Cohen, E., Department of Entomology, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76-100, Israel
Podoler, H., Department of Entomology, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76-100, Israel
El-Hamlauwi, M., Department of Entomology, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76-100, Israel

Experiments with laboratory populations have shown that immature stages of the hymenopterous parasitoids, Aphytis holoxanthus DeBach and Pteroptrix smithi (Compere) were unaffected by a malathion-bait mixture (0 · 13% malathion) applied to their host, Chrysomphalus aonidum (L.). However, the emerging adults of A. holoxanthus were highly susceptible and it was clearly shown that malathion absorbed in the scale cover of the host was responsible for the lethal effect. Similar results were obtained with parasitized scale insects which were exposed in a commercial citrus grove to ground spray of malathion (1 · 3% malathion). An experiment with natural populations of both parasitoids in the citrus grove clearly demonstrated the susceptibility of A. holoxanthus to malathion-bait spray and were consistent with the above-mentioned laboratory experiments. The results are discussed in the light of the predominant status of P. smithi as a natural enemy of the Florida red scale in citrus groves in Israel. No delayed effects related to fertility, sex ratio and life expectancy of both parasitoid species which survived the malathion treatment were observed. © 1988.