Strip cropping wheat and alfalfa to improve the biological control of the wheat aphid Macrosiphum avenae by the mite Allothrombium ovatum

Strip cropping of wheat (Triticum aestivum) and alfalfa (Medicago sativa) as a mechanism for improving the effectiveness of biological control of the wheat aphid (Macrosiphum avenae) by the mite (Allothrombium ovatum) was studied from 2002 to 2004 in Luancheng County, Hebei Province, China. Results showed that the strip cropping of wheat and alfalfa significantly increased the egg and larval densities of A. ovatum and the percentage of M. avenae parasitized by larval mites compared with the monoculture of wheat. The mean number of mites per parasitized aphid was also significantly higher in strip cropping than in wheat monoculture. The percentage of parasitized aphids was shown as a negative and logarithmical function of the mean population growth rate of wheat aphid, however such a relationship was statistically significant only in the strip cropping plots. The higher incidence of parasitism on alate than on apterous aphids indicated that parasitism of alate aphids by A. ovatum play an important role in facilitating A. ovatum dispersal and limiting wheat aphid population increase.