The mean global surface temperatures have increased since the late 19th century by approximately 0.3-0.6 o C (IPCC, 2007). The linear warming trend over the last 50 years is nearly double that of the last 100 years. The impact of climate warming on insects was studied using the southern green stink bug Nezara viridula as a model insect. This bug is known as a cosmopolitan agricultural pest that damages diverse crop plants worldwide. Effects of climate warming on insects include distribution, abundance, phenology, voltinism, physiology, behaviour, and community structure. It should be noted here that climate warming affects insects not only directly but also indirectly through interaction of species in ecosystem.

1. Northward range expansion of N. viridula in Japan.
Past and current limits of the distribution range of N. viridula were compared (Tougou et al., 2009). It was found that the climate warming promoted northward range expansion of N. viridula by providing favourable overwintering conditions close to the specie’s range limit. The past and the current limits of the distribution range of N. viridula in central Japan were investigated. In the early 1960s, the northern limit of the species’ range was in Wakayama Prefecture and was limited by a +5 o C isothermal line for the mean January temperature. Forty five years later (2006–2007), a new survey in Wakayama and five neighbouring prefectures demonstrated that this northern limit shifted northward by 85km, at a rate of 19.0km/decade. The shift northward was likely promoted by milder winter conditions. The mean January–February temperature in the region was 1.03–1.91ºC higher in 1998–2007 than in 1960–1969. In the 1960s, the mean January temperature exceeded +5 ºC only in Wakayama city, but by now it has surpassed +5 ºC in Osaka and Tsu cities, located further north (52 and 59 km respectively), which corresponds to the current distribution of N. viridula. The number of cold days (with mean temperature below +5ºC) also significantly decreased, while the annual lowest temperature significantly increased. N. viridula was found mostly at or close to those locations where (1) the mean January temperature exceeded +5ºC, (2) the mean number of cold days did not exceed 26 during January–February, and (3) where the mean annual lowest temperature did not drop below –3.0 ºC. The general linear model shows that the mean January temperature and number of cold days are the most important factors controlling the northern limit of distribution of N. viridula.
All the climatic data suggest that over the last 45 years environmental conditions have become more favourable for overwintering of N.viridula at many locations in central Japan. This has likely promoted the northward spread of the species, representing the direct response of the species to climate warming. A sympatrically distributed congeneric N. antennata is responding to the warming by retreat from the ocean coast towards cooler elevated habitats, which might be a complex response to elevated temperature and interspecific mating with N. viridula. This range expansion of N. viridula is now accelerating the extinction of N. antennata by the infertile interspecific copulation.

2. Ecophysiological responses of N. viridula to simulated warming.
The effect of simulated climate change on N. viridula was studied close to the species’ northern range limit in Japan (Musolin et al., 2010). Insects from the same egg masses were reared for 15 months in 10 consecutive series under quasi-natural (i.e. outdoor) conditions and in a transparent incubator, in which climate warming was simulated by adding 2.5ºC to the outdoor temperature. The warming strongly affected all life-history and phenological parameters. In the spring, the simulated warming advanced the timing of body colour changes and post-diapause reproduction. In the early summer, it increased egg production and accelerated nymphal development. In the late summer (the hottest season), the effect of the simulated warming was strongly deleterious: nymphs developed slowly, suffered higher mortality and had difficulties during final moulting; the emerged females were smaller, some exhibited abnormal cuticle, produced fewer eggs and had a decreased life span.
Our current studies are going to clarify that such an adverse effect of high temperature is caused by the decrease in bacterial symbiont which is harboured in the midgut of stink bugs. As in many phytophagous heteropteran species, N. viridula possesses a number of sac-like outgrowths, called crypts, in a posterior section of the midgut, wherein a specific bacterial symbiont is harbored. In previous studies on N. viridula from Hawaiian populations, experimental elimination of the symbiont caused little fitness defects in the host insect (Prado, et.al., 2006, Prado et al., 2009). N. viridula from Japanese populations consistently harbor the same gammaproteobacterial gut symbiont. However, in this case, contrary to the previous works, experimental sterilization of the symbiont resulted in severe nymphal mortality, indicating an obligate host-symbiont relationship (Tada et al., 2011). Considering the worldwide host-symbiont association and those experimental data, N. viridula is considered to be generally and obligatorily associated with the gut symbiont, while the symbiont effect on the host performance may be different among geographic populations.
In the autumn, the warming accelerated nymphal development, resulted in larger female size, affected the timing of the diapause-associated adult body colour change from green to russet and enhanced preparation for overwintering. Larger females had higher winter survival rate than smaller females. The warming strongly increased survival rate in both size classes and allowed smaller females to reach the same winter survival rate as larger females had under the quasi-natural conditions. The winter survival also differed between the green and dark-coloured females under the quasi-natural, but not under the warming conditions. However, under the warming conditions, green females survived the winter even better than dark-coloured females did under the quasi-natural conditions. Moreover, the warming shortened the life span of females from the summer generations and prolonged it in those from the autumn generation.
It is concluded that even a moderate temperature increase (+2.5 ºC) in the future is likely to have a complex influence upon insects, strongly affecting many of their life-history and phenological parameters.

• Kenji Fujisaki(Emeritus Professor of Entomology, Kyoto University, Kyoto, 606-8502, Japan) Corresponding author