Studies On The Host Specificity And Formation Mechamism Of Cotton Aphid, Aphis Gossypii Glover

The cotton aphid Aphis gossypii Glover is one of the main pests of cotton, cucurbits and other crops. Many researches have confirmed that there are various host biotypes in wild populations of the cotton aphid. There are differences in morphological and ecological adaptability between different host biotypes. Until now, we have not clarified the host biotype formation mechanism of cotton aphid, though it is important for us to know the evolution of feeding differentiation, and it can provide us with theoretic foundation to prevent cotton aphid. So it is valuable to study on the host biotype formation mechanism of cotton aphid. In this paper, the adaptability and host transfer way, the differences of esterase activity between different host biotypes aphid, the influence of host secondary chemical compounds to the fecundity of different host biotypes and the inheritance differentiation between host biotypes were studied, and the results were summarized as follows:Host transplantation and life table methodology were used in the paper, and the results showed that the survival and reproduction of the cotton and cucumber biotype aphids declined dramatically when their host plant was exchanged.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 cotton and cucumber aphid could not live on plantain and E.japonica, because offspring production and survival was very low. Both the cotton and cucumber biotype aphids were able to utilize pumpkin and hibiscus. But the cotton biotype aphids were more suitable for feeding on hibiscus because it could produce more offspring on hibiscus. And the cucumber biotype aphid were more capable of using pumpkin, the intrinsic rate of natural increase was 0.19±0.05 to cotton biotype aphid and 0.28±0.02 to cucumber biotype aphid. The vegetable plant pumpkin played an important role in the reciprocal transfer of the cotton aphids between hibiscus, cotton and cucumber plants.There were differences between the aphid headstream of the two biotype aphids. Results showed that aphids from hibiscus could directly transferred to cotton, but it could not directly transferred to cucumber and needs to rear on muskmelon or pumpkin for several generations. Aphids from E.japonica could survive and build population on cotton, but could not survive on cucumber. After being reared on pumpkin, the two biotype aphids were able to live on their original host, and the specialization of the two biotype aphids persist well. The result suggested that host specificity could not modified by experience.The result of esterase activity mensuration showed that there are significant differences on the carboxylesterase and Acetylcholinesterase activities between the two biotype aphids. The carboxylesterase and Acetylcholinesterase activities of cucumber biotype were 3.5 times and 6.6 times as high as the cotton biotype aphid respectively. The secondary metabolities of cotton extracted by ethanol and acetone have repellent effect on the fecundity of cucumber biotype aphid, and the secondary metabolities of cucumber extracted by ethanol have obverse effect on the fecundity of cucumber biotype aphid. Though the response to the secondary metabolities of cotton aphid was not as sensitive as the cucumber biotype aphid, it also prefer ovipositing on cotton.This repellent effect could result in the failure of building population on the alternative host plant for the two biotype aphid and it would facilitate the formation of host specificity.The result of RAPD-PCR illustrated that DNA polymorphism was increasing with the population gradations. The DNA diversity among different clones of Aphis gossypii from same host population was lower than that of the different host populations. The genetic differentiation among different host biotypes and different host populations were only at population lever but had not achieved the subspecies lever. Based on the analysis on Nei's genetic distance(D), it was assumed that the population source on cotton might come from maily on hibiscus, and some come from E.japonica. The result of similarity index and cluster analysis showed that the differentiation between cotton and cucumber aphid were at high lever, but in low lever among cotton, hibiscus and E.japonica populations. This genetic differentiation between host biotype aphids would accelerate the formation of host specificity.