Populations of the leaf-miner Leucoptera meyricki Ghesq. and its parasites, of which Zagrammosoma variegatum (Masi), Achrysocharella ritchiei Ferriére and Parahormius leucopterae Nixon were the most abundant, were studied for 40 consecutive weeks during 1960–61 in Kenya, in four experimental blocks of coffee (Coffea arabica), two of which received several applications of parathion sprays and the other two none. Each week eight different trees within each block were covered with a tent, pyrethrum spray was atomised within, and the adult moths and parasites were recovered from the polyethylene sheeting used as a floor to the tent. Fortnightly, counts of leaves, mined and unmined, and collections of leaves with leaf-miner larvae and parasites within them were made. These intensive studies were supplemented with two special spray tests, in which different dosages of insecticides were used, and with collections of leaves made periodically over a rather extensive area in Kenya, from which adult leaf-miners and parasites were reared to gain information on the parasite/host relationships over a wide range of ecological conditions.

Parathion used at 1 lb. 20 per cent, wettable powder in either 65 or 80 gal. water per acre appeared to kill all adult leaf-miners and parasites present. Possibly between 50 and 75 per cent, of the leaf-miner larvae, and appreciably fewer of the immature stages of the parasites within the mines, were killed. The sprays apparently had even less effect on the leaf-miner prepupae and pupae and the parasites within the cocoons, which are usually formed in the litter and in crevices in the soil.

Leaf-miner populations within the sprayed blocks were somewhat lower than those in the unsprayed blocks, but the fluctuations were remarkably similar. In general, this was also true for the parasites.

Assessment of the effects of the sprays on the efficiency of the parasites was made on the basis of the ratios of adult parasites to moths obtained from tented trees and from infested leaf samples held for adult emergence, as well as in terms of percentages of larvae parasitised in the samples dissected. It was found that the sprays killed a greater proportion of the leaf-miner larvae than of parasites within the mines, and thus increased the ratios of living parasites to leaf-miners immediately following the spray applications. During the period from October to January, when the sprays in the Eukera plots were synchronised with the peak populations of the moth, parasitisation was significantly higher in the sprayed plots than in the unsprayed plots. Furthermore, in the special spray tests, where different dosages of parathion and diazinon were used, parasitisation within the surviving populations was higher in the plots that received the higher dosages. These findings suggest that with proper timing of spray applications the efficiency of leaf-miner parasites may be improved.