Lymantria dispar (Lepidoptera, Lymantriidae) is one of the most destructive insect pests in Algerian cork oak forests. The identification of areas infested between 1968 and 2013 by this insect indicates three main peaks of culminations in 1977, 1987 and 1996 during which 13,445, 28,157 and 20,793 ha of forests were defoliated, respectively. Eastern coastal forests seem to be the most vulnerable. For the first time in Algeria, we conducted a test on first larval instars with two commercial products based on Bacillus thuringiensis Berliner var. kurstaki (Btk) serotype 3a-3b at a concentration of 16,000 International Units per milligram (IU/mg) in three infested areas in order to determine the optimal conditions for using bio-insecticides in cork oak forests. Surveys conducted 20 d after helicopter spraying showed that mortality from the first to the third instars reached at least 90% in the areas treated. Rearing caterpillars and pupae that survived the treatment revealed a delayed mortality of 16.0 to 38.8% in the treated areas. Pupal mortality was mainly due to Hymenoptera, Diptera and pathogenic microorganisms.
