INTERACTIVE EFFECTS OF ELEVATED CARBON DIOXIDE AND TEMPERATURE ON HELICOVERPA ARMIGERA HUB. AND SPODOPTERA LITURA FAB. ON BT COTTON
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Date
2023-11-30
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Acharya N G Ranga Agricultural University
Abstract
The present investigations titled „Interactive effects of elevated Carbon dioxide and Temperature on Helicoverpa armigera Hub. and Spodoptera litura Fab. on Bt cotton‟ of Department of entomology, Agricultural college, Bapatla, Acharya N. G. Ranga Agricultural university (ANGRAU) were conducted during 2018-2020 at ICAR - Central Research Institute for Dryland Agriculture (CRIDA), Hyderabad, Telangana. The two test insects, H. armigera and S. litura are major polyphagous cotton pests, causing heavy yield losses. In the context of global warming, largely driven by elevated carbon dioxide (eCO2) and elevated temperatures (eTemp), understanding the future status of these pests is an important task. Hence, growth, development and feeding indices of H. armigera and S. litura for three successive generations on cotton and also the biochemical constituents of cotton foliage and the larvae were elaborately studied under ambient carbon dioxide (aCO2: 380 ± 25 ppm) and elevated carbon dioxide (eCO2: 550 ± 25 ppm), at five temperatures (28, 29, 31, 33 and 35 ± 1°C). Growth parameters in test insects, H. armigera and S. litura, viz., food ingestion, larval weight and larval excretion increased under eCO2 (550 ppm) + eTemp (35 °C) whereas, pupal weight and fecundity decreased. Food ingestion capacity of the larvae increased significantly in every next generation, thus indicating greater adaptation of the test insects to climate change with advancement of generations. Notably, larval and pupal weights in the three successive generations were relatively higher for S. litura, probably due to good food ingestion. The durations of larvae, pre-pupae and pupae increased significantly under eCO2 + eTemp. The major reproductive parameter fecundity decreased under eCO2 + eTemp. In Bt cotton, mean fecundity of H. armigera under eCO2 was reduced by 7.29, 9.03, 9.58, 9.93 and 6.39 % in fiber comparison to 1.90, 3.54, 5.18, 1.62 and 1.53 % in S. litura.
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Among feeding indices of both the test insects, with increase in CO2 and temperature, Approximate Digestibility (AD) and Relative Consumption Rate (RCR) increased whereas Efficiency of Conversion of Ingested food (ECI), Efficiency of Conversion of Digested food (ECD) and Relative Growth Rate (RGR) decreased. In Bt cotton, under eCO2, the mean decrease of RGR of H. armigera was higher by 14.79, 9.80, 10.36, 7.11 and 7.81 % compared to 11.50, 13.08, 14.39, 13.65 and 13.75 % in S. litura compared to that of aCO2. At elevated conditions, H. armigera has higher ECI in Bt cotton and high RGR in both non-Bt and Bt cotton, compared to S. litura. It implies that H. armigera could be a potential key pest in future climate stress scenarios even in Bt cotton. The biochemical constituents of non-Bt and Bt cotton were down regulated by eCO2 + eTemp i.e., nitrogen, proteins, Cry1Ac and Cry2Ab toxin contents were reduced, there by the future cotton production may challenged by these polyphagous insect pests under stressed environmental conditions. Commensurate with the decline in leaf Bt endotoxins, the larval mortality of both the test insects also reduced. Among the biochemical components in test insects, carbohydrate content in H. armigera increased at elevated conditions, whereas the protein content in H. armigera and S. litura decreased. Succinctly, anticipated eCO2 necessitates the test insects to attain more nitrogen from the under nutritious foliage by huge consumption as evident from the enchanced foliage ingestion and AD and RCR. Apart from the other growth and feeding indices, higher foliage consumption itself is a major threat of future crop production. Further, increased larval and pupal durations may play a key role in gaining biomass and sufficient energy for transforming into adults with improved fitness. Besides, they could adapt to changing climate as evident from the higher values of all the parameters in the third generation under eCO2 +eTemp. Another intriguing issue is the decrease in Bt toxin production in Bt cotton leaves under eCO2 +eTemp, thus cotton crop can become more vulnerable for these pests, especially for H. armigera.