Impact of insecticide and tillage methods in wheat on the populations of aphid and their coccinellid predators in rice-wheat cropping system of India

The tillage methods are changing in north-western plains of India and conservation tillage is emerging as an important alternative for wheat cultivation. Tillage is known to reduce insect-pests through mechanical damage or burying them deep or bringing them on the surface. The present investigations were undertaken to know the effect of different tillage conditions viz. conventional tillage (CT), Zero tillage (ZT), Zero tillage + mulch (ZT+ mulch) and rotary tillage (RT), on the population build-up of wheat aphids and their natural enemies in rice-wheat cropping system. The aphid incidence was not influenced by tillage type while the population of coccinellid predators increased significantly in all conservation tillage practices. At the same time, the application of insecticide used for aphid control resulted in significant decline of coccinellid predators in reduced tillage conditions. The interactions of insecticide × time indicated that effect of insecticide applied for control of aphids (13 WAS) lasted for 2-4 weeks. More than 12 species of predators and parasitoids of wheat insect pests including coccinellids, syrphid flies, parasitic wasps, spiders and insects belonging to chrysopidae family were recorded under different tillage conditions. Number of coccinellids/m2 in insecticide protected conditions (2.19-2.29) was significantly lower than unprotected ones (2.52-3.55). Significant differences in coccinellids population were also recorded in different tillage conditions. Number of coccinellids/m2 were minimum in CT (1.95-2.27) followed by RT (2.15-2.66), ZT (2.72-3.69) and ZT + mulch (2.80-2.86). Coccinellid population in CT and RT was on par with each other; however, it was significantly lower than in ZT and ZT+ mulch. The results advocate that insecticide application in CT and RT has no effect on incidence of coccinellid population whereas in other tillage conditions application of insecticide significantly reduced their population. Thus, utmost care should be taken while advocating the use of insecticide for aphid control in conservation tillage.