Biological Evidences For The Primary Role Of Aphid Flight Dispersal In The Dissemination Of The Aphid Parasitoids

Hymenopterous parasitoids are of great significance for natural control of aphid populatons and have received intensive studies on multitrophic interactions. However, no effort has been directed toward a peep at the possible dispersal of parasitoids with host flight although specialists such as Aphidiidae are tightly associated with host aphids. This ignorance retards understanding the potential role of the host dispersal flight in aphid-parasitoid interactions. The study take use of green peach aphid Myzus persicae Sulzer and its parasitoid Aphidius gifuensis Ashmed as research system and sought to test this hypothesis by demonstrating the capabilities of host alates in post-parasitization flight, postflight colonization and fecundity, and the development of parasitoids in alate aphids.1 Simulation flight experiments with host alates parasitized by Aphidius gifuensisOver several batches of≤2-days-old M. persicae alates (≤16 alates per batch) were parasitized individually, and then the alates individually flew for 1～5 h in a computer-monitored flight mill system and then individually reared at 20～23℃and L: D 14: 10 for colonization and the development of parasitoids on green grocery for 14 days. A total of 378 parasitized alates successfully flew for an average of 2.6 (0.01～10.7) km in 3.2 (1～5) h. Among them, 239 alates were mummified and 139 alates not mummified during the post-flight colonization experiment. For those mummified alates, they successfully flew for an average of 2.63 h and 2.16 km, and suvived for an average time of 6.11 days and laid 8.5 nymphs per alate during 6-day postflight period, and reached an average fecundity of 162 nymphs during 14-day postflight period. For those unmummified, they successfully flew for 3.2 h and 2.60 km, and laid 8.5 nymphs per alate during 6-day postflight period, and reached an average fecundity of 156.3 nymphs during 14-day postflight period. The differences in flight capability and fecundity between the mummified alates and unmummified alates are not significant. 205 parasitoid adults successfully emerged from 239 mummified alates, of which 33 were female and the others 172 were male, the sex ratio is 1: 5.2; the emerging ratio is 85.8%. It is thus proposed that dispersal flight not only enables the parasitized aphids to disperse themselves to sutable plants, but this dispersal is also utilized by aphid parasitoids to readily locate hosts for attack.2 Development of Aphidius gifuensis in alate and apterous aphidsThe experiments were carried out in the lab under 20～23℃and L:D 14:10 condition. The results showed that the egg stage of A. gifuensis last 3 days. The larvae appeared to have 4 instars; the first, second, third and fourth instar last 2.5, 1.5, 1.5 and 1.5 days, respectively. The first instar larvae were charactered with a larger head, a pair of reaphook-like mandibles and a tail. The mandibles disappeared and the tail was significantly shorten at the second instar. The third instar were without mandibles, but the tracheal system completely formed. At the fourth instar larvae, the mandibles appeared again, but shorter and stronger then that of the first instar larvae. At the end of fourth instar, the larvae began to bite a hole on the abdomen of green peach aphid and then form a cocoon; the midgut and hindgut began to fuse and the meconium was excluded; the coumpound eyes is clear and salmonn pink. With the development of pupae, the coumpound eyes gradually changed from rosiness, henna to puce, the antennae, legs and wings were transparent and adhered to the body, then separated the body when the pupae matured.3 Conclusions and implicationsSimulation flight experiments with M. persicae- A. gifuensis system have proven that the parasitized alates were capable of flying for dispersing themselves and initiating colonies before they became mummies. The development of A. gifuensis in alates and apterous aphids have proven that the parasitoids could be able to develop successfully in alate aphids. These experiments successfully proved that the hypothesis of aphid dispersal in the dissemination of aphid parasitoids is correct. This may help to explain why no association between mtDNA haplotypes of Diaeretiella rapae and host aphid species was found in its ancestral or introduced ranges and why genetic variation were not detected in the different geographic populations of the same wasp species. The lack of D. rapae host races probably relates to the flight dispersal potential of its hosts. If the dispersal of parasitoids with aphid flight was a general phenomenon, it could have a profound influence on the distribution, coevolution, and interactions with host aphids and also would provide a complete new study method on molecular ecology and aphid-parasitoid coevolution. Therefore, aphid dispersal biology merits attention in the future studies of aphid-parasitoid interactions.