Biological Dissemination Of Aphid Pathogens (Entomophthorales) And Parasitoids With Dispersal Flight Of Green Peach Aphid Myzus Persicae

Dispersal flight is well known a strategy for aphids to locate suitable plants but has not been recognized to take part in dissemination of fungal pathogens and parasitoids that are of significance for natural control of aphid populations. Aphid-pathogenic fungi, mostly in the Entomophthorales, can cause aphid epizootics worldwide but initiation of their seasonal prevalence in host populations has not been well understood. Due to the presence of resting spores in most of the obligate aphid pathogens, primary infection is considered rising from resting spores or unknown form of inoculum in soil. However, this plausible postulation never suits to Pandora neoaphidis, an obligate pathogen that is most prevalent in global aphid populations but has no form of resting spores discovered. Hymenopterous parasitoids also are of great importance in natural control of aphids 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 present study sought to identify fungal pathogens and parasitoids borne by air-trapped alates, to elucidate whether, why and how both types of aphid mortality agents can be spread with their host dispersal flight based on the observed and fitted performances of captured alates during the early period of postfiight colonization. One of fungal isolates from the infected alates was bioassayed for its virulence to the aphid species.From October 2002 to early June 2005, 3183 migratory alates of green peach aphid Myzus persicae from air were captured daily using a yellow-plus-plant trap set up on the top platform of a six-story building in an urbanized area and individually reared in laboratory for 7-12 days. Of those, 29.0% died from mycoses attributed to obligate or non-obligate aphid pathogens. Among the alates killed by the Entomophthorales, P. neoaphidis took a proportion of 81.9%, Conidiobolus obscurus 8.4%, Zoophthora spp. (including mainly Zoophthora anhuiensis and occasionally Z. radicans) 3.8%, Entomophthora planchoniana 2.9% and Neozygities fresenii 0.67%, respectively. A very few alates suffered from cross infection of P. neoaphidis with E. planchonian, C. obscurus or Z anhuiensis.Among the infected alates, 83.2% died from mycosis within three days and the rest died in the following days. On average, all infected alates survived for 2.49 days despite more or less variation among the involved fungal pathogens. This survival period was shorter than those known for the latent period of entomophthoralean infection, indicating that the alates got infected before they were trapped from air.Contagious transmissions of the mycoses were observed in the progeny colonies whose mother alates were mycosed at least 2 days earlier. On day 12, 13.2% of their progeny colonies suffered from secondary infection and 4.4% from tertiary infection, which was equivalent to 33.3% of the secondarily infected colonies. In the secondary infection, P. neoaphidis took a proportion of 82.4%, Z anhuiensis 7.8%, C. obscurus 5.9%, and E. planchoniana 3.9%. This indicates successful transmission of the fungal pathogens from theinfected alates to their progeny.Another 106 alates trapped from spring 2003 onwards survived averagely for 3.75±1.47 days, followed by mummification due to parasitism of Aphidius gifuensis (52.9%) and Diaeretella rapae (47.1%). This took a proportion of 4.4% from the air-trapped alates. The parasitized alates survived significantly longer than those infected by fungal pathogens (2.49±1.18days).The increasing trends of progeny colonies individually initiated from three groups of alates (infected, n=473;parasitized, n=86;healthy, n=381) over days after colonization fit very well to a logistic equation (r2>0.985), yielding the estimated K values of 25.7, 38.6 and 92.8 (maximal mean colony sizes to be achieved potentially during a period of 12 days) for the three groups, respectively. Moreover, the cumulative fecundity probability, P(m0.99). The fitted m and P(m