Agriculture Reference
In-Depth Information
time in the more vulnerable, early stages of
development and thus may suffer greater mor-
tality from natural enemies.
• Herbivores may be physically weakened while
feeding on poor hosts under elevated CO
2
con-
ditions and are thus less able to defend them-
selves against predators and parasitoids; and
enriched CO
2
may alter enemy-avoidance
behavior; some aphids, for example, show
reduced responses to alarm pheromones under
elevated CO
2
, potentially making them more
susceptible to enemy attack (Awmack et al.
1997
).
Such effects would increase the susceptibility
of herbivores to natural enemies, reducing herbi-
vore population size under elevated CO
2
condi-
tions (Coll and Hughes
2008
).
Elevated temperature basically favors adult
hunting insects and spiders, and it seems that
the lethal temperature of many spiders is much
above the temperature expected by climate
change. Skirvin et al. (
1997
) modeled the
interaction of ladybird (
Coccinella septem-
punctata
) with aphid populations (
Sitobion
avenae
) and predict that in hot summer cocci-
nellids reduce aphids more strongly than in
moderate summers.
7.10
Natural Enemies
Biological control of insect pests is one of the
important components of integrated pest man-
agement, safeguarding the ecosystem. Natural
enemies of crop pests, viz., predators, parasit-
oids, and pathogens, are prompt density respon-
sive in their action subjected to the action of
abiotic components. Being tiny and delicate,
natural enemies of the insect pests are more sen-
sitive to the climatic extremes like heat, cold,
wind, and rains. Precipitation changes can also
affect predators, parasitoids, and pathogens of
insect pests resulting in a complex dynamics.
With changing climate, incidence of entomo-
pathogenic fungi might be favored by prolonged
humidity conditions and obstinately be reduced
by drier conditions (Newton et al.
2011
). Natural
enemy and host insect populations may respond
differently to changes in climate. Hosts may pass
through vulnerable life stages more quickly at
higher temperatures, reducing the window of
opportunity for parasitism which may give great
setback to the survival and multiplication of para-
sitoids (Petzoldt and Seaman
2010
).
Ecologists argue that the tritrophic interac-
tions between plants, herbivorous insects, and
their natural enemies (predators, parasitoids, and
pathogens) result from a long coevolutionary
process specifi c to a particular environment and
relatively stable climatic conditions (Hance et al.
2007
). Abrupt environmental changes as induced
by current climatic change and elevated CO
2
may
infl uence the biology of each component of a sys-
tem differently, provoking a destabilization in
their population dynamics that may lead to the
extinction of part of the system. Specialists, for
example, many host-specifi c parasitoids, which
evolved under rather stable conditions might be
especially endangered.
Atmospheric CO
2
levels may affect the perfor-
mance of natural enemies and/or susceptibility of
prey via a variety of indirect effects. Some of
these impacts, which potentially make prey more
susceptible to their enemies, include:
•
7.10.1 Pathogens
Fungi, bacteria, microsporidia, and viruses can
successfully affect rodents, insect pests, and
mites. They are widely used in biological control,
with the bacteria
Bacillus thuringiensis
and the
fungi
Beauveria bassiana
being prominent
examples.
Effects of climate change on the effi ciency of
pathogens depend on the environment they live
in. In general, fungi and bacteria benefi t from
warm and moist environments; therefore, mild
and wetter winters as predicted in temperate
zones will benefi t them, especially those living in
the soil (e.g.,
Beauveria bassiana
). Since many
larvae or pupae of pests also overwinter (pass
through or wait out the winter season) in soils,
fungi and bacteria might affect them more
strongly.
Herbivores that feed on poor host plants under
elevated CO
2
conditions often spend more
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