Contents - Faperta

Host Plant Resistance to Insects: Potential and Limitations 109
TABLE 4.4
Effect of Different Levels of Protection on Insect-Resistant and Insect-Susceptible Cultivars of
Sorghum on Insect Damage and Grain Yield
Genotype
IS 17610 and IS 21443 (Sharma and Lopez, 1993). Head bug-resistant genotypes not only
reduce the rate of population increase of the insect, but are also able to tolerate greater
head bug densities.
High Levels of Plant Resistance and Chemical Control
High levels of resistance can be quite effective in controlling certain insect pests. However,
there may be need to protect such varieties against secondary pests. HPR in combination
with insecticide application (use of carbofuran granules) has a dramatic effect in reducing
shoot fl y (A. soccata) damage in sorghum (Sharma et al., 1999b) (Table 4.4). However, HPR
alone is quite effective for the control of sorghum midge, S. sorghicola. Insecticide application
alone is ineffective in reducing the shoot fl y damage on the susceptible cultivars.
The corn earworm-resistant untreated maize hybrid, 471-U6 81-1 had 48% more undamaged
ears under artifi cial infestation (heavy population pressure) than the susceptible
hybrid treated with seven applications of tetrachlorvinphos (GardonaR ). However, one
insecticide application on the resistant hybrid gives earworm control equal to that achieved
with seven insecticide applications on the susceptible hybrid (McMillian et al., 1972;
Wiseman, Harrell, and McMillian, 1973). One of the most successful examples of drastic
reduction in insecticide use with resistant varieties is the large-scale planting of rice gall
midge-resistant varieties in India (Panda and Khush, 1995), and sorghum midge-resistant
hybrids in Australia.
Advantages of HPR
Shoot Fly
DH (%)
Head Bugs a
(Milk Stage) Midge Score b
Grain Yield
(kg ha –1 )
P UP P UP P UP P UP
ICSV 197-MR 29.7 98.2 83 197 1.3 1.3 5,000 1,333
ICSV 700-SBR 7.6 72.5 4 163 5.3 9.0 1,000 0.000
ICSV 745-MR 30.1 98.4 4 101 1.0 1.7 5,000 1,000
PS 30710-SFR 8.0 73.7 6 55 3.7 8.7 4,333 667
Swarna-S 24.5 97.9 0 80 4.7 9.0 2,000 0.000
SE ± 3.00 61 0.73 433
MR, midge resistant; SBR, stem borer resistant; SFR, shoot fl y-resistant; S, susceptible check; P, protected; UP, unprotected;
and DH, dead hearts.
a Number of head bug nymphs and adults per panicle.
b Midge score (1 10% spikelets with midge damage, and 9 80% spikelets with midge damage).
Utilization of plant resistance as a control strategy has enormous practical relevance and
additional emotional appeal (Davies, 1981; Sharma, 1996). It is in this context that HPR