecticides were more effective than wettable powders. The inclusion of additional wetting agents increased the efficiency of commercial formulations of these two insecticides. These results are subject to confirmation in field trials.

The mode of action of parathion was investigated. Dosage/mortality lines were constructed for parathion, methyl-parathion and diazinon so that resistance to these insecticides by L. meyricki could be detected should it develop at some future date. L. caffeina was found to be apparently more susceptible to these insecticides than L. meyricki and this was believed to be due to its greater rate of feeding.