Key aphid natural enemies showing positive effects on wheat yield through biocontrol services in northern China

Aphids constitute a group of major wheat crop pests in northern China and severe infestations at heading and flowering stages lead to important yield loss. Natural enemies may increase yield by reducing aphid density but alteration in cropping pattern can cause changes in their species composition. We observed strong evidence for top-down control of the wheat aphid population growth in winter wheat during 2011-2013 at Langfang Experimental Station (CAAS, China), e.g., a significant increase in the control efficiency of natural enemies in open field cages accounted for 98, 53.8 and 59.3% in the 2011, 2012 and 2013, respectively as compared to exclusion cages. We observed peak population of wheat aphids in the middle growth stage in open field survey. Overall, the population densities of wheat aphids and natural enemies were highest in the middle and late growth stages, respectively while lowest densities recorded in late stage for aphids and in early plant stage for natural enemies. The potential natural enemies were coccinellids, and Aphidiinae aphid parasitoids, especially Aphidius gifuensis (Ashmead) following the aphid peaks each season. Yield difference among the three exclosure cages were substantially different during 2011-2012, the highest yield was recorded in the treatment without aphids as compared the others. Compared to previous studies conducted in northern China, the control efficiency of natural enemies has been greatly increased by coccinellids and parasitoid community in the absence of chrysopids and spiders. These biocontrol services will provide improved IPM strategies for the wheat ecosystem in northern China.