Impacts on Insect and Mite Pests - Climate Resilient Agriculture for Ensuring Food Security - page 119

(cabbage maggot, onion maggot, European corn
borer, Colorado potato beetle). Increased tem-
peratures will accelerate the development of
these types of insects - possibly resulting in more
generations (and crop damage) per year.
“Migratory” insects (corn earworm in north-
ern parts of the northeast) may arrive in the
Northeast earlier, or the area in which they are
able to overwinter may be expanded. Natural
enemy and host insect populations may respond
differently to changes in temperature. Parasitism
could be reduced if host populations emerge and
pass through vulnerable life stages before para-
sitoids emerge. Hosts may pass though vulnera-
ble life stages more quickly at higher
temperatures, reducing the window of opportu-
nity for parasitism. Temperature may change
gender ratios of some pest species such as thrips
(Lewis 1997 ) potentially affecting reproduction
rates. Insects that spend important parts of their
life histories in the soil may be more gradually
affected by temperature changes than those that
are aboveground simply because soil provides an
insulating medium that will tend to buffer tem-
perature changes more than the air (Bale et al.
2002 ).
A few pest species/groups have been investi-
gated more thoroughly and the cotton bollworm/
pod borer ( Helicoverpa armigera ), a widely
occurring lepidopteron pest, might give some
idea what impact climate change might have on
this species. Larvae of H. armigera feed on many
vegetables, cotton, and cereals (CPC 2007 ). The
adult moth lays eggs on the plant, and after the
eggs are hatched, the caterpillars feed. The dura-
tion of larval development depends on the tem-
perature (to a maximum of 35 °C in South and
Southeast Asia) and on the quality of the host
food. On completion of growth, the fully fed
larva enters the soil to pupate. The pupal dia-
pause is induced by short day lengths (11-14 h/
day) and low temperatures (15-23 °C) experi-
enced as a larva (CPC 2007 ). After a number of
days, depending on the environmental condi-
tions, the butterfl y will emerge from the pupae
and the cycle begins again.
Some insects are closely tied to a specifi c set
of host crops. Temperature increases that cause

farmers not to grow the host crop any longer
would decrease the populations of insect pests
specifi c to those crops. The same environmen-
tal factors that impact pest insects can impact
their insect predators and parasites as well as
the disease organisms that infect the pests,
resulting in increased attack on insect popula-
tions. At higher temperatures, aphids have been
shown to be less responsive to the aphid alarm
pheromone they release when under attack by
insect predators and parasitoids - resulting in
the potential for greater predation (Awmack
et al. 1997 ).

7.4 CO 2 Enrichment

Direct effects of higher CO 2 concentrations on
insects are basically not investigated. It seems
that insects can detect CO 2 sources such as plants
and elevated levels might affect the insect's CO 2 -
sensing system.
There is a general agreement between scien-
tists that the reduced nutrient quality of C3 plants
might lead to a compensation by increased feed-
ing of many, but not all, herbivorous species
(DeLucia et al. 2008 ). Under elevated CO 2 , popu-
lation densities of chewing insects are unaffected
or decrease, but do not increase while sap sucker
(phloem feeder) population densities might
increase.
However, the results from experiments with
aphids (phloem feeders) feeding on plants grown
under elevated CO 2 and/or at elevated tempera-
ture have not shown consistent results. In some
cases, aphid performance was not (signifi cantly)
infl uenced by either elevated temperature or
elevated CO 2 (Diaz et al. 1998 ); in one research
trial, two species responded very differently
under same conditions (e.g., Brevicoryne
brassicae vs. Myzus persicae on Brassica napus
ssp . oleifera ). Increased aphid infestation and
reduced infestation in response to elevated CO 2
have been observed. Experiments by Awmack
and Harrington ( 2000 ) showed signifi cant effects
through aphids on the peas (shoot, root weight,
fl ower number) at elevated CO 2 , although the
aphid density was unaffected.