Influence of intercropping systems of corn and sunflower in control of the European corn borer, Ostrinia nubilalis (Hübner)

Background and Objectives
The European corn borer, Ostrinia nubilalis (Hübner), is an important pest of corn in Iran. Intercropping systems has been proposed as one approach that can be used for reducing pest density and improving biodiversity of natural enemies and crop yield. Therefore, the present research assessed the densities of immature stages of O. nubilalis, biodiversity of its predators, and parasitism rates of immature stages in experimental plots where corn was grown either as monoculture or strip-intercropped with sunflower. We also evaluated the effect of the intercropping systems with natural infestation of O. nubilalis on corn yield.
Materials and Methods
In this research, the influence of five strip-intercropping systems of corn (C) and sunflower (S)including 2C: 2S, 4C: 2S, 8C: 2S, 4C: 4S, and 2C: 4S along with corn monoculture was studied on density of imaginal stages of O. nubilalis, its natural enemies biodiversity, the percentage of parasitized eggs and larvae, and the grain yield of corn and the percentage of grain yield loss in an experimental field in Ardabil region in 2016 and 2017. These experimets were conducted in a randomized block design with four replicates.
Results
The densities of imaginal stages (eggs, larvae and pupae) of O. nubilalis in intercrops were significantly lower than corn monoculture. Among the natural enemies of this moth, Orius niger (Wolff), Lydella thompsoni Herting, Bracon hebetor Say and Trichogramma brassicae Bezdenko had the high relative abundance in the tested cropping systems. The Shannon diversity index (H') in intercrops was significantly greater than in corn monoculture. The calculated values of Morisita–Horn index for the predator’s species composition between intercrops were C_MH ≥ 0.946 in 2016 and C_MH ≥ 0.934 in 2017. In both years, the percentage of parasitized eggs and larvae of O. nubilalis in intercrops was significantly higher than in corn monoculture. The highest grain yield of corn and the lowest percentage of grain yield loss of corn were found in 8C: 2S intercrop.
Discussion
Based on the resource concentration hypothesis, the lower density of O. nubilalis and the lower percentage of infested plants in the intercropping systems of corn and sunflower could be demonstrated by overlapping of the secreted volatile compounds of two crops that caused disruption in the host finding. Moreover, based on the enemies hypothesis, the higher attraction and conservation of predators and parasitoids in intercrops caused lower density of O. nubilalis than in corn monoculture. In this study, the competition for nutriants between corn and sunflower was also reduced with increasing of the corn rows between the sunflower rows; the percentage of grain yield loss of corn was reduced in 8C: 2S copmpared to other intercrops. Therefore, it could be concluded that 8C: 2S intercrop is the most suitable intercropping system for use in integrated management of O. nubilalis in corn fields.