## 4 Discussion  
Plant viruses affect host plants in different ways to mediate vector attraction to, arrestment on, and dispersal from infected plants [9; 16; 19; 25; 26; 27]. For example, aphids are attracted to plants infected with the non-persistent CMV and quickly left the plant after landing and avoided prolonged feeding. Such behavior presumably would enhance virus transmission particularly effectively with non-persistent viruses [14; 25]. Persistently-transmitted viruses often promote long-term feeding as shown in _Sitobion avenue_, which is engaged in faster phloem-finding behavior and longer feeding time on plants infected with BYDV or PLRV than on non-infected plants [25; 28]. A similar phenomenon has been observed with _R. padi_. Initially, non-infected _R. padi_ significantly preferred wheat plants infected with BYDV compared to mock-inoculated plants, but ultimately viruliferous _R. padi_ significantly preferred mock-inoculated wheat to virus-infected plants [16], which also would promote the spread of this persistently transmitted virus. In our MMWV-_M. micrantha-M. persica_ system, a change of plant host preference was also observed, supporting the \"Host Manipulation Hypothesis\" [14]. Gustatory, olfactory, and visual cues may have contributed to this preference switch and promotion of MMWV transmission. Modified VOCs emissions due to virus infection can potentially serve as important mediators in decision making by insect vectors when selecting a host plant [25; 29]. CMV-infected _C. pepo_ in the field released significantly greater quantities of volatiles than healthy plants [14], presumably being responsible for attracting aphids to CMV-infected plants [14]. The total concentration of headspace VOCs differed between PLRV-infected and mock-inoculated potato plants as well. The concentration of monoterenes, green leaf volatiles, and sesquiterpenes from PLRV-infected potato plants were higher than those of mock-inoculated plants [19]. Mock-inoculated tomato plants emitted significantly higher amounts of the terpen \(\alpha\)-pinene, 4-carene, \(\alpha\)-phellandrene, terpinene, and \(\beta\)-phellandrene than _Tomato severe rugose virus_ (ToSRV)-infected plants, indicating that ToSRV infection suppresses some volatile terpen [25]. Pea plants inoculated with persistently transmitted viruses (e.g., _Bean leaf roll virus_ (BLRV) or _Pea enation mosaic virus_ (PEMV)) had significantly higher concentrations of green leaf volatiles relative to concentrations of monoterenes, compared to either sham-inoculated or control plants [30]. In our study, _M. persica_ apparently could discriminate between healthy, MMWV-infected, or mock-inoculated plants possibly based on characteristics of the VOCs blends. Virus-host plant-insect vector interactions could affect population growth of the insect vectors which would further influence virus transmission [16; 31; 32; 33]. While some studies indicate that virus-infected plants are superior hosts for aphid reproduction [34; 31; 35], others show the opposite outcome [14; 34]. In most cases, non-persistently transmitted viruses have negative effects on vector survival, fecundity or longevity, while persistently and semi-persistently transmitted viruses positively impact on one or more of these parameters [9]. For example, TYLCV mediated alterations to plant nutritional traits and defensive responses improve the growth and reproduction of _B. tabaci_ Mediterranean (MED) [31]. MED feeding on TYLCV-infected plants gained 68% more weight and laid 81% more eggs than those feeding on mock-inoculated tomato plants. MED can both rapidly acquire and effectively transmit the persistently transmitted TYCLV. The beneficial effects of TYLCV infection on MED fitness should in turn favor TYLCV transmission [31]. Compared with non-infected sweet potato (_Ipomoea batatas_) plants, _M.