It is generally expected that with climate warming in mid- to high-latitude regions, insect pests become more abundant. The main effect of climate warming in the temperate zone is believed to be a change in winter survival, while in the northern latitude shifts in phenology in terms of growth and reproduction, may be of prime importance (Bale et al. 2002). Shifts in climate are also likely to affect the geographical distribution of insects. Species-specific responses of pests to increasing O3 concentrations were observed (see Fuhrer 2003).
Most studies have related changes in insect performance to changes in foliar concentrations of nitrogen, carbohydrate and phenolics. In contrast to elevated CO2, O3 tends to increase the N concentration in leaves (Pleijel et al. 1999). However, the relevant plant nutritive factor in the response of herbivores to O3 might be the carbohydrate rather than the N concentration in leaves, as O3 stress increases herbivore development as well as foliar sugar and starch in some host-plant systems (Heagle et al. 1994; Hummel et al. 1998). Glucosinolate content of rape seed oil has been reported to be influenced by O3 exposure (Gielen et al. 2006). These nitrogen- and sulfur-containing secondary plant metabolites are primarily involved in plant-insect interactions, but in high concentrations, they may become toxic for animal feed, whereas moderate consumption has beneficial effects on human health (Stoewsand 1995). Elevated CO2 and O3 may alter the performance of insects through changes in bottom-up (plant) and top-down (natural enemy) control. In summary, in an atmosphere containing higher levels of both O3 and CO2, increased populations of some insect pests can be expected.
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