Personal musings about studies on the pink bollworm

 

19 August 2022

Why is the PBW a key pest of Bt cotton? This question often comes to my mind. And it may be bothering you too. The fact that it is the only insect pest that has evolved resistance to protein toxins in the Bt cotton and that too only in India, sets in a tone for our failure to manage it and know of its key pest status in the late 2000s and early 2010s.

The indication of incidence of the PBW larvae of 3^rd instar, about 2-3 mm size and pinkish in colour in Bt cotton in Delhi was our initial observation during 2009 and 10, when we saw them in the cut green bolls. Prior to that we just hardly saw them or saw few of them less than a mm size which did not grow well and hence, their impact in damaging bolls did not become clear.

We started working on the PBW resistance monitoring since mid-2000s. We requested colleagues for infested bolls from different localities then onwards and started rearing PBW emerging from the infested bolls. Even some of the green bolls which looked healthy were infested as we examined on cutting them. As our studies on resistance monitoring against the larvae of PBW progressed, we found that the Amreli population of PBW collected in 2008 showed more than 40-fold more resistance than the most susceptible field population. The boll damage correlated with our observations on relative resistance to Cry1Ac. During the course of publication of these results, the reviewer who later disclosed his identity as he enquired about delay in publication, said that this is indeed the first case of field-evolved resistance, a practical resistance, in the bollworm to Bt cotton.

Unfortunately, resistance monitoring programme for PBW did not take off primarily because the cotton bollworm, Helicoverpa armigera was a key pest for the past two decades in India. It had developed resistance to most of insecticides and was difficult to control. Further, its outbreak occurred quite frequently, which meant great loss of crop yields despite maximal use of insecticides. We too had started this monitoring studies on H. armigera mid-1990s and were first to report the variation in susceptibility of larvae of H. armigera to Bt Cry1Ac toxin as well as Bt products in 2000, two years prior to the commercialization of Bt cotton. These were the baseline susceptibility studies that served purpose for resistance monitoring in the following years.

Another reason for late monitoring of PBW was inadequate knowledge about artificial diet and rearing method. There were reports of PBW rearing but these were few and did not amount to much in terms of mass rearing of this species.

Sanyasi Dhurua who joined us as a Ph.D. student in 2006 was therefore hesitant to take up the thesis work on resistance monitoring in the PBW to Bt Cry1Ac in view of the above constraints. He was worried about completion of Ph.D. within a stimulated deputation period as he was in service candidate. I remember what he said that time. I explained and prevailed. Eventually he worked hard and succeeded on perfecting a meridic diet and developing rearing methodology. We were glad that we developed it ourselves sourcing dietary ingredients locally, rather than importing them from abroad. A major part of his thesis was thus related to the development of diet and rearing methods and bioassays. Later, we published the first ever report on the field-evolved resistance to Bt cotton in the PBW in 2011. It tops my most quoted papers and so of my co-author, Sanyasi Dhurua. Prior to our publication, Monsanto had released a press release in 2010 in this regard.

Politics of the day played its role. Anti-GM lobby was too glad with the failure of Bt technology as it made out, following a press release. And the GM lobby was defending the technology as it attributed resistance due to refusal of farmers to implement refuges. Very few talked about need of high expression of toxin in the target plant part. Furthermore, with so many hybrids being in the market, the quality of some was definitely questionable. Of course, later, the Government prescribed minimum expression levels of toxins and also revised guidelines for IRM. But, for those times in 2010, Bt cotton had come under attack in public. The ICAR called a meeting of some experts to discuss resistance evolution in the PBW as it became a major news on the front pages of many newspapers. I was too invited because my name appeared favourably in the press for advocating integrated pest management, and not associated with resistance evolution in the PBW. Fortunately for me, I was not asked anything nor I talked or gave my opinion in presence of other seniors. Our paper on PBW resistance was at the back of my mind and was being reviewed. For me, keeping quiet was the best option. It was alleged that the company had reported PBW resistance to single-toxin Bt cotton with an ulterior motive, as it wanted to promote dual-toxin Bt cotton belonging to its BGII event, MON15985. The latter was being slowly adapted in Gujarat. Furthermore, since some of native seed producing companies too had their own event of a single-toxin Bt cotton, this report was meant to malign them, hurt their business prospects vis-à-vis that of multinational company. And this is what was all in the meeting.

As the events unfolded, this native vs foreign battle took a broad turn in respect of breach of IPR, royalties, seed price control and other in the years to come, putting halt to the development of new versions of GM cotton.

Coming back to PBW, our paper on field-evolved resistance in the PBW published in 2011 in the prestigious journal, Pest Management Science [ https://onlinelibrary.wiley.com/doi/10.1002/ps.2127] did not create any ripple, like those of Monsanto press release in 2010. I sighed a relief. I was not called to explain our studies. We were not asked to retract the publication, alleging its’ being premature like Monsanto’s. But we did not escape searching questions of the reviewer who asked us if we informed GEAC about these findings on the field-level evolution of resistance in PBW and what pro-active measures being taken to control the spread of resistance, besides others. Fortunately for us, while casually talking to high official of ICAR, he mentioned that 40-fold resistance in the PBW is natural phenomenon, not to worry about. It was just fine for me. This observation of high official of ICAR in 2011 was in sharp contrast to those of academicians [K Chandrashekara and ARV Kumar] who reviewed and commented upon our studies on ‘Spatial and temporal variation in susceptibility of the American bollworm…….in India’ published in 2000, the first publication that we have credit for on this aspect. I quote what appeared in that preface to our publication [https://www.jstor.org/stable/24103737]. ‘Their data seem to suggest high level of tolerance [resistance] to Bt toxins…..The detailed insights into population genetics of resistance which in turn will benefit development of improved transgenic insecticidal cultivars. More importantly, it will enable development of resistance management strategies against pests’.

The resistance evolution in the PBW spread too fast throughout the country. By 2014, the PBW had evolved high levels of resistance to both toxins, Cry1Ac and Cry2Ab in the dual-stacked Bt cotton. We estimated monetary cost of resistance as reports of failure of Bt cotton crop came in from many localities. Some state Governments offered millions of rupees in compensation to the farmers. Obviously, Bt resistance spread with the spread of incidence. Was the spread of incidence led to spread of resistance or spread of resistance led to spread of incidence or was the pest incidence was concurrent with spread of resistance? In retrospect, PBW threatened the sustainability of cotton cultivation at the farmers’ level.

PBW incidence in the cotton growing regions prior to Bt cotton was restricted to some localities in the country. It was common in some localities like Faridkot in North India, Rajkot and Nemad in Central India and Raichur in South India. PBW incidence did not spread over many decades like H. armigera prior to Bt cotton. Besides differences in their biology and ecology, other reason could be attributed to the fact that cotton hybrids and varieties were more restricted to their regions and did not spread across the regions. So, cotton varieties continued to rule in North India, while cotton hybrids in Central and South India. And as many as four different species were cultivated with predominance of Gossypium hirsutum.

The area under cotton prior to Bt cotton continued to linger around 8-9 million hectares as productivity was mostly restricted due to damage caused by the cotton bollworm.

I remember once decades ago talking to a breeder in PDKV Akola who said that best of genotypes available are used for breeding high yielding cotton varieties and hybrids, sans for their biotic and abiotic stresses. So, cotton had limited breeding options so as to increase yields. As cotton bollworm continued to prevail upon with limited cotton germplasm; the only option left was to, use insecticides which had limited impact. Other options were agronomic that farmers followed depending upon their ability to spend money on agri-inputs.

With Bt cotton, the dramatic increase in yield took place as key pest was controlled to the great extent. So also, crop husbandry technology too took off as Bt technology was considered premium one, to be taken seriously. Now, almost all of cotton belonged to G. hirsutum hybrids and cotton seeds and produce moved across the country as cotton cultivation had revived.

With Bt cotton, area increased from about 8 million hectares to about 12 million hectares. Imagine increase in bolls per hectare with Bt cotton and total number of bolls throughout the cotton season providing congenial environment for the PBW to multiply, once it crossed the boll rind that posed a barrier for its neonates. Furthermore, PBW is native to this subcontinent offering genetic diversity which favoured its adaptation faster to the changing agro-ecological conditions. Coupled with that was low expression of Bt toxin in Bt cotton bolls, which further helped it to evolve resistance to Bt Cry1Ac toxin.

It is well known that mosaic of Bt events like presence of a single- as well as dual-toxin Bt events provide a congenial environment for the target pests. This was perfect situation until 2010 for the PBW to evolve resistance first to Cry1Ac and then to Cry2Ab in next few years. Over the past decade, the number of mutants of PBW are reported to be resistant to these toxins.

In the past decade, sucking pests like thrips, whiteflies, mites, mirids jassids have become key pests. Whitefly caused major yield losses in some years in the North. While some attention was given to screen cotton hybrids against CLCV-whitefly incidence; this aspect did not get much attention for PBW. Furthermore, the horizontal plant resistance was totally ignored, as again dependency on insecticides was assumed to be a natural course of pest management.

Many years ago, I read the paper on synergism of Bt and phenolics against bollworms. But the significance of presence of phenolics and other secondary metabolites was missed largely by most of us. As early as 1950s, importance of these metabolites was elucidated in determining host plant insect relationships [Fraenkel G, The raison d’être of secondary plant substances, Science 129: 1466, 1959 https://www.science.org/doi/10.1126/science.129.3361.1466]. Role of phenolics and their regulating genes in cotton is recently revisited rekindling hope of this kind of crop resistance [ https://www.tandfonline.com/doi/full/10.1080/15592324.2020.1747689]. Is it possible to upregulate these genes with boll rind specific promotors to express phenolics to kill neonates of PBW? Short-sightedness of only using insecticides for the control of PBW will be realised soon. With mating disruption technology, there is hope of its management. But long-term plan would need developing cotton plant’s own defences and the genetic engineering technologies like those in the developed countries to control resistance evolution and a spectrum of pest insects.

 PS: Cotton boll rind as a physical and biochemical barrier for the neonates of pink bollworm which feed later on inside of boll essentially of carbohydrates and the other nutrients. Is there any study on this aspect? Please spare your valuable reprints to me, Do you think that could be a basis for identifying tolerance or resistance to pink bollworm?

Thanks a lot

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