Agriculture Reference
In-Depth Information
relationship between
H. armigera
and
M. mediator
is unlikely to vary due to future elevated atmo-
spheric CO
2
concentrations.
The development of a parasitoid wasp
(
Glyptapanteles liparidis
) of gypsy moth
(
Lymantria dispar
) feeding on three different tree
species fumigated with 540 ± 20 ppm CO
2
was
not adversely affected by changes in food quality
when compared to ambient CO
2
. However, it
must be taken into account that the effects of ele-
vated CO
2
on mature trees might not be compa-
rable to annual plants or tree seedlings.
The precipitation variability seems to be a key
factor infl uencing parasitism in 15 Lepidoptera
(butterfl y) rearing programs from a broad
spectrum of climatic regimes and locations, from
the region between southern Canada and central
Brazil. A higher variability led to a decrease in
parasitism. These fi ndings basically support the
theory that interaction, which evolved due to sta-
ble conditions, is weakened when frequent
changes occur.
In general, host-specifi c parasitoids should be
more sensitive to variations in host emergence
time or developmental rate when compared to gen-
eralists. Specialist parasitoids may miss narrow
windows of vulnerability of their particular hosts.
In contrast, because generalists exploit a variety of
hosts that might individually respond to climatic
cues in different ways, they should be less suscep-
tible to the host population's lags and asynchronies
associated with climatic unpredictability.
mechanisms like antixenosis, antibiosis, and tol-
erance (Dhaliwal and Dilawari
1993
). However,
expression of the host plant resistance is greatly
infl uenced by environmental factors like temper-
ature, sunlight, soil moisture, air pollution, etc.
Under stressful environment, plant becomes
more susceptible to attack by insect pests because
of weakening of their own defensive system
resulting in pest outbreaks and more crop dam-
age (Rhoades
1985
). Thermal and drought stress-
associated breakdown of plant resistance has
been widely reported (Rhoades
1985
; Sharma
et al.
2005
). With global temperature rise and
increased water stress, tropical countries like
India may face the problem of severe yield loss in
sorghum due to breakdown of resistance against
midge
Stenodiplosis sorghicola
and spotted stem
borer
Chilo partellus
(Sharma et al.
2005
).
Development of insect-resistant transgenics
opened new avenues for exploiting host plant
resistance in integrated pest management. A gene
encoding delta-endotoxin proteins from entomo-
pathogenic soil bacterium
Bacillus thuringiensis
is deployed in transgenic plants (Kranti et al.
2005
). However, expression of Bt toxins in trans-
genic plants is greatly infl uenced by environmen-
tal factors like temperature, soil moisture, and
plant age (Dhaliwal and Dilawari
1993
; Kranti
et al.
2005
). The environmental factors like high
temperature have been found affecting transgene
expression in Bt cotton resulting in reduced pro-
duction of Bt toxins. This leads to enhanced sus-
ceptibility of the crops to insect pests like
bollworms, viz.,
Heliothis virescens
(Kaiser
1996
),
Helicoverpa armigera
, and
H. punctigera
(Hilder and Boulter
1999
).
European large raspberry aphid
(
Amphorophora idaei
) is the most signifi cant
insect pest of raspberry production. Aphids are
vectors of at least four plant viruses that reduce
plant vigor and can cause death. Two resistance
genes have been introduced to overcome the
aphid damage. But the aphids have partially over-
come the resistance induced by the above two
genes.
Global warming may result in breakdown of
resistance to certain insect pests. Sorghum
varieties exhibiting resistance to sorghum
7.10.3 Predators
Like parasitoids, predators which prey on crop
pests belong to the third trophic level. Thus, they
are indirectly or directly affected by any changes
of the fi rst (plant) and second level (herbivore).
7.11
Breakdown of Host Plant
Resistance
Host plant resistance is one of the eco-friendly
options for managing harmful insect pests of
crops wherein the plant can lessen the damage
Search WWH ::
Custom Search