ULTRASTRUCTURAL EFFECTS OF A NONSTEROIDAL ECDYSONE AGONIST, RH-5992, ON THE 6TH INSTAR LARVA OF THE SPRUCE BUDWORM, CHORISTONEURA-FUMIFERANA

Force feeding of RH-5992 (Tebufenozide), a non-steroidal ecdysone agon ist to newly moulted sixth instar larvae of the spruce budworm, Choris toneura fumiferana, (Lepidoptera: Tortricidae) initiates a precocious, incomplete moult, Within 6 h post treatment (pt) the larva stops feed ing and remains quiescent, Around 12 h pt, the head capsule slips part ially revealing an untanned new head capsule that appears wrinkled and poorly formed, By 24 h pt, the head capsule slippage is pronounced an d there is a mid-dorsal split of the old cuticle in the thoracic regio n but there is no ecdysis, The larva remains moribund in this state an d ultimately dies of starvation and desiccation. The temporal sequence of the external and internal changes of the integument were studied u sing both scanning and transmission electron microscopy. Within 3 h pt , there is hypertrophy of the Golgi complex indicating synthetic activ ity and soon after, large, putative ecdysial droplets are seen, Within 24 h, a new cuticle that lacks the endocuticular lamellae is formed, The formation of the various cuticular components, the degradation of the old cuticle and changes in the organelles of the epidermal cells o f the mesothoracic tergite are described, The difference between the n atural moult and the one induced by RH-5992 are explained on the basis of molecular events that take place during the moulting cycle, The pe rsistence of this ecdysone agonist in the tissues permits the expressi on of all the genes that are up-regulated by the presence of the natur al hormone but those that are turned on in the absence of the hormone are not expressed. (C) 1997 Elsevier Science Ltd.