One reason that DDT has stopped being used to combat the boll weevil was of the boll weevil.

One reason that DDT has stopped being used to combat the boll weevil was increased resistance of the boll weevil.

Explanation:

Boll weevil is a pest that feeds on the cotton crop plant. Farmers often use pesticides such as DDT to kill this pest. But farmers observe that in the long run, DDT becomes ineffective against these pests. This is because the boll weevil develops resistance for the pesticide. As organisms which have better qualities to live in an environment survive and pass on their characteristics to their offsprings, hence with the passage of time boll weevils with resistant genes for DDT become abundant in number.

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