Abstract: The aphid quantity ratio (AQR) is defined as the number of aphids on each cultivar divided by the number of aphids on all cultivars.  AQR is based on the correlation between aphid populations and their host plants and is an important tool that has been utilized in evaluating Medicago sativa (alfalfa) cultivar resistance to aphids.  However, assessment of alfalfa resistance to aphids can be confused by the presence of aphid predators, causing the assessment of plant resistance to aphids to be based on incorrect aphid population data.  To refine the AQR and account for the effect of predators on aphid population assessments, we introduced a parameter ‘α’, corresponding to the predator quantity ratio, and used αAQR as the ratio to quantify aphid populations.  Populations of both aphids (4 species) and their predators (12 species) occurring in 28 M. sativa cultivars were sampled over two years at a research station near Cangzhou, Hebei Province, China.  Results showed that the most suitable evaluation period was from May to June, as the aphid population was stable during this period.  Compared with the AQR method, the predator population numbers based on the αAQR had a significant inverse relationship with aphid population numbers and the 28 cultivars were clustered into three classes: the resistant class, tolerant class, and susceptible class.  In addition, 17 cultivars were reassigned when evaluated using αAQR.  All numerical values calculated by αAQR were displayed as a Gaussian distribution, which showed that the 28 cultivars could be clustered into nine groups using a median value (±SE) of 1±0.1.  Hence, ongoing alfalfa breeding trials will be assessed using the αAQR to establish a robust system that includes agronomic performance parameters in order to generalize the new method for further studies.

Key words: aphid quantity ratio (AQR) model ,  alfalfa cultivar resistance to aphids ,  natural enemy