Change in significance of feeding during larval development in the yellow-spotted longicorn beetle, Psacothea hilaris

Abstract

Larvae of the west-Japan type yellow-spotted longicorn beetle, Psacothea hilaris (Coleoptera: Cerambycidae), show a long-day type photoperiodic response at 25 °C; under long-day conditions, larvae pupate after the fourth or fifth instar, while under short-day conditions, they undergo a few nonstationary supernumerary molts and eventually enter diapause. In the present study, the effect of food on the development and photoperiodic response of the larvae was examined with special reference to molting and pupation. Although the pupal body size was greatly affected by the food quality and the length of feeding, the critical day length for induction of metamorphosis at 25 °C was always between 13.5 and 14 h. Exposure to starvation of larvae reared on the standard diet revealed that the capability to pupate is acquired after a few days of feeding in the fourth instar. In the larvae that had acquired the capability to pupate, premature pupation was induced by exposure to starvation, indicating that feeding becomes dispensable long before it is normally terminated.

Introduction

The yellow-spotted longicorn beetle, Psacothea hilaris (Pascoe) (Coleoptera: Cerambycidae), is widely distributed in easternmost Asia, and infests Moraceae trees. This species is an important pest of sericulture in Japan because larvae bore tunnels in the trunks and adults feed on the leaves of the mulberry trees. Due to its large geographic variation in morphology, this insect is divided into many subspecies (Kusama and Takakuwa, 1984). One subspecies, P. hilaris hilaris, inhabits three of the four main islands of Japan, Honshu, Shikoku and Kyushu. Within this subspecies, two morphological types have been recognized, i.e. ‘east-Japan type’ and ‘west-Japan type’. The two types are distinguished by the spot patterns on the pronotum of adults (Iba, 1980).

The larval photoperiodic response of P. hilaris has been examined in detail in the west-Japan type population (Shintani et al., 1996a, Shintani et al., 1996b, Shintani and Ishikawa, 1997a, Shintani and Ishikawa, 1997b, Shintani and Ishikawa, 1998). Larval development of this insect at 25 °C is under photoperiodic control with a critical day length between 13.5 and 14 h. Under long-day conditions, pupation occurs after the fourth or fifth instar, while under short-day conditions, the larvae undergo a few supernumerary molts and eventually enter diapause. Long-day strongly promotes metamorphosis; the larvae reared under short-day conditions until the fourth or later instars pupate in the current or next instar if the photoperiod is changed to long-day (Shintani and Ishikawa, 1998).

As is the case with other cerambycid species, P. hilaris larvae do not move from one host plant to another; they have to complete their larval development in the trunk in which the eggs were deposited. Therefore, larval development largely depends on the nutritional value of the host tree, which varies from tree to tree. Furthermore, since all branches of mulberry trees are cut off in autumn, as is the practice in sericulture, the majority of larvae that are left in the cut off branches are confronted with deterioration of their food. However, it has been well documented that adults of P. hilaris emerge from the cut branches. P. hilaris larvae therefore must have mechanisms for adapting to the changes in food quality. In the present study, the effects of diet quality and starvation on the larval development were examined with special reference to the photoperiodic response. We also demonstrate that feeding becomes dispensable long before it is normally terminated.