Geographic variation of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) susceptibility to Cry toxins in Bt cotton, in Punjab Pakistan

Abstract

Bt transgenic cotton, being a vital part of pest management effectively controls the target pest, Helicoverpa armigera. However, the success of Bt technology depends on the persistent vulnerability of target pests to Bt insecticidal protein The results of field studies exhibited a parallel trend of H. armigera egg laying in all six cultivars whereas larval incidence found minimum in dual toxin genotype followed by single Bt genotypes (FH-118 followed by FH-Lalazar, MNH-886 and FH-142, respectively) and maximum on non-Bt (FH-942) at Faisalabad, Multan and Bahawalpur during both study years. The baseline susceptibility of H. armigera larvae conducted in 2013, showed LC50 ranging from 0.123 to 1.026 µg ml-1, 0.148 to 1.675 µg ml-1 and 0.210 to 2.761 µg ml-1, against 1st, 2nd and 3rd instar larvae respectively to susceptible and field collected populations (Faisalabad, Multan and Bahawalpur). These results gave an indication that Bahawalpur population was found 8.34, 11.32 and 14.71-fold resistant, followed by Multan (5.54, 7.44 and 8.99 fold) and Faisalabad population (4.08, 4.88 and 5.23 fold), for 1st, 2nd and 3rd instar larvae respectively in comparison with susceptible population. The MIC50 was 0.003 to 0.006 µg ml-1, 0.009 to 0.088 µg ml-1 and 0.014 to 0.206 µg ml-1 for 1st, 2nd and 3rd instar larvae respectively. whereas highest number of BWP population survived and showed resistance (23.33, 9.77 and 14.71-fold), MLT (10.67, 5.88 and 6.21) and FSD (6.67, 2.44 and 2.71 fold). This lethal concentration increased gradually and similar trend were found during 2014. In case of detached plant bioassay, 1st and 2nd instar larvae were found to be more susceptible and highest mortality was recorded on different plants parts of CEMB33 (leaves > square > flower > bolls), compared with single Bt cotton genotypes (FH-118 > FH-Lalazar > MNH-886 > FH-142, respectively). All 1st and 2nd instar larvae were unable to pupate. The order of mortality was: susceptible > Faisalabad > Multan > Bahawalpur. In case of bioassay on 3rd, 4th and 5th instar, the percent mortality and female longevity was consistently decreased whereas the pupation, adult emergence, fecundity and male longevity was consistently increased. The data also indicated that 1st instar neonate died (i.e. 100% cumulative mortality) after one day, three days and six days when assayed by 4, 1 and 0.25 µg ml-1 toxin respectively. However, maximum survival was recorded when neonate assayed at 25ᴼC followed by 45, 30, 35 and 40ᴼC, in case of all preceding concentrations (0.063, 0.016 and 0.004). The pupation (%), adult emergence (%), pupal weight (mg), larval and pupal duration (days) was recorded maximum at 25ᴼC followed by 45, 30, 35 and 40ᴼC. The Relative Enhanced Toxicity Factor was increased gradually and consistently up to 40ᴼC but decreased suddenly when assayed at 45ᴼC, in case of LC50 and LC90 estimation. The results of asymmetric fluctuation indicated that right side of tibia, fore wing and hind wing length was larger than the left side. However, the susceptible population possess minimum length of tibia, fore wing and hind wing than population of Faisalabad, Multan and Bahawalpur. The data showed that all the morphological traits including FMR ratio was found maximum from Bahawalpur population followed by Multan, Faisalabad and susceptible population.