New Insights into the Life History Changes Can Enhance Control Strategies for Therioaphis trifolii

The spotted alfalfa aphid (SAA), Therioaphis trifolii, is a phloem-feeding pest with a complex life history, and it causes a tremendous global loss of crop yields. A large number of previous studies focused only on few-generation life tables, whereas multi-generation life tables could explore aphid adaptation processes that are poorly understood. In this study, we investigated the effects of physical characters of alfalfa on SAA growth and development and used life-table parameters to evaluate the transgenerational effects of SAA populations on highly resistant (HA-3) and highly susceptible (Hu) alfalfa cultivars. The results indicated that alfalfa waxy content, anatomical structure of vegetative organs, and density and length of leaf hairs were significantly positively correlated with resistance to the SAA. In terms of the developmental time of total preoviposition period (TPOP), no significant differences were observed between two populations; compared to the HA-3 population, the G2-G8 Hu populations were markedly lower and G9-G10 Hu populations were visibly higher. Meanwhile, the reproductive days, mean longevity, and fecundity (offspring) of the HA-3 population were obviously higher than those of the Hu group after G5. Furthermore, the finite rate of increase (λ) and intrinsic rate of increase (r) of HA-3 were significantly higher than for the Hu population after G7. Meanwhile, the net reproductive rate (R0) and mean generation time (T) of HA-3 were significantly higher than for the Hu population after G5. On the whole, the SAA had a high survival rate, strong reproductive capacity, long life span, and high population growth parameters on Hu in the early stage, while the SAA had better growth and development on HA-3 in the late stage. The physical characteristics of alfalfa leaves could be used as one of the indicators of aphid resistance. However, the coevolutionary coupling was broken with the gradual adaptation of SAA, which provides an empirical basis for further exploring the mechanisms of alfalfa resistance to aphids and the integrated control of pests.