Contents - Faperta

Host Plant Resistance to Insects: Potential and Limitations 91
the herbivore, Pieris rapae L. (van Poecke and Dicke, 2002). Besides exogenous application of
jasmonic acid or salicylic acid, use is also made of transgenic A. thaliana that does not show
induced jasmonic acid levels after wounding (S-12) and transgenic A. thaliana that does not
accumulate salicylic acid (NahG). Treatment of A. thaliana with jasmonic acid resulted in an
increased attraction of parasitoid wasps compared with untreated plants, whereas treatment
with salicylic acid did not. Transgenic plants impaired in the octadecanoid or the salicylic
acid were less attractive than wild-type plants (van Poecke and Dicke, 2002).
Factors Affecting Expression of Resistance to Insects
The most desirable form of insect resistance is the one that is stable across locations and
seasons. However, several climatic and edaphic factors infl uence the level and nature of
resistance to insect pests (Kogan, 1982). Inherited characters, especially those involving
physiological characteristics, are infl uenced by the environmental factors. Some of the factors
that infl uence plant resistance are discussed below.
Soil Moisture
Moisture stress alters the plant’s reaction to insect damage, leading either to an increase or
decrease in susceptibility to insect pests. Populations of Aphis fabae (Scop.) have lower rates
of reproduction on water-stressed plants (McMutry, 1962). High levels of water stress also
reduce damage by sorghum shoot fl y, A. soccata (Soman et al., 1994). However, waterstressed
plants of sorghum suffer greater damage by the spotted stem borer, C. partellus
and sugarcane aphid, Melanaphis sacchari (Zehntner) (Sharma et al., 2005c). Atmospheric
humidity also interferes with insect-plant interactions (Sharma et al., 1999a). High humidity
increases the detection of odors and, thus, may infl uence host fi nding by the insects.
In cotton, frequent irrigation increases vegetative growth and subsequent damage by
H. armigera (B. Singh et al., 2005).
Plant Nutrition
Nutrients play an important role in plant resistance to insects. In some instances, high
levels of nutrients increase the level of plant resistance to insects, and in others they may
increase the susceptibility. Application of nitrogenous fertilizers decreases the damage by
shoot fl y, A. soccata, and spotted stem borer, C. partellus, in sorghum (Reddy and Narasimha
Rao, 1975; Chand, Sinha, and Kumar, 1979). A decrease in shoot fl y damage has also been
observed after application of phosphatic fertilizers (Channabasavanna, Venkat Rao, and
Rajagopal, 1969; Sharma, Singh, and Nwanze, 1997). Changes in nutrient supply also affect
the sorghum resistance to greenbug, S. graminum (Schweissing and Wilde, 1979). Application
of potash decreases the incidence of the top borer, Scirpophaga excerptalis (Walker), in sugarcane.
High levels of nitrogen lead to greater damage by the cotton jassid, A. biguttula
biguttula (Purohit and Deshpande, 1992).
Temperature
Temperature is one of the most important factors affecting the behavioral and physiological
interactions of insects and their host plants (Benedict and Hatfi eld, 1988). Temperature-induced