Capacity Of Cucurbit-Specialized Aphid Aphis Gossypii To Use Cotton With Different Growth Stages Or Infested With Whitefly

The cotton-melon aphid Aphis gossypii Glover is one of the polyphagous insect pests, but it also shows strong feeding preferences for some specific hosts. A. gossypii can be classified into the cotton- and cucurbit-specialized biotypes in Nanjing. The cotton-specialized aphids use cucurbit crops very difficultly, while the cucurbit-specialized ones use cotton difficultly too. However, the growth cycles of the cucurbit crops are very short, and these crops are changed frequently very year. When the cucurbit crop harvest, the cucurbit-specialized aphids may be suffered from food deficiency. Then, in order to resolve the temporary food shortage for ensuring the long-term survival and reproduction for cucurbit-specialized aphids, we presumed that the aphids would use the cotton temporarily. Therefore, the performances of the cucurbit-specialized aphids on different growth stages and leaf positions of cotton, and on the whitefly-stressed cotton were studied. Meanwhile, the genetic structures of the cotton aphid populations on different crops in the field at different dates were detected in order to unfold the possible transfer pathway of cotton-melon aphids among different crops. This study obtained the following main results:The ability of the cucurbit-specialized aphids to use cotton plants was strongly dependent on the growth stages of cotton. The cucurbit-specialized aphids could not use the cotton plants with less than six leaves. But they could establish populations slowly on cotton plants with more than nine leaves. And some cucurbit-specialized aphids established population on the cotton with nine leaves. When the cucurbit-specialized aphids have colonized on the cotton plant, they inhabited mainly on the basal old leaves, and rarely on the upper and middle leaves.The cucurbit-specialized aphids could live on the basal and middle leaves of the older cotton. The ability of the cucurbit-specialized aphid to use the cotton leaves was closely related to the age of cotton leaves. The cucurbit-specialized aphids could not survive on young leaves of the cotton plant, but the senescent leaves were benefit from the cucurbit-specialized aphids to use.The damage of whiteflies to cotton plants improved the fitness of the cucurbit-specialized aphids on these plants. The cucurbit-specialized aphids could survive a long time on the cotton with six leaves which was infested with whiteflies. And some of them could establish populations predominantly on the lower and middle leaves.The cucurbit-specialized aphids established populations on the whiteflies stressed cotton were still maintained a strong preference for the cucumber. The net reproductive rate and intrinsic rate of cucurbit-specialized aphids were very low when they were transferred onto new six-leaf cotton or excised cotton leaves, even if they were reared on six-leaf cotton for 61 days, and they could not establish populations on the new cotton. But when the cucurbit-specialized aphids were transferred onto cucumber leaves again, their net reproductive rate and intrinsic rate were very high. They performed strong preference on cucumber. The feeding experience of the cucurbit-specialized aphids on cotton which infested with whiteflies did not alter the host specialization to cucumber.Five genotypes of cotton-melon aphids in fields were identified from different host plants in Nanjing. They were genotype A, B, C, D and E. The genotypes of aphids on the hibiscus, an overwintering host and cotton were similar, mainly belonging to the genotype A and B. The genotypes of aphids on cucumber and pumpkin were similar, mainly belonging to the genotype C. The main genotypes of aphids on zucchini were genotype A, B and C. After checking the genotypes of aphids sampled from different hosts and dates, we found that the aphids with genotype A and B increased on the cucumber and pumpkin during the peak period of aphids in field. Meanwhile, the aphids on the zucchini owned the genotypes of aphid from cotton and cucumber at different collection dates. The results showed that aphids both from cotton and cucumber could transfer onto zucchini. The zucchini might act as a bridge host for aphids from cotton to cucumber. After the cucumber withered, the genotype C aphids which belong to the cucumber were found on cotton plants. The cucurbit-specialized aphid could temporarily transfer to cotton plant from cucumber when they meet a food shortage. In addition, the ability of cucurbit-specialized aphids to use cotton plants was different among different genotypes, and the capacity of the genotype C (strain 2) was stronger than that of the genotype E (strain 1).