DETERMINATION OF PREMATURE LARVAL DROP AND OTHER CAUSES OF LARCH SAWFLY MORTALITY

Abstract

Fifth-instar larvae of the larch sawfly, Pristiphora erichsonii (Hartig), of different weights, were deprived of food and placed in containers with moist moss. A low percentage of the early fifth-instar larvae were able to spin cocoons, but the percentage rapidly increased to a plateau as the larval weight increased. All but the heaviest larvae suffered appreciable mortality before first-year adult emergence was complete. A curvilinear relationship between larval weight and survival to the adult stage or to larvae in prolonged diapause is evident. The percentage of mature ova in adult females was reduced at the lower larval weights, with corresponding increases in the percentages of near-mature and large immature ova. There were linear relationships between larval weights, adult weights, and numbers of oöcytes. A method for assessing premature larval drop and other causes of mortality under field conditions is described and data for a number of years and plots are presented. Variations in the percentages of unparasitized normal late fifth-instar larvae were large. Data indicate that parasitism by Olesicampe (Holocremnus) sp. nr. nematorum (Tschek) and Bessa harveyi (Townsend) is the most important factor, probably a key factor, contributing to this variation.