Impacts on Insect and Mite Pests - Climate Resilient Agriculture for Ensuring Food Security - page 129

Guiterrez et al. ( 2008 ) found that during the
normally wet Northern California winter, the
fungal pathogen ( Pandora neoaphidis ) causes
catastrophic mortality to pea aphid ( Acyrthosiphon
pisum ), but during hot dry periods, the impact of
the pathogen declines.
Most entomopathogenic fungi have optimal
growth temperatures between 25 and 35 °C.
Beauveria bassiana grows at a wide temperature
range (from 8 to 35 °C) with a maximum thermal
threshold for growth at 37 °C. Higher tempera-
tures, low humidity, as well as direct exposure to
UV radiation reduce effi ciency of pathogens.
However, each pathogen responds to tempera-
tures differently and behavior of the host in
response to temperature is important as well
(Blanford and Thomas 1999 ). Manning and von
Tiedemann ( 1995 ) showed that direct exposure
of bacteria and fungi to high levels of CO 2 con-
centrations often inhibits their growth.
Some pathogens, which always live in the host
body, might not be affected directly by climatic
changes, they basically follow the development
of their hosts. The effects of higher temperature
on the impact of the microsporidia, Nosema
lymantriae , on the gypsy moth ( Lymantria dis-
par ) clearly showed a much higher and earlier
mortality of gypsy moth larvae at higher temper-
atures (Pollan 2009 ).
Pathogens, especially viruses, become more
deadly if the vector/host is weakened; therefore,
environmental stress such as high or low tempera-
ture might lead to higher mortality. Considering
that herbivorous pests are potentially weakened by
the lower nutritional quality of (C3) plants grown
under elevated CO 2 , it could be assumed that mor-
tality of pests feeding on C3 crops increases when
infected with pathogens (with potentially serious
consequences also for some natural ecosystems).
However, it seems that no one has investigated this
kind of interactions so far.

(plant) and second level (herbivore). It is not at
all clear what happens to herbivores under cli-
mate change; therefore, conclusions for parasit-
oids are speculative. However, there are some
ecological “laws” which imply certain scenarios.
If a herbivore reproduces less, because of low
nutritional value, less potential hosts are avail-
able for the parasitoid. If the host changes its sea-
sonal appearance or behavior due to climatic
changes, the parasitoid might not be able to locate
the host. Finally, parasitoids might be adversely
affected, if the host dies too early due to addi-
tional environmental stress. However, in temper-
ate zones, milder winters might enhance survival
of parasitoids. Legrand et al. ( 2004 ) have shown
that parasitoids of cereal aphids are active in win-
ter and this winter activity can considerably
reduce spring aphid populations.
No experiments have been conducted to inves-
tigate changes of all three trophic levels together
(plant-herbivore-parasitoid) under climate
change (elevated CO 2 and temperature). Bezemer
et al. ( 1998 ) conducted an experiment involving
several plant species, aphids, and parasitoids
under elevated CO 2 (+200 ppm of ambient con-
centrations) and showed that elevated CO 2 did
not infl uence parasitism. Elevated temperature
(+2 °C of ambient temperature) increased para-
sitism by about 300 % on average, but due to high
variation between the replicates, no signifi cance
could be detected.
A mathematical model has been developed
that predicts responses of grasses, cereal aphids,
and parasitoids to combined effects of elevated
CO 2 and elevated temperature. Their results sug-
gest that aphid and parasitoid populations will
develop more similar to current ambient condi-
tions than expected from the individual effects of
CO 2 or temperature increases.
In one experiment with cotton bollworm, larvae
reared on milky wheat grain under 750 ppm CO 2 ,
researchers included a parasitoid wasp ( Microplitis
mediator ) widely used as biocontrol agent of the
cotton bollworm ( Helicoverpa armigera ). The
researchers found no signifi cant changes in wheat
consumption by H. armigera population under
elevated CO 2 or in the parasitic rate of M. media-
tor . The researchers concluded that the population

7.10.2 Parasitoids

Parasitoids which live on crop pests belong to the
third trophic level. Thus, they are indirectly or
directly affected by any changes of the fi rst