Critical Evaluation of GM Cotton

Abstract

Over the past 24 years, biotech cotton has been influencing global cotton production systems. Genetically modified (GM) cotton or biotech cotton has thus far been approved in 19 countries. In 2018, biotech cotton was cultivated in 24.9 million hectares, which comprised 76.0% of the global cotton area. More than 94.0% of the global biotech cotton area is located only in five countries: India, the USA, Pakistan, China, and Brazil. Thus far only two traits, namely, insect resistance and herbicide tolerance (HT), are commercially available in GM cotton. Herbicide-tolerant cotton has been developed to survive six different herbicides, namely, glyphosate, glufosinate, dicamba, 2,4-D, isoxaflutole, and bromoxynil (HT not in use). Insect-resistant cotton has been based mainly on seven Bt toxins, cry1Ac, cry1Ab, cry1C, cry1F, cry2Ab, cry2Ae, and vip3a expressed by genes derived from the soil bacterium Bacillus thuringiensis (Bt). Though cowpea trypsin inhibitor (CpTi) protein was also deployed in insect-resistant biotech cotton in China, the area under cultivation appears to be negligible. Biotech cotton is available in different forms with either a single gene trait or with combinations of genes and traits. Initially Bt cotton was commercialized with a single gene cry1Ac. The second-generation biotech cotton contained cry1Ac + cry2Abcry2Ab, cry1Ac + cry1F, cry1Ab + cry2Ae, cry1 + cry2 + vip3A, and Bt + HT (epsps). Bt cotton and HT cotton are grown in Argentina, Australia, Brazil, Colombia, Mexico, Paraguay, South Africa, and the USA, while developing countries such as India, Burkina Faso, China, Pakistan, Myanmar, and Sudan approved only Bt cotton and have not approved HT cotton as yet. Two bollworm species, namely, Helicoverpa zea (in the USA) and the pink bollworm Pectinophora gossypiella (in India), have developed resistance to cry2Abcry2Ab and cry1Actoxins in Bt cotton. Glyphosate resistance was recorded in 13 weed species each in the USA and Australia and 8 each in Argentina and Brazil. Reports indicate substantial economic benefits due to the adoption of GM cotton. Economic benefits from Bt cotton can arise from higher yields due to effective protection from lepidopteran larval damage and from savings due to reduced insecticides for bollworm control. Research reports show that adoption of Bt cotton resulted in an initial decrease of insecticide usage followed by an increase in some countries. Insecticide usage has been increasing constantly over the past 10 years in India, Pakistan, China, Brazil, and the USA for the control of thrips, whiteflies, mealybugs, boll weevils, and pink bollworms. Insecticide use for boll weevil control is a major concern in Brazil. Enhanced use of herbicides to control resistant weeds in the USA and Brazil is an emerging concern. GM technologies so far have assisted growers in managing bollworms and weeds. Target insects and weeds have been developing resistance, thereby necessitating the need for new genes or new technologies. New cotton varieties have been developed using CRISPR and RNAi and offer promise in combating weeds, insect pests, diseases, and drought.