Prothoracicotropic Hormone (PTTH) Regulation of the Larval Pupal Molt in the Gypsy Moth, Lymantria Dispar: Observations on the Development of a Larval in Vivo Bioassay

Abstract

Since the initial discovery of Kopec (1922), the impor­tance of a head factor to initiate molting/metamorphosis in lepidopterous insects has been studied by several workers either by neck ligation or brain removal (see references in Smith, 1985; also Kiguchi et al., 1985; Negate et al., 1987). These findings lead to the develop­ment of bioassays for characterization of this factor, PITH, from the brain (Kobayashi and Burdette, 1961; Kimura et al., 1962; Gibbs and Riddiford, 1977; Ishizaki et al., 1983; Suzuki and Ishizaki, 1986). To determine the feasi­bility of developing a similar assay, with Lymantria, the physiological and endocrinological promotion of larval-pupal development was examined by neck ligation of female fifth instar (last-stage) larvae on consecutive days. Newly molted larvae were collected between 2:00 pm to 8:00 pm and weighed daily. Weight and growth rate were maximum (4.7 times) on day 7 (Fig. 1). Larvae neck-ligated on day 7 did not pupate, even after 22 days, while larvae ligated on day 8 to 9 shrunk and pupated within 2 to 3 days (Fig. 2). A small pre-peak of hemolymph ecdysteroid (ca. 450 pg.) was recorded by radioimmunoassay (RIA) on day 7 and followed by another increase on day 8 which reached a maximum on day 9, two days before pupation (Fig. 2).