The cotton aphid Aphis gossypii Glover is one of the main pests of cotton Gossypium hirsutum L., cucurbits and other crops. Many researches have confirmed that there are various host biotypes in wild populations of the cotton aphid, but the capacity of the biotypes to survive on other host plants differed between them. In the present study, the fitness of two host biotypes, the cotton and cucumber biotypes, and their reciprocal transfer pathways were investigated. Survival、reproduction and other life table parameters of the cotton and cucumber biotypes on six host plants were measured by host transplantation and life table methodology. The results showed that the survival and reproduction of the cotton and cucumber biotype aphids declined dramatically when their host plant was exchanged. The net reproductive rate of the cotton biotype aphid decreased from 19.67±6.22 on the original host plant cotton to 0.02±0.05 on cucumber, and the cucumber biotype aphid decreased from 32.65±3.72 on cucumber to 2.49±0.88 on cotton. Compared with the original hosts, the net reproductive rate of the cotton and cucumber aphids declined 980 times and 12 times, and the average life span decreased by 5 and 12 days, respectively on the alternative host plant. Both the cotton and the cucumber biotype aphids could survive on Hibiscus syriacus L., and the fitness on H. syriacus L. was not significantly different. But the two biotype aphids could not survive on Plantago centralis and Euonymus japonica, because offspring production and survival was very low. The remarkable result was that both the cotton and cucumber biotype aphids were able to survive and establish populations on Cucuibita pepo L., and the intrinsic rate of natural increase was 0.19±0.05 and 0.28±0.02, respectively. Moreover, the vegetable plant C. pepo played an important role in the reciprocal transfer of the cotton aphids between hibiscus、cotton and cucumber plants.
The results of host transfer pathways indicated that the cotton aphids originally living on hibiscus could survive and establish populations on cucumber plants after being reared on Cucumis melo L. for three generations, and the intrinsic rate of natural increase (rm) of the subsequent three generations of aphids on cucumber was not significant differently (the values of rm were 0.23±0.03, 0.20±0.03 and 0.19±0.03, respectively). On the other hand, cotton aphids on hibiscus could also attain the capacity to use cucumber plants after their transfers from hibiscus to cotton and then to C. pepo. At the same time, the cotton biotype aphid and the cucumber biotype aphid could transfer between the cucumber and cotton plants via the intermedial transfer on C. pepo, and values of rm were significantly different when compared with the direct transfers. The values were 0.25±0.07 for the cucumber biotype aphids indirectly transferred to cotton and 0.27±0.02 for the cotton biotype aphids indirectly transferred to cucumber. The results suggest that there are reciprocal transfer pathways for the host specialized cotton aphids between cotton and cucumber plants, and these pathways will facilitate the dispersion of the cotton aphid and aggravate the damage caused directly by their probe penetration for sap-sucking and the transmission of viral diseases.