Toxicity of biocides to native cyanobacteria at different rice crop stages in wetland paddy field

Biocides (herbicides and insecticides) are intensively being used in rice cultivation, despite several adverse non-target effects on native cyanobacteria. Comparative effects of three herbicides (benthiocarb, butachlor, and 2,4-D) and four insecticides (furadon, phorate, methyl parathion, and ekalux) were studied at field-recommended doses. Field experiments of rice were carried out for three consecutive seasons in completely randomized block designs in both rainy and winter croppings at the Central Rice Research Institute, Cuttack, India. Toxicities of seven biocides individually to native cyanobacteria growing with rice crops in wetland fields were assessed for growth and acetylene reduction activity (ARA); several crop parameters of paddy were estimated concomitantly. It was observed that applications of butachlor and benthiocarb individually significantly decreased growth (15.86 and 17.50%), ARA (0.16 and 0.68%), and N-yield (0.26 and 0.85%) of the native cyanobacteria, whereas these parameters increased (24.04% in growth, 0.25% in ARA, and 0.40% in N-yield) due to the application of 2,4-D. Of the tested insecticides, furadon, phorate, and methyl parathion enhanced the growth (17.8, 10.7, and 11.7%), N2-fixation (0.10, 0.097, and 0.074%), and N-yield (0.16, 0.18, and 0.20%) of cyanobacteria, whereas ekalux had no significant effect on cyanobacteria. On the basis of cyanobacterial parameters, biocides could be recommended for rice crops in the order furadon < phorate < methyl parathion < ekalux. Applications of 2,4-D and furadon individually in comparison to the rest other biocides enhanced rice yield. These finding could help to revise integrated strategies of weed and pest management in wetland rice agriculture to save non-target soil microorganisms, particularly cyanobacteria.