Studies onLaricobius erichsonii rosenh (Coleoptera: Derodontidae) a predator on Chermesids

Abstract

1. Laricobius erichsoniirosenh, occurs in Central Europe from the Italian Alps to Denmark and from Normandy to the Carpathian Mountains. The species is a predator on Chermesids which invaded the northern European plain after the replacement of the natural hardwood stands by coniferous forests. 2. There is a reasonable correlation between the area of precipitation exceeding 600 mm. per annum and the area of distribution of the bettle. Other climatic factors are discussed and the main study area described. 3. The life-history and ecology are treated in some detail. The activity of adults in spring is governed by temperature. The beetles can support starvation for over a week and consume, on an average, approximately 33 eggs ofA. piceae per day. The maximum amount of eggs laid per female was 41 in the rearings. There is one generation per annum only, and the young beetles appear for some weeks in July and August, after emergence from the pupa. Hibernation of the adults takes place in the forest floor. 4. The eggs are deposited singly on the bark and develop, in the forest, in about 12 to 16 days in May. 5. The general behaviour of the larva is described in some detail with regard to movements, dropping, skin cover, and moulting. The food consumption of the larval stage ranges from 7.2 eggs ofA. piceae for L I to 37.5 eggs for L IV, per day. The developmental period of the various instars, and the increase of total food consumption per instar is compared with the growth of head capsule and body length. Counts in various regions suggest that there is a regular decline in the population density of the predator after some years in infestations ofA. piceae, caused by increasing mortality (parasitism and disease) and the reduction of the host insect. 6. Pupation takes place in the forest floor. The depth and position depends on moisture conditions. The duration of the instar is about 13 days. Mortality cause by biotic factors is assessed to be low. 7. The host specificity ofL. erichsonii is considered to be slight, but observations in mixed infestations indicate that preference is given toA. piceae. The ability to find Chermesid as food insects is well marked in the adults. Larvae searching on the stems do not appear to be capable of perceiving food at distances beoond a few millimeters. 8. The economic importance of the species is discussed based partly on a rough approximation of the number necessary to keep a certain square unit of infested bark free of Chermesids throughout the year. This species alone is not considered to be capable of suppressing a heavy outbreak of woolly aphids. The moderate host specificity made it possible for the beetle to invade new areas and reach rather isolated fir stands where other, more spezialized predators onA. piceae, could not follow. 9. The collecting and shipment to Canada of 7,790 living beetles from Germany, 1951 and 1952, is described.