Growing insect-free cotton

17 Sep 2007

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COTTON, the backbone of our agriculture, is being affected by increasing population of insect pests, mainly sucking and chewing insects.

In commercially grown cotton fields, 1,326 species of insect have been reported. Of these, 30 species are common. Pink bollworm, spotted bollworm, American bollworm, army worm, white fly, jassid, mealy bug and aphid are very harmful pests. Recently, mealy bug alone has ruined cotton fields in some of the cotton- growing areas, because of its great range of host plants which is about 215 plant species.

There are several characteristics both in wild and cultivated species of cotton that repel insects. These traits are often collectively called “defence umbrella” against insects in cotton. These traits include hairs on plant parts, absence of nectars, reddish colour of the stem, presence of fregobraits, okra type leaves and concentration of gossypol chemical in the sap.

Different studies have depicted that hairs on the plant surface are not liked by insects mainly by bollworms as they cause a problem in their chewing process. Similarly, redness of the stem and branches is not liked by sucking insects like white fly, jassid and aphids.

Nectars in plants attract various insects, so plants without nectars will not be preferred by the sucking and chewing insects. Fregobraits are closed type bracts, if present in a flower of the plant it provides some escape from bollworms which often make their abode in the open bract of the flowers. It is also a means to introduce resistance against bollworms.

Similarly, okra type leaves provide less space for the reproduction and multiplication of insect on leaf surface. In addition okra type leaves give a chance to sunlight to reach the leaf surface inhibiting insect multiplication by destroying their eggs and larvae.

Cell sap with high content of gossypol is not liked by insects, especially by the sucking ones. Gossypol is a chemical present in all plant parts with a great amount in leaves, floral buds, flower and seed. It may also become a source of insect resistance. In contrast to hairiness, plants with faborous leaves do not allow eggs of insect to stay on its surface.

Although a lot of research work has been done in this field, the objectives are far from being achieved. The first and foremost objective of the plant breeders and researchers should be to identify sources of these traits and then to incorporate all of them into a single plant by using appropriate breeding method.

There may be some sort of linkages and associations which hinder the transfer of these characters to a single variety. These can be overcome by use of genetic engineering techniques.

Now, this is the best time of breeding cotton. It is a challenge for the plant breeders to develop such a variety of cotton which possesses all these characteristics.