Contents - Faperta

92 Biotechnological Approaches for Pest Management and Ecological Sustainability
stress changes the levels of biochemicals, enzymes, morphological defenses, or nutritional
quality of the host plant. Temperature affects not only plant growth, but also the biology,
behavior, and population dynamics of the insects (Tingey and Singh, 1980). In general, low
temperatures have a negative effect on plant resistance to insects (Kogan, 1982). However, lower
temperatures resulting in reduced rate of growth of the developing grain increase sorghum
susceptibility to midge, S. sorghicola (Sharma et al., 1999a). Differences in genoptypic susceptibility
to greenbug in sorghum increase with an increase in temperature (Schweissing and
Wilde, 1978). In alfalfa, the level of resistance to pea aphid, Acyrthosiphon pisum Harris and
alfalfa aphid, T. maculata is enhanced at higher temperatures (Kogan, 1982). In sorghum, expression
of resistance to sorghum midge, S. sorghicola is infl uenced by temperature and the relative
humidity (Sharma, Venkateswarulu, and Sharma, 2003). There is considerable variation in the
infl uence of temperature on expression of resistance to insects, and such interactions need to
be kept in mind while identifying sources of resistance to insect pests for use in crop improvement
programs.
Photoperiod
Photoperiod infl uences plant growth and physicochemical characteristics of crop plants
and, thus, infl uences the interaction between insects and crop plants. Failure or inability
to grow certain crop plants during the off season at times is largely associated with
increased susceptibility to insects and diseases. Intensity and quality of light infl uences
the biosynthesis of phenylpropanoids (Hahlbrock and Grisebach, 1979) and anthocyanins
(Carew and Krueger, 1976). Prolonged exposure to high-intensity light induces susceptibility
in PI 227687 soybean plants to the cabbage looper, Trichoplusia ni (Hubner) (otherwise
resistant) (Khan et al., 1986). Susceptibility in sorghum to midge, S. sorghicola, increases
under long day length in Kenya near the Equator (Sharma et al., 1999a).
Insect Biotypes
Biotypes are populations of insects capable of damaging and surviving on cultivars known
to be resistant to other populations of the same species (Kogan, 1982). The term biotype is
used for a group of insects primarily distinguishable on the basis of their interaction with
genetically stable varieties or clones of host plants. Most biotypes have been recorded in
aphids because of parthenogenesis. Even a single mutant aphid capable of feeding on a
resistant genotype can build up into a new biotype (Pathak, 1970). Instances of emergence
of new biotypes have been documented in the case of gall midge, O. oryzae in rice (Bentur,
Srinivasan, and Kalode, 1987), Hessian fl y, M. destructor in wheat, and brown planthopper,
N. lugens in rice (Smith, 2005). To overcome the problem of biotypes, genotypes with different
mechanisms of resistance should be utilized in a breeding program to have stable and
high levels of resistance against a number of prevalent insect biotypes in a region.
Influence of HPR on Pest Population Dynamics and
Economic Injury Levels
Economic injury levels (EILs) can be used to determine the levels of host plant resistance
that can be practically attainable and economically rewarding in crop improvement