The capacity of an herbivore
insect to complete its development depends on the adaptation to both,
the environmental conditions and the host plant. The plant may respond
to changes in temperature by varying its growth rate, as it is commonly
observed along latitudinal or altitudinal gradients, when the
conditions become progressively less favorable and the herbivores are
more limited in the host’s exploitation. An example is given by the
willow psyllid (Cacopsylla spp.), which is restricted to feed on a low number of species or types of plant tissues as the range edge is approached (, ).
The change of temperature, which promotes the expansion of the insect’s
range, may also involve a new association between a herbivore and its
host, as it has been shown by the pine processionary moth attacking the
mountain pine (Pinus mugo) in the southern Alps. The large
outbreaks observed in the expansion areas on the new hosts may be
explained either by the high susceptibility of the hosts or by the
inability of natural enemies to locate the moth larvae on an unusual
hosts or environment (, ).
An elevated concentration of CO2 may affect the performance of phytophagous insects through the modification of the nutritional properties of the host plant (, , ). As CO2
is the main carbon source for photosynthesis, its increase could alter
the carbon/nutrient balance of plants, increasing the C/N ratio and
thus diluting the nitrogen content of the tissues. However, the
response of plants to increased CO2 varies among species. A
high concentration causes an increase of tannins in the leaves of
birch, poplar and maple, but not in the eastern white pine (Pinus strobus) ().
The first reaction expected from herbivores to the increase of the C/N
ratio is compensatory feeding, in other words they should eat more to
accumulate enough nitrogen for their development. Thus, plant damage
may increase, but plant biomass could remain stable if we assume that
the plants exposed to high CO2 grow more. Phytophagous insects may also develop adaptations to overcome higher C/N ratios, such as the pine sawfly Neodiprion lecontei, which shows an increase in the efficiency of nitrogen utilization when reared on plants treated with high CO2 concentration ().
However, other insect species seem unable to compensate the lower
nutritional quality of the plants by increasing the efficiency of
nutrient utilization (, ). The experiments of Lindroth et al. (),
on three species of saturnid moths, show that the performance of the
caterpillars is only marginally affected when the nitrogen content of
the leaves is reduced by 23% and the C/N ratio increased by 13-28%.
Experiments combining different concentrations of both nitrogen and
carbon dioxide supplied to Norway spruce showed that a high nitrogen
level may compensate the effects of CO2 on the concentration of nutrients and defence compounds in the shoots, limiting the negative effects on the test insect Lymantria monacha ().
The effects of a modified atmosphere on herbivore insects could also
involve the third trophic level, i.e. their parasitoids and predators.
As we are expecting a delay in the developmental time of the herbivores
after exposure to high CO2 (, , ),
the probability of parasitism and predation should increase as well.
Experimental evidence of such a hypothesis is contradictory, as Roth
& Lindroth () did not find higher parasitism by the hymenopteran Cotesia melanoscela on the larvae of Lymantria dispar raised at high CO2, whereas Stiling et al. () found higher mortality of oak leaf miners by parasitoids on two species of oak (Quercus myrtifolia, Q. geminata) grown at high CO2 level.
Laboratory or greenhouse experiments provide valuable data, but it is
difficult to derive conclusions applicable to the natural environment.
For example, high CO2 levels are known to increase the temperature and, indirectly may affect the host-herbivore interaction.
Dury et al. () showed that an increase of 3°C of the temperature might lead to the same effects of an increase of CO2
(decrease of nitrogen, increased of condensed tannins) on oak leaves.
However, an increase of temperature may enhance the feeding of the
herbivore and thus compensate for the negative effects of a lower food
quality. An experiment that tested simultaneously the effects of
different levels of CO2, nitrogen and temperature on the monoterpene production of Pseudotsuga menziesii (),
indicated that the synthesis of these defence compounds was more
affected by individual tree variability than by the treatments.
The response of herbivore insects to increased CO2 may also differ among the feeding guilds, as suggested by Bezemer & Jones ().
Defoliators are generally expected to increase leaf consumption by
about 30%, but leaf miners showed a much lower rate. Phloem-sucking
insects appear to take the greatest advantage from increased CO2, as they grow bigger and in a shorter time. In a FACE (Free Air Carbon Enrichment) experiment carried out in Wisconsin (),
the activity of all guilds of herbivores, combined with the effect of
increased ozone, may be compensated by the beneficial consequences of
enriched CO2 on growth of Populus tremuloides.
More research is clearly needed to make reliable predictions about the
effects of climate change on the relationships between the forest trees
and phytophagous insects. Good conceptual frameworks, such as the
carbon/nutrient balance () and the growth/differentiation balance ()
theories are available to interpret experimental results and to
formulate new hypotheses. Hopefully, this understanding of the effects
of climate change on forest pests will enable us to take the necessary
measures to counteract or mitigate the possible negative consequences
on the forest ecosystems.