THE INFLUENCE OF NATURAL ENEMIES ON BROOD PRODUCTION IN IPS-TYPOGRAPHUS (COL, SCOLYTIDAE) WITH SPECIAL REFERENCE TO EGG-LAYING AND PREDATION BY THANASIMUS-FORMICARIUS (COL, CLERIDAE)

The predator Thanasimus formicarius (L.) (Coleoptera, Cleridae) and its prey Ips typographus (L.) (Coleoptera, Scolytidae) were studied in the laboratory and the field. In the laboratory, 11 T. formicarius laid 71-132 eggs (mean = 162) during 66-123 days. During this time they ate 66-132 I. typographus adults per pair (male + female). The number of eggs laid per female was not correlated with life span or the number of Ips eaten. In the field, predation by T. formicarius larvae and other natural enemies on I. typographus brood was studied in the last year of an outbreak. Caged and uncaged spruce bolts attacked by I. typographus were used, and pairs of T. formicarius were released in the cages. The treatments were : uncaged bolts, caged bolts without T. formicarius, caged bolts with 4 T. formicarius pairs, and caged bolts with 8 T. formicarius pairs. The productivity of I. typographus was highest in the caged bolts without T. formicarius (mean = 4.5 offspring/female) and lowest in the uncaged bolts (mean = 0.9 offspring/female). The density of I. typographus galleries was similar in the different treatments. Hence, the variation in productivity between treatments could not have been due to differences in the levels of intraspecific competition. There was no difference in bark beetle productivity or density of T. formicarius larvae between bolts with 4 pairs of T. formicarius and bolts with 8 pairs (mean = 2.5 offspring/female). This indicates that some kind of interference occurred between T. formicarius individuals (e.g. cannibalism) and that a maximum level of predation was reached. Predation by larvae of Medetera spp. (Diptera, Dolichopodidae), Thanasimus spp. and other beetles, and parasitism by wasps (Hymenoptera, Pteromalidae) probably caused the low productivity in the uncaged bolts.