Contents - Faperta

126 Biotechnological Approaches for Pest Management and Ecological Sustainability
Mechanisms of Resistance to Insects
Knowledge of insect-plant relationships and the mechanisms that contribute to insect
resistance is critical for developing germplasm with high yield and durable resistance
(H.C. Sharma, 1994). In view of limited success in the past in developing crop cultivars
with resistance to insect pests by using known sources of resistance, there is a need to
identify genotypes with diverse mechanisms (genes), and pyramid the resistance genes
to increase the levels and diversify the bases of resistance. Information on different mechanisms
of resistance to insects, such as antixenosis or nonpreference, antibiosis, and tolerance
is discussed below.
Antixenosis
Antixenosis or nonpreference is expressed in terms of unsuitability of the host plant for
oviposition or feeding. The word antixenosis is derived from the Greek, xeno, meaning
“guest.” It describes the inability of a plant to serve as a host to a herbivore insect, and
forces the insect to change its host plant for feeding and oviposition. Antixenotic (nonpreference)
resistance reduces the rate of both initial and successive insect population buildup.
Antixenotic resistance may also shift the insect population to other fi elds of the same crop
or to other host plants of the insect. It is due to physico-chemical characteristics of the host
plant that affect insect behavior adversely, resulting in selection of an alternative host
plant. Absence of physico chemical stimuli that are involved in selection of the host plant
or presence of repellents, deterrents, and antifeedants contribute to the antixenosis mechanism
of resistance. Sensory cues that mediate host selection for oviposition include visual,
tactile, and chemical stimuli (Thompson and Pellmyr, 1991). Antixenosis is an important
component of resistance to the Hessian fl y, Mayetiola destructor (Say) in wheat (Roberts
et al., 1979), the onion fl y, Hylemyia antiqua (Meigen) in onion, the carrot fl y, Psila rosae (Fab.)
in carrot (Stadler and Buser, 1984), and the Asiatic stem borer, Chilo suppressalis (Walker)
in rice (Saxena, 1986). Antixenosis for oviposition is a major component of resistance to the
shoot fl y, Atherigona soccata (Rondani) and midge, Stenodiplosis sorghicola (Coquillett) in
sorghum (Figure 5.1) (H.C. Sharma and Vidyasagar, 1994; H.C. Sharma and Nwanze, 1997;
H.C. Sharma, Franzmann, and Henzell, 2002). Cotton genotypes with red and okra leaf
No. of eggs/100 spikelets
180
160
140
120
100
80
60
40
20
0
DJ 6514
AF 28
TAM 2566
IS 15107
FIGURE 5.1 Oviposition by the females of sorghum midge, Stenodiplosis sorghicola, on different sorghum genotypes
under no-choice conditions in the headcage.
CSH 1
Swarna