Vector-borne pathogens can alter the phenotypes of their hosts and vectors in ways that influence the frequency and nature of interactions between them, with significant implications for the transmission and spread of diseases [27]. Previous studies have shown that plant viruses can affect the insect vectors, but the degrees of influence of different virus-vector combinations are not identical. There have been many reports on alteration of physiology, molecular biology or feeding behaviors in insect vectors by persistently transmitted plant viruses, for example, Begomovirus on B. tabaci, but few or no studies are available on impact of semipersistent viruses on vectors [3–5, 14]. In our present study, we reported the effects of semi-persistent virus CCYV on the biological characteristics of the vector B. tabaci. Although the nymphs play no roles in virus transmission, their immobile stages (esp. 2nd to 4th instar) encounter the plant viruses when feeding on the plant. Nymphs can be affected, more or less, by virus particles taken with plant sap, and thereby may affect the status of the adults responsible for virus transmission. Therefore, this study comprehensively investigated the biological effects of CCYV on nymphs and adults of B. tabaci, with a view to fully obtaining the biological effects of the virus. The results indicated that all nymph instars can be infected with CCYV, and the virion titer amount varies with instars. The 2nd -instar has the highest virion titer amount (1424.54 copies) among the nymphs, followed by the 3rd -instar and the 1st -instar, and the 4th -instar (112.34 copies) has the lowest virion titer, which may be related to the behavior characteristics of each instar nymph. The 1st -instar nymphs have tentacles and feet and can crawl over a short distance to find a suitable feeding site and then settle down and start feeding. The tentacles and feet of the 2nd and 3rd -instar nymphs were degraded, and they had no crawling ability. They were fixed on the back of the leaves for feeding with the stylets [28]. The 4th -instar nymphs, also known as pseudo pupal stage, basically stopped feeding [29, 30], which may be the reason for the low virion titer of the 4th -instar nymphs. The virion titer of the adults was much greater than those of the nymphs, and the virion titer of the individual adult was up to 21360.08 copies. Adult B. tabaci is highly active and can even migrate over long distances with the assistance of air currents, becoming the main cause of the CCYV pandemic.
By comparing the development period of B. tabaci, it was found that CCYV could significantly extend the development period of female nymphs (P < 0.01) and the longevity of female adults (P < 0.01), but not significantly affect the development period of male nymphs (P = 0.391) and the longevity of male adults (P = 0.136). The influence of CCYV on the growth and development of females are much greater than that of males. This may be because females are larger than males and require more nutrients to reproduce, so females ingest more viruses than males, which in turn has a more significant impact on their growth and development. Longer development period and longevity means more possibility of virus transmission. Therefore, we speculate that females are more conducive to the transmission of CCYV virus than males.
Through a comparative analysis of the body length and oviposition of B. tabaci, we found that CCYV significantly increased the body length of female adults (P < 0.01) and male adults (P < 0.01), and increased the oviposition of individual female adult (P < 0.01). It may be due to an extended developmental period and a higher intake of nutrients. The size of insect is an important factor affecting population development potential and community structure and function [31–33]. Relevant studies have shown that, compared with smaller individuals within the same species, larger insects often have advantages in reproduction, flight, competition, stress resistance and other aspects, contributing to the improvement of population fitness [34].
CCYV significantly increased the proportion of female adult from 50.53% on healthy plants to 66.40% on CCYV-infected cucumber plants. There are two reproductive modes of B tabaci, including parthenogenesis and amphigenesis. The female offsprings of B tabaci are all developed from fertilized eggs, while the male offsprings may come from fertilized eggs and parthenogenesis [35]. The increase of female proportion of whitefly may be due to the increase of body length caused by CCYV, which enables it to have comparative advantages in mating process and obtain more mating opportunities, so as to increase the proportion of female offspring by increasing the number of fertilized eggs, thus ensuring the reproduction of its offspring population.