Agriculture Reference
In-Depth Information
A meta-analysis of studies on elevated tem-
perature and elevated CO
2
suggests that insect
herbivore performance is adversely affected by
elevated CO
2
, favored by elevated temperature,
and not modifi ed when both parameters (temper-
ature and CO
2
combined) were elevated.
It seems that current knowledge does not
allow a generalization regarding the impact of
climate change in herbivorous insects, espe-
cially not for the tropics. Even the trend of a
northward shift of insects must not coercively
translate into a pest problem - ecosystems are
not that simple and human infl uence is quite
strong. Basically, it would be necessary to inves-
tigate at least over three trophic levels with sev-
eral generations of plants, herbivores, and
predators/parasites, under elevated temperature
and elevated CO
2
.
Research with three generations of
H. armig-
era
reared on milky grains of spring wheat grown
in ambient CO
2
concentrations and at 750 ppm
showed again that bollworm fecundity was sig-
nifi cantly decreased for the second and third gen-
erations under elevated CO
2
levels. While the
consumption per larva and relative consumption
rate signifi cantly increased in elevated CO
2
, the
potential population consumption was signifi -
cantly reduced by elevated CO
2
in the second and
third generations. Therefore, the researcher sug-
gests that net damage of cotton bollworm on
wheat will be less under elevated atmospheric
CO
2
levels because increased consumption is off-
set by slower development and reduced fertility
(Chen et al.
2005
).
In a similar experiment (larvae reared on
milky wheat grain under 750 ppm CO
2
), the
researcher included a parasitoid wasp (
Microplitis
mediator
) widely used as biocontrol agent of
H.
armigera
. The researcher found no signifi cant
changes in wheat consumption by
H. armigera
population under elevated CO
2
or in the parasitic
rate of
M. mediator
. The results indicate that the
population relationship between
H. armigera
and
M. mediator
is unlikely to vary due to future ele-
vated atmospheric CO
2
concentrations.
A multiple generation experiment compared
consumption, growth, and performance of
H.
armigera
feeding on transgenic Bt cotton versus
conventional cotton grown under elevated CO
2
(750 ppm) versus ambient CO
2
(375 ppm). The
results suggest that on the one hand damage
caused by the cotton bollworm might be higher
under elevated CO
2
conditions, regardless of the
cotton variety. On the other hand, population
abundance might be lower under elevated CO
2
compared to that under ambient CO
2
(Chen et al.
2007
). The researcher explains both observations
with nutritional changes under elevated CO
2
(compensatory feeding), but did not determine
the nutrient content of the different experimental
cotton groups.
An experiment by Coll and Hughes (
2008
)
investigated the effects of elevated CO
2
on
H.
armigera
and an omnivorous bug, which feeds on
plants but also preys on the bollworm. Bollworm
larvae feeding on elevated CO
2
-grown pea plants
(at 700 ppm) were signifi cantly smaller than
those grown on ambient-grown plants. The
omnivorous bug required prey to complete its
development and performed best on a mixed
plant-prey diet, regardless of CO
2
level. The bugs
performed best when fed larvae from the elevated
CO
2
treatment apparently because these prey
were smaller and thus easier to overcome. Taken
together, results indicate that elevated CO
2
may
benefi t generalist predators through increased
prey vulnerability, which would put pest species
under higher risk of predation.
Recently, free air gas concentration enrich-
ment (FACE) technology was used to create an
atmosphere with CO
2
and O
2
concentrations sim-
ilar to what climate change models predict for the
middle of the twenty-fi rst century. FACE allows
for fi eld testing of crop situations with fewer lim-
itations than those conducted in enclosed spaces.
During the early season, soybeans grown in ele-
vated CO
2
atmosphere had 57 % more damage
from insects (primarily Japanese beetle, potato
leaf hopper, Western corn rootworm, and
Mexican bean beetle) than those grown in today's
atmosphere and required an insecticide treatment
in order to continue the experiment. It is thought
that measured increases in the levels of simple
sugars in the soybean leaves may have stimulated
the additional insect feeding (Hamilton et al.
2005
).
Search WWH ::
Custom Search