​which characteristic features of cotton plant dont attract bollworms 

This is not a characteristic of all cotton plants but bollworms population can be controlled by the Bt cotton plant.

• Bacillus thuringiensis is a bacterium which produces proteins that attack insects such as lepidopterans (armyworm), coleopterans (beetles), dipterans (flies/ mosquitoes) etc. It does so by producing a  protein crystal that contains a toxic protein in an inactivated state. A gene known as CRY in the bacterium encodes this toxin. Cotton bollworms are controlled by Cry IAc and Cry II Ab. These genes when introduced in cotton leads to the formation of insect resistant variety of cotton, known as Bt cotton.

• Inactivated toxin gets converted to Activated toxin in the gut of insect

• This activated toxin then binds to the epithelial cells in the midgut of insect, creates pores that cause lysis and ultimately the death of insect.