NON-TARGET EFFECTS OF INSECTICIDES AND GENETICALLY MODIFIED CROP ON ECOLOGICAL ROLE OF COLLEMBOLANS IN COTTON ECOSYSTEM OF PUNJAB, PAKISTAN

Abstract

The present research was planned to determine the effects of insecticides and genetically modified cotton on the abundance of Collembola and on their ecological role as these are the most abundant group of organisms and also used as bio-indicators in disturbed soil habitats. Base line data of Collembola demography were determined in four different crops i.e. sugarcane, cotton, clover (Trifolium alexandrinum) and wheat under conventional agronomic practices. The results revealed that maximum mean abundance of Collembola was found in clover (T. alexandrinum) (36.74) followed by sugarcane (29.15), cotton (20.79) and wheat (16.52). Four species of Collembola (Isotoma decorata, Xenylla indus, Seira indra and Sminthurus mime) were abundant in cotton followed by three (Xenylla indus, Seira indra and Sminthurus mime) in sugarcane and two (Xenylla indus and Seira indra) each in cotton and wheat. The proportionate percentage of the abundance of Xenylla indus was 65% followed by 20% of Seira indra, 13% of Isotoma decorata and 2% of Sminthurus mime throughout the sampling period. The principal component analysis revealed significant effect of soil moisture and soil pH on the abundance of Collembola while the effect of soil temperature, relative humidity and organic matter was non-significant. The abundance of Collembola was positively correlated with soil moisture and organic matter in all the crops, soil temperature in wheat, relative humidity in sugarcane and cotton, soil pH in cotton while negatively correlated with soil temperature in sugarcane, cotton and clover (T. alexandrinum), relative humidity in clover (T. alexandrinum) and wheat, soil pH in sugarcane, clover (T. alexandrinum) and wheat. A laboratory test was conducted to determine the effect of different insecticides on the survival rate of Isotoma decorata after exposure to various doses for the period of eight weeks. Endosulfan, chlorpyrifos, chlorfenapyr and acetamiprid were the highly toxic, imidacloprid, bifenthrin and pyriproxifen were moderately toxic while emamectin benzoate, spinosad and cypermethrin were the least toxic at their recommended doses against I. decorata. No individual of I. decorata was alive when they were exposed to recommended doses of endosulfan and chlorpyrifos for two weeks, acetamiprid for three weeks, chlorfenapyr for four weeks, imidacloprid for five weeks and bifenthrin for eight weeks while mortality of all the released individuals during the period of study, was not observed for pyriproxifen, emamectin benzoate, spnisoad, and cypermethrin. Field experiments consisting of 3 field trials were conducted for two consecutive years (2012 and 2013) at Faisalabad and Multan to determine the effects of insecticides and Bt cotton on the population abundance of Collembola. In the first field trial, the repeated applications of acetamiprid followed by imidacloprid were found the highly toxic while spinosad was the least toxic insecticide against the Collembola. At Faisalabad, the population means of Collembola in the plots that received five applications of acetamiprid reduced to 0.22 and 0.33 individuals per trap during 2012 and 0.33 individuals per trap during 2013 as compared to the control treatments in which means of population were 21.55 and 21.77 individuals per trap during 2012 and 18.55 and 19.11 individuals per trap during 2013 in Bt and non-Bt varieties, respectively. At Multan, the population means of Collembola in the plots that received five applications of acetamiprid reduced to 0.11 and 0.22 individuals per trap during 2012 and 0.11 individuals per trap xiv during 2013 as compared to control treatments in which means of population were 15.33 and 15.88 individuals per trap during 2012 and 13.11 and 13.55 individuals per trap during 2013 in Bt and non-Bt varieties, respectively. No recovery of Collembola population was observed during this study. In the second field trial, the applications of different insecticides in the sequence also caused the significant reduction in the abundance of Collembola. A reduction in the population was observed after 7 and 15 days of treatments while slight increase or recovery in the population was observed after the 30 days of treatments. The abundance of Collembola was reduced significantly in T3 that was consisted of two applications of acetamiprid followed by one application of bifenthrin. After 30 days of treatments at Faisalabad, the population means in T3 were 1.33 and 3.77 individuals per trap during 2012 and 1.88 and 3.00 individuals per trap during 2013 as compared to the T10 (control) in which means of population were 20.33 and 21.33 individuals per trap during 2012 and 19.88 and 19.00 individuals per trap during 2013 in Bt and non-Bt varieties, respectively. After 30 days of treatments at Multan, the population means in T3 were 3.00 and 3.66 individuals per trap during 2012 and 2.33 and 3.77 individuals per trap during 2013 as compared to the T10 (control) in which means of population were 14.55 and 14.66 individuals per trap during 2012 and 13.66 and 13.11 individuals per trap during 2013 in Bt and non-Bt varieties, respectively. In the third field trial, the high doses of different insecticides caused more reduction in the abundance of Collembola as compared to the applications on the recommended dose. A recovery in the population was observed after the 25 days of treatments and onward as the period of study progressed. Fast recovery of population to control level was observed in those plots that were treated with the recommended doses while slow rate of recovery was observed in the plots that were treated with the high doses of insecticides. It is also clear from the results of field trials that there was no adverse effect of Bt cotton on the abundance of Collembola. Litterbag experiment was conducted to determine the effects of insecticides and Bt toxin on the rate of decomposition of litter by Collembola. The results showed significant effects of the mesh size of bags and treated litter on the decomposition rate, while non-significant effect of the litter type from Bt and non-Bt cotton after 60, 120 and 180 days of incubation in the soil was observed. Litter from insecticide treated Bt and non-Bt cotton fields decomposed at slower rate as compared to the untreated litter in both large and small mesh size bags. The percent mass loss of treated litter after 60, 120 and 180 days was 11.97%, 32.28% and 43.73% while mass loss of untreated litter was 16.54%, 41.15% and 56.67%. The litter of Bt and non-Bt cotton was decomposed identically and thus disapprove level of toxin in the litter which can influence Collembola.