Impacts on Insect and Mite Pests - Climate Resilient Agriculture for Ensuring Food Security - page 125

Table 7.6 Estimated impact of climate change on
Western corn rootworm infestations in maize

brings increased insect pest outbreaks. It is well
known that drought can change the physiology of
host species, leading to changes in the insects
that feed on them (Mattson and Haack 1987 ).
Abnormally cool, wet conditions can also bring
on severe insect infestations, although excessive
soil moisture may drown out soil-residing insects.
Chen and McCarl ( 2001 ) investigated the rela-
tionship of precipitation and pesticide costs for
several crops in the USA and concluded that
increases in rainfall lead to increases in average
pesticide costs for corn, cotton, potatoes, soy-
beans, and wheat.
Distribution and frequency of rainfall may
also affect the incidence of pests directly as well
as through changes in humidity levels. It is being
predicted that under climate change, frequency of
rainfall would decline while its intensity would
increase. This would lead to heavy showers and
fl oods on one hand and drought spells on the
other. Under such situations, incidence of small
pests such as aphids, jassids, whitefl ies, mites,
etc. on crops may be reduced as these get washed
away by the heavy rains.
Armyworm, Mythimna separata , reaches out-
break proportions after heavy rains and fl oods.
Lever ( 1969 ) had analyzed the relationship
between outbreaks of armyworm and to a lesser
extent Spodoptera mauritia and rainfall from
1938 to 1965 and observed that all but three out-
breaks occurred when rainfall exceeded the aver-
age 89 cm. The effect of rainfall on pests can be
studied by simulating various rainfall intensities
through sprinklers. Aphid population on wheat
and other crops was adversely affected by rainfall
and sprinkler irrigation (Chander 1998 ).
Masters et al. ( 1998 ) have carried out novel
manipulations of local climate to investigate how
warmer winters with either wetter or drier sum-
mers would affect the homopteran insects - a
major component of the insect fauna of grass-
lands. Direct and indirect effects of climate
manipulation were observed. It was observed that
the supplemented summer rainfall resulted in an
increase in the vegetation cover, leading to an
increase in the abundance of the insects. Summer
drought, however, caused a decrease in the
vegetation cover, but this did not lead to a

Crop area
(million ha)

Crop area infested
(million ha)

Region 2004 2020 2004 2020

Europe 15.1 18.1 <0.1 2.1

North
America

30.8 32.3 12.0 19.0

the year. Analysis of historical light trap data vis-
à-vis current data can provide important informa-
tion on the impacts of climate change on rice
pests.

7.7 Varying Precipitation

Patterns

There are fewer scientifi c studies on the effect of
precipitation on insects than temperature. Some
insects are sensitive to precipitation and are killed
or removed from crops by heavy rains - in some
northeastern US states, this consideration is
important when choosing management options
for onion thrips (Reiners and Petzoldt 2005 ). For
some insects that overwinter in soil, such as the
cranberry fruit worm and other cranberry insect
pests, fl ooding the soil has been used as a control
measure. One would expect the predicted more
frequent and intense precipitation events fore-
casted with climate change to negatively impact
these insects. Other insects such as pea aphids are
not tolerant of drought (Macvean and Dixon
2001 ). As with temperature, precipitation changes
can impact insect pest predators, parasitoids, and
diseases resulting in a complex dynamic. Fungal
pathogens of insects are favored by high humid-
ity, and their incidence would be increased by
climate changes that lengthen periods of high
humidity and reduced by those that result in drier
conditions. Guiterrez et al. ( 2008 ) found that dur-
ing the normally wet Northern California winter,
the fungal pathogen ( Pandora neoaphidis ) causes
catastrophic mortality to pea aphid ( Acyrthosiphon
pisum ).
Precipitation - whether optimal, excessive, or
insuffi cient - is a key variable that also affects
crop-pest interactions. Drought stress sometimes