Fungal Pathogens Of Cotton Aphids Spread With Host Dispersal Flight In The Tarim Basin Of Xinjiang, China And Opportunistic Features Of Conidiobolus Obscurus In Infective Biology

Entomophthorales(Zygomycota:Zygomycetes)includes a large number of insect pathogens,which can forcibly discharge conidia into host colonies around mycosed cadavers and often cause host epizootics.Aphids can be infected by over 30 species of fungal pathogens,of which most are entomophthoralean fungi,including obligate or non-obligate species in the genera Conidiobolus,Entomophthora,Pandara,Zoophthora and Neozygites.The Entomophthorales-caused mycoses have been generally concerned in the natural control of aphids,attracting more and more researchers in the field.This study sought to survey fungal pathogens present in migratory alates of the cotton aphids Aphis gossypii and Acyrthosiphon gossypii trapped from air in the Tarim Basin,Xinjiang,and explore possible mechanisms involved in the opportunism of some aphid pathogens.Fungal pathogens borne by migratory alates trapped from air.To collect daily migratory alates of cotton aphids from air,a yellow-plus-plant trap was set up on the roof platform of a six-story teaching building on the campus of Tarim University(N 40°33′,E 81°18′)located at the Alaer City,Xinjiang,northwest China.During two summers from June through August of 2005 and 2006,9438 migratory alates of Ac.gossypii were trapped from air in the urbane area and individually reared in laboratory for 7 days.Of those,6.5% died from mycoses attributed to obligate or non-obligate aphid pathogens.Conidiobolus spp. (mainly C.obscurus and less frequently C.thromboides)took 71.4%among the infected alates,followed by Entomophthora planchoniana of 21.3%,Zoophthora radicans of 5.4% and Pandora neoaphidis of 1.5%,respectively.During the same periods,4336 migratory alates of Ap.gossypii were trapped from air and also individually reared for 7 days in laboratory.As a result,2.4%of the alates were killed by Conidiobolus spp.(taking 85.8% among the mycosed alates and E.planchoniana(13.2%).Additionally,four alates of Ac. gossypii or Ap.gossypii were found infected by Beauveria bassiana,a well-known insect pathogen in Hyphomycetes.More than 90%of the infected alates(91.7%for Ac.gossypii and 92.5%for Ap. gossypii)became mycosed within three days during colonization after trapping and the rest died in the following one or two days.On average,the infected alates survived for 2.05 and 1.78 days for Ac.gossypi and Ap.gossypii,respectively,despite some variation among the involved pathogens.Both survival periods were shorter than those known for the less than 6-day latent periods of entomophthoralean infection towards aphids.This indicates that the mycosed alates were infected before they were trapped from air or where they emigrated. Moreover,the infected alates of both Ac.gossypii and Ap.gossypii were capable of producing a certain number of nymphs before death although their fecundity were significantly reduced compared to those not infected by fungal pathogens.Resting spores (azygospores)were found in some of the alates killed by C.obscurus or Z.radicans.Simulation flight experiments with C.obscurus infected alates.Conidiobolus (Entomophthorales:Ancylistaceae)includes common aphid pathogens but causes sporadic mycosis worldwide.This epizootiological opportunism was explored herein by examining the potential of mycosis transmission in the progeny colonies of 513 Myzus persicae alates as disseminators of C.obscurus often infecting aphids.The alates exposed to spore showers were flown for 2.05(0.01-8.95)km on flight mills and then reared individually on cabbage leaves at 20-23℃for 14 day colonization.All truly infected alates were mycosed within 6 days and averagely left 3.9(0-15)nymphs while those uninfected produced 11.6(0-35) nymphs during the same period.Secondary and tertiary infections occurred only in 16.2% and 4.8%of the progeny colonies of the mycosed alates,respectively,due to ca.60%of the cadavers forming resting spores.Most of the contagious infections appeared on days 4-8 after colonization and no more occurred from day 11 onwards.Trends of colony sizes (last-day averaging 51.5 aphids)and mycosis transmission(sixth-day maximum 6.3%)were fitted well to logistic(R~2=0.99)and Gompertz growth models(R~2=0.91),respectively.The results confirm that the opportunism of C.obscurus stems from depression of contagious infection after dissemination by host alates and suggest that it be overwhelmed by the prevalence of other non-resting fungal species if they occurred simultaneously.