April 29, 2002	Monday	Safar 15, 1423

COTTON being the leading cash crop substantially contributes to food, textile industry and foreign exchange earnings. It is grown over an area of 3 million hectares with an average yield of 610kg of cotton lint per hectare.

Among many factors, the insect pests are of significant importance, which result in quantitative and qualitative losses. Among various species the American bollworm, Helicoverpa armigera Hubner (Lepidoptera: Noctuidae) have reached to alarming levels. This insect is globally reckoned to cause severe losses. As a typical of the Noctuidae, the American bollworm is highly polyphagous which attacks a wide variety of crops including cotton, tomato, tobacco, sunflower and legumes. The damage is frequently localized on the reproductive parts and thus influences the yield directly. Insect pest management programmes are being developed to combat these problems with minimum loss to the environment.

Pesticides are widely used to control the American bollworm (ABW) and other pests in cotton, indiscriminate use of which has resulted in resistance. Insecticides are providing poor control of this pest and its excessive use has flared up other species like armyworms. This has created health hazards to human and animal lives along with speeding up environmental pollution. Crop failures due to the outbreak of the secondary pests is the result of the misuse of pesticides. Many scientists have documented pyrethroids resistance in the field populations of the ABW. This insect has a great importance and its management has become a challenge. Genetically modified crops are being introduced which make plant resistant to pests and the new genes also improve other characteristics leading to better quality. Many scientists advocate use of resistant varieties as the most promising method for reducing infestation. The search for pest resistance in cotton germplasm by transferring resistant genes has attained great momentum during the past few years. The extent of damage caused by the insect pests to the plants depends upon their morpho-physical and chemical composition.

Considering the importance of host plant resistance in the integrated pest management (IPM), a project was planned to screening nine cotton genotypes for resistance against the ABW. These nine genotypes were selected after preliminary screening of 25 commercial varieties and new lines. The studies monitored various genotypes of cotton for ascertaining resistance to the ABW. Resistance implies an indirect evaluation i.e. lesser the oviposition, number of surviving insects and infestation of fruiting parts, the greater would be the resistance. Data on the number of eggs laid by the ABW were collected from 10 plants taken at random from each plot. Upper 45cm of each plant was searched for data collection. Based on the results, the genotypes were divided into five categories i.e., susceptible, moderately susceptible, intermediate, moderately resistant and resistant in the preliminary trails.

When the criterion of the larval infestation was used to determine the resistance, the genotype FH-645 showed maximum infestation of fruiting bodies (11.28 per cent) caused by the ABW, which was similar to those recorded on the BH-36 and the MNH-554 with 9.96 and 10.06 per cent infestation, respectively. The minimum infestation of 5.27 per cent was recorded on the RH-295, which did not differ significantly from the FH-682 with 6.43 per cent infestation. The S-12 had 6.99 per cent infestation, which was also at par with the 6.43 per cent infestation recorded on the FH-682. The genotypes FS-628, FH-634 and VH-137 showing 9.80, 9.84 and 9.92 per cent infestation of fruiting parts respectively were similar with each other, as well as the BH-36 and the MNH-554.

The abiotic factors have very weak effect. Relative humidity plays a negative role while the effect of temperatures and rainfall were positive. Relationship between the relative humidity and the temperature was strongly negative whereas the relationship between the rainfall and the humidity was positive. The regression model did not strongly describe these relationships. These studies also include morpho-physical plant characters to find out their role in consumption of leaves by the ABW larvae in the laboratory. In this context the trichome density, trichome length, thickness of leaf lamina and the number of gossypol glands were investigated to determine their role in the resistance mechanism. These characters could be exploited in the development of resistant varieties of cotton.

It was evident from the results that the trichome density on midrib, vein and leaf lamina, thickness of leaf lamina and gossypol glands on leaf lamina play a negative role on the consumption. Length of trichome on leaf lamina was significant and played a positive role towards consumption. the gossypol glands on the midrib and veins were not so important because of their non-significant effect on the consumption. Trichome density on lamina, length of trichome on lamina, thickness of leaf lamina, gossypol glands present on vein and lamina had significant effect on the coefficient of the utilization. The cumulative effect of all morpho-physical characters on the consumption and coefficient of the utilization was calculated to be 97.5 and 88.7 per cent, respectively.

Based on oviposition, larval population and infestation of fruiting bodies, it was concluded that the FH-628, the RH-295 and the S-12 were resistant genotypes against the American bollworm. By growing these varieties the farming community can minimize the damages to cotton. The morpho-physical plant characters such as the trichome density and the thickness of leaf lamina were found very important in their contribution. These plant characters could effectively be utilized by plant breeders to develop new varieties resistant against the ABW.