The effects of global
warming on living organisms have now been recognized from the level of
individual species to communities, most notably in the form of
temperature-related range shifts (, , ). As the number of insects per unit area is inversely related to latitude and elevation (),
we may assume that the increase of temperature would allow the
spreading of insect species northward and upward, especially for those
species that have wide ranges, as many forest pests have. This
assumption is supported by fossil data related to the forest insect
response to climatic changes of the past. Higher damage and insect
diversity was recorded during the global warming which occurred during
the Paleocene - Eocene transition, relative to other periods ().
With mean global temperatures increasing over the past 100 years by about 0.8 °C and projected to continue (),
widespread climate-related changes in the biosphere can be expected.
There are various ways by which the insects may react to climate change
(, , , , ),
and it seems reasonable to assume that an increase of temperature
within the vital limits of a species implies a faster development. The
species ready to expand are those characterized by high growth
potential, multivoltinism and absence of diapause, whereas those that
could be restricted show slow development rate and long cycles. The
reduction of the period of time spent as a larva or pupa may improve
survival, as these are the stages more subjected to predation and other
mortality factors ().
The increase in population density may in turn promote a further
expansion of the range. Some species would be simply limited in their
survival at the southern edge of their range and would shift the range
northward. Switching to new hosts may occur among non-specialist
herbivores, and can be the first consequence of the strong selection on
colonizers (, ).
Parmesan & Yohe ()
have provided a quantitative assessment of the biological impact of
climatic change, using data from different types of organisms,
including insects. This analysis concerned the spatial (range shift)
and phenological (advancement of spring events) data, averaging 6.1
km/decade and 2.3 days/decade, respectively. Eighty percent of the
studied species (n = 434) showed a consistent range shift and 87% an
advancement of spring events, such as flowering or migration.
However, the response of insects to climatic change may not always be linear (, ).
For example, the developmental stages of the insects can be
differentially affected by the climate change, i.e. the growth can be
accelerated by higher temperature, but at the same time the length of
diapause may be extended.
Those insects developing
without winter diapause, which are active during this season and are
protected from the low temperature, are the best candidates for range
expansion if the winter temperature maintain the current increasing
trend (). The increase in winter temperature is a key factor for the survival of the lepidopteran Atalopedes campestris in the new colonization areas (). A good example concerning a forest pest is the case of the pine processionary moth Thaumetopoea pityocampa (Box 1, Fig. 1), reported also in the last IPCC report ().
However, most forest insects of temperate regions have a winter
diapause, which in some cases can last several years. Temperature plays
a major role in the induction and maintenance of this diapause. An
increase of the temperature would modify the induction and maintenance
of the diapause, involving changes, which could affect the development
of the insect, making predictions about population dynamics quite
unreliable. Two examples are reported here, which illustrate how high
temperature during the larval development has caused lower diapause
rate and higher damage by the spruce webspinning sawfly Cephalcia arvensis
(Box 2), and how high winter temperature has disrupted the maintenance
and termination of the egg diapause in the larch bud moth Zeiraphera diniana (Box 3), causing a poor synchronization with the host and the absence of an expected outbreak.
A different situation is presented by species that are already adapted to the cold environments, such as Aglais urticae.
These would probably undergo a restriction of the range if they become
limited at their southern boundary by increasing temperature ().
For insects that are heavily dependent on a favourable synchronization
between bud breaking and hatching, such as the winter moth Operophtera brumata,
it appears that there may be a compensation between a faster egg spring
development and a delayed pupation in autumn, both triggered by an
increase of temperature (). Therefore, phenology is not affected, allowing Bale et al. () to conclude that the effects of the global warming would not be so evident in some species.
Finally, the natural enemies of forest insects may be affected by the
temperature change in different directions or extent. The expansion of
the host may not be promptly followed by that of its enemies, as in the
case of the pine processionary moth (),
or the synchronization between host and parasitoid may not be
maintained under new temperature conditions. This seems to be the case
of a parasitoid of the winter moth Operophtera brumata, which is effective at low elevation but that is almost absent at high elevation ().
All the examples cited above illustrate how insects may react to the
climate change, however they also have a great potential to develop
physiological and behavioural adaptations, which may improve their
fitness under new conditions. This would ultimately lead to the
formation of genetically differentiated populations and possibly new
species, especially when the climatic change is associated with range
expansion and host switch.