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A small number of larval survival and reproduction of  Helicoverpa armigera on Bt-cotton is not unexpected 

This has reference to the article entitled by certain NGOs and the media that this adopted, be it chemical insecticides or ‘Survival and reproduction of natural pest has already developed resistance to Bt-proteins, a small proportion of target populations of Helicoverpa armigera on Bt-cotton and the technology is not effec-pests can escape mortality and complete Bt-cotton hybrids in Raichur, India’ by tive! I wish to clarify the status. their life cycle. These may represent rare Ranjith et al.. The authors’ observations It is known in plant protection practice tolerant individuals or they may be have been misinterpreted and publicized that no matter what control measure is insects exposed to lower toxin levels because of particular environmental circumstances. If they are allowed to breed among themselves, their progenies are also to be tolerant to the ‘insecticide’ or the ‘protein’ in question. In fact, a ‘refuge’ (non-Bt cotton and other host crops) can break this cycle of mating among the ‘tolerant’ individuals by making available a large number of susceptible moths. The planting of ‘refuge’ crops has been recommended as a proactive insect resistance management (IRM) strategy since the introduction of Bt-cotton in 1996 in USA and Australia, and subsequently in other countries, including India (in 2002).

Ranjith et al. collected H. armigera larvae that survived on Bt-cotton (Bollgard and Bollgard II) and reared them to moth stage. Such moths were mated among themselves in confined conditions (tolerant males with tolerant females) with no choice and their progeny was shown to feed and breed on Bt-cotton. As explained earlier, this result is not surprising. In fact, such ‘selective breeding’ of a tolerant strain for several generations is a standard practice to evolve resistant strains in the laboratory to study resistance mechanisms and develop models to predict resistance development. However, field realities are different. In the field situation, the rare tolerant insects are most likely to mate with the abundant susceptible populations of H. armigera generated from ‘refuge’ crops and nearby host crops (chickpea, pigeon pea, tomato, etc.), thereby resulting in susceptible progenies. 

Mere larval survival on Bt-cotton does not automatically mean that resistance has developed. For example, unusually high larval survival of Helicoverpa zea,a close relative of H. armigera, was recorded in 2002 in Mississippi and Arkansas, but the pest has not developed field resistance so far. In fact, in the year of Bollgard’s commercial release in the US in 1996, unusual H. zea survival led to errant claims of resistance in spite of previously published data demonstrating the potential for such survival of nonresistant H. zea. Similarly, in Australia, H. armigera larvae were observed in some areas in Queensland on Bollgard II during 2005–2006, 2006–2007 and 2007–2008, but there is no sign of resistance development or increase in such populations till date. It is to be realized that Bt technology is expected to provide effective protection of cotton crop against bollworms, but cannot ensure 0% survival and damage by bollworms. A small proportion of bollworm population survives on Bt-cotton and damages the crop to a limited extent. Ranjith et al. have also acknowledged the lower damage on Bollgard and Bollgard II plants (4–8%) when compared to non-Bt cotton (30%). 

Survival percentage on Bt plants also depends on as how well the recommended agronomic practices like scouting for insects twice a week, taking other control measures if the population exceeds the economic threshold level, etc. have been followed in the field, which are not revealed in the article. H. armigera survival or its reproduction under confinement, therefore, is not a new issue. How far these insects could proliferate under field conditions would be of interest. The present authors could have subjected the F1 progeny population (assuming the field survivors were F0) to bioassays (termed as reactive monitoring) with the Bt proteins. If there was a significant shift in LC50 relative to the established baseline susceptibility values for H. armigera in India, then there is a valid reason for concern. It is to be noted that the authors of the subject publication are not directly attributing the survival on Bollgard and Bollgard II fields to resistance to Bt proteins, but have rightly suggested to be watchful for signs of development and spread of resistance. Meanwhile, the opponents of Bt technology are exploiting the present publication to renew their attack on Bt-cotton to declare that bollworms have developed resistance and it is ineffective. This claim is receiving wide coverage in print and electronic media. Such hasty conclusions are against scientific principles and should be avoided. 

Large-scale commercial planting of Bollgard (expressing Cry1Ac protein) has been in practice since 2002 and Bollgard II (expressing both Cry1Ac and Cry2Ab) since 2006 in India. It is acknowledged that Bt-cotton has been mainly responsible for doubling the cotton yield due to effective control of bollworms and turning India from an importer of cotton to a major exporter. It has also significantly improved the socio-economic status of our farmers in the last eight years. Resistance development is a natural phenomenon and efforts should be made to delay it as far possible through proper IRM strategies. The fact that no field resistance to Btcotton has developed in USA, Australia and other countries in the last 15 years is a testimony to this. Further work is already in progress to develop and utilize additional technologies to use proteins, Bt and non-Bt, with other modes of action and pyramiding with additional genes in Bt-cotton. Continuous research is needed to help farmers improve farm productivity and manage various challenges they face in their fields.

Source : Current Science



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