Contents - Faperta

Host Plant Resistance to Insects: Potential and Limitations 97
TABLE 4.2
Population Increase of Sorghum Midge, Stenodiplosis sorghicola, on Three Sorghum Genotypes*
Generation
No. of Midge Flies ha –1 a
CSH 1 IS 12664C DJ 6514
First year P1 100 100 100
F1 600 300 100
F2 3600 900 100
F3 21600 1700 100
F4 129600 8100 100
Diapause populationb (1%) c 1554 120 4
Second year P2 1554 120 4
F1 9324 360 4
F2 55944 1080 4
F3 335664 3240 4
F4 2113984 9720 4
Diapause populationb (1%) 25149 142 1
* CSH 1, susceptible; IS 12664C, moderately resistant; and DJ 6514, resistant. A hypothetical example.
a Midge population multiplies by 6 times on CSH 1 and 3 times on IS 12664C as compared to that on DJ 6514.
The midge population at the beginning of the season is assumed to be 100 fl ies ha 1 (P 1).
b In each generation (F1–F 4), 1% of the total population enters diapause.
resistance, and over 100 generations to overcome the antixenotic resistance. If the insects
are exposed to toxin-producing plants, a biotype capable of overcoming the resistance can
emerge quickly. Addition of 10% and 30% susceptible plants in the fi eld can delay the
development of a new biotype by 150 and 500 generations, respectively.
The impact of growing a highly resistant, a moderately resistant, and a susceptible variety
on insect populations over a period of time has been explained for sorghum midge,
S. sorghicola in Table 4.2. At the end of four generations during the fi rst year, there would
be 129,600, 8100, and 100 midges ha 1 on CSH 1, susceptible; IS 12664C, moderately resistant;
and DJ 6514, a highly resistant cultvar, respectively. By the second year, there would
be 2,113,984, 9,720, and 4 midges ha 1 in areas planted to CSH 1, IS 12664C, and DJ 6514,
respectively. More importantly, there would be 25,149, 142, and one diapausing midge larvae
ha 1 in areas planted to CSH 1, IS 12664C, and DJ 6514, respectively. Even moderate
levels of resistance, such as those available for the polyphagous pest H. armigera in chickpea,
can have considerable effect on the population increase of this pest (Table 4.3). The
effect of growing a resistant cultivar would be similar for other insects depending on the
level of resistance and the mechanisms involved. If the mortality of H. armigera larvae is 15%
on ICCC 37, 35% on ICCV 2, and 40% on ICC 506, and assuming that there are 10 female
moths per hectare in the beginning of the season, each female moth lays an average of 500
eggs, and there are three generations in a cropping season, then there will be 191,914,063
moths in an area planted with the susceptible cultivar ICCC 37 as compared to 85,820,313
moths in the area planted with the moderately resistant cultivar ICCV 2, and 67,500,000
moths in the area planted with the resistant cultivar ICC 506. Based on rates of insect multiplication,
there would be 2.84 and 1.27 times as many insects in areas planted to ICCC 37
and ICCV 2, respectively, as compared with areas cropped with ICC 506. Thus, even moderate
levels of plant resistance exercise considerable infl uence on insect populations, which
is cumulative over time. These models can also be used to explain the situations where