Yoshinori Shintani

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

Larvae of the west-Japan type yellow-spotted longicorn beetle, Psacothea hilaris (Coleoptera: Cerambycidae), show a long-day type photoperiodic response at 25 degrees 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 degrees 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.

Geographic variation in cold hardiness of eggs and neonate larvae of the yellow-spotted longicorn beetle Psacothea hilaris

Cold hardiness of eggs and neonate larvae of the yellow‐spotted longicorn beetle, Psacothea hilaris (Pascoe) was examined using six geographical populations in Japan. Particular attention was paid to cold hardiness of eggs and neonate larvae of the subtropical population (Ishigaki), because the east Japan populations are considered to have been introduced from a subtropical area, and the overwintering stage in the east Japan populations is incidentally shifted from the original mature larval stage to the egg or neonate larval stages. When the eggs were exposed to low temperatures for 1 h, the decrease in hatchability became significant at –12°C in the southernmost two populations (Ishigaki and Naze), and at –16°C in the northern populations. After 1 h exposure to –20°C, few eggs could hatch in the Ishigaki population, whereas 27–55% of the eggs survived in the northern populations. Pre‐chilling of the eggs at 10°C for 10 days enhanced the cold hardiness in all populations. This effect was particularly distinct in the subtropical population; the eggs of the Ishigaki population became as cold hardy as those of the northern populations after acclimation. These results suggest that the subtropical population is capable of establishing itself in east Japan, where the winter is cold.

Effects of photoperiod and low temperature on diapause termination in the yellow-spotted longicorn beetle (Psacothea hilaris)

Abstract. .The effects of photoperiod and low temperature on diapause termination in the yellow‐spotted longicorn beetle, Psacothea hilaris (Pascoe) (Coleoptera: Cerambycidae), were examined using a population from Ino, Japan. Diapausing insects obtained by rearing larvae under short daylength (12 or 13 h) at 25oC were subjected to various treatments. When the photoperiod was changed at the same temperature, diapausing larvae showed a long‐day response with a critical daylength between 13.5 and 14h. The diapause was terminated and consequently pupation occurred if the daylength was longer than 13.5 h. Chilling the diapausing larvae at 10oC for 30 or more days also terminated diapause in most larvae irrespective of the photoperiods during and after chilling treatment. In contrast, the post‐chilling photoperiod had a critical effect on development of diapausing larvae chilled for only 15 days.

Photoperiodic control of larval diapause in the yellow-spotted longicorn beetle, Psacothea hilaris: analysis by photoperiod manipulation

The photoperiodic control of diapause induction in the larvae of the yellow‐spotted longicorn beetle, Psacothea hilaris (Pascoe), was investigated using a west Japan‐type population collected from Ino, Kochi Prefecture, Japan. In this population, the larvae expressed a long‐day photoperiodic response with a critical daylength between 13.5 and 14 h at 25 °C; under a long daylength, the larvae pupated after the 4th or 5th instar, while the larvae entered diapause under a short daylength after 2.3 additional molts on average. When the photoperiod was changed from a short (L12:D12) to a long (L15:D9) daylength, pupation occurred in most of the individuals irrespective of the time of the change. When the photoperiod was changed from long to short at 1 or 2 weeks after hatching, all of the larvae entered diapause, whereas when the photoperiod was changed at 5 weeks after hatching or later, most of the larvae pupated. The 2 weeks exposures to a long daylength against a ‘background’ of a short daylength at various times revealed that the larvae of this insect are most sensitive to the photoperiod from 4 to 6 weeks after hatching.

Interactive effects of photoperiod and temperature on diapause induction and termination in the yellow-spotted longicorn beetle, Psacothea hilaris

Abstract.  The interactive effects of temperature (20 °C or 25 °C) and photoperiod (LD 12 : 12 h or LD 15 : 9 h) on diapause induction and termination are investigated in the west‐Japan type yellow‐spotted longicorn beetle, Psacothea hilaris (Pascoe) (Coleoptera: Cerambycidae). Larval diapause of P. hilaris is induced under three diapause‐inducing conditions (20 °C–SD, 20 °C–LD and 25 °C–SD), and the diapause larvae are transferred to one of four conditions (20 °C–SD, 20 °C–LD, 25 °C–SD or 25 °C–LD) for observation of pupation, which indicates termination of diapause. The intensity of diapause induced under the three conditions increases in the order 20 °C–SD < 25 °C–SD < 20 °C–LD, when assessed by the time course of pupation after the transfer. On the other hand, the effectiveness of the temperature–photoperiod combinations to terminate diapause is in the order 25 °C–SD (ineffective) < < 20 °C–LD < 25 °C–LD < 20 °C–SD. Among the temperatures (5, 10, 15 and 20 °C) examined, 15 °C is the most effective in terminating diapause under the short day; diapause in most larvae appears to have been completed in 15 days.

Diapause avoidance induced by low temperature in the yellow-spotted longicorn beetle, Psacothea hilaris

The diapause‐averting effect of low temperature on pre‐diapause larvae was examined in the yellow‐spotted longicorn beetle, Psacothea hilaris. Larvae that had been reared under diapause‐inducing conditions (25 °C, L12:D12) were temporarily exposed to 10 °C for various periods, and returned to the initial condition. Diapause was not averted by chilling for 15 days irrespective of the age of the larvae at chilling. After a 30‐day chilling treatment, all of the 40‐ and 60‐day‐old larvae averted diapause, while diapause was averted in only one‐third of the 10‐ and 20‐day‐old larvae. None of the pre‐diapause larvae chilled for 60 days entered diapause irrespective of the age at chilling. With diapause avoidance, larvae that overwintered in earlier instars can start growing in earliest spring without any arrest; this phenomenon probably subserves the synchronization of larval development in a population.

Effect of seasonal variation in host-plant quality on the rice leaf bug, Trigonotylus caelestialium.

The rice leaf bug, Trigonotylus caelestialium (Kirkaldy) (Heteroptera: Miridae), is an important insect pest, causing pecky‐rice damage in Japan. This bug has a multivoltine life cycle and feeds on various species of the Poaceae. The phenology of the host‐plants differs between species and the plant quality of a single host species may change with the seasons. The fitness of the bug may be affected by these seasonal changes in host‐plant quality. In the current study, the effects of such quality changes on adult and nymphal performance were examined using a population of bugs collected in Joetsu, Niigata, Japan. Rearing of newly emerged adults collected from spring to autumn from Italian rye grass, Lolium multiflorum Lam., and southern crabgrass, Digitaria ciliaris (Retz.), showed that body size and performance of adults differed between collection seasons and plant species, indicating that nymphal conditions including host‐plant quality have crucial effects on their fitness. Rearing nymphs from hatching to adult emergence on various field‐collected grasses under laboratory conditions showed that the performance of nymphs depends on the plant species and varies greatly with the season, within a given species. In general, nymphs performed more poorly on most of the plants collected late in the season, but they performed well on some plants unavailable early in the season. Therefore, in T. caelestialium, seasonal and spatial variations are important factors that affect fitness of nymphs and adults, and this suggests that oviposition of diapause eggs in summer is an adaptation to deteriorating food conditions.