Larval Olfactory Preference And Adult Mating Habits Of Spodoptera Frugiperda

Spodoptera frugiperda(Smith),the so-called tall armyworm,is native to America and specially prefers to damage Poaceous crops such as the corn.Since it invaded in Yunnan Province in China in Janury 2019,now extended to several provinces.This invasive species,along with a native species,Mythimna separata(Walker),have many similar habits.They both have the characteristics of long migration,strong fecundity,polyphagous on poaceous leaves,and rangely outbreak.The author studied the difference between the larval olfactory preference of these two lepidopteron species to corn volatiles using a tracksphere,and then examined the major morphological traits(body weight,body length,forewing length,and abdominal width)possibly involved in the pre-copulatory mate choice of the fall armyworm under the environments of two operational sex ratios(2♀1♂ and 1♀2♂);finally,I studied the potential synergistic effect of different corn volatiles on the sex lure of S.frugiperda.The main conclusions are summarized as follows:(1)Comparison of the olfactory responses of S.frugiperda and M.separata larvae to corn volatiles on a tracksphereI tested the behavioral responses of the 3rd larvae of these two moth species to 11 corn volatiles emitted by the leaves of different corn varieties using a tracksphere,simplified the seven original walking parameters recored by the principal component analysis(PCA),and compared the attractiveness of these corn volatiles as well as the inter-specific difference of the walking response to different corn volatiles.The PCA results show that extraction of two principal components could explain a most part of total variance in both species.Among the components,the biological significances of the first and the second components are "activity factor" and "attractivity factor",respectively.I further compared the differences of the scores of the activity factor,the attractivity factor,and the integrative evaluation of the two lepidopteron species to different corn volatiles,and made three conclusions as follows:firstly,the larvae of S.frugiperda were more active than the larvae of M.separata when clean air without corn volatiles was supplied(t17.56=3.41,P=0.0032);secondly,once corn-related volatiles were introduced in the air stream,the walking behavior of M.separata larvae often could be significantly changed,while that of S.frugiperda larvae could only be changed by a limited range of molecules,suggesting that the olfactory receptor range of S.frugiperda larvae is much narrower than that of M.separata larvae.Finally,the biological significance of different corn volatiles were different.As for S.frugiperda larvae,(E)-β-caryophyllene and carvacrol mainly exhibited strong active walking responses,while β-pinene and nonylacetate mainly stimulated the directional walking response of the larvae.When the two components were integrated,the strongest compounds was carvacrol.(2)Mating-choice of S.frugiperda in close rangeWe studied the morphological traits(body weight,body length,forewing length,and abdominal width)possibly involved in the pre-copulatory mate choice of the fall armyworm under the environments of two operational sex ratios(2♀1♂,1♀2♂).The results indicate that the body lengths of female mates being selected were significantly shorter than non-selected females(t14=3.57,P=0.0031).Abdominal width was the only signigicant trait for assortative mating when the morphological traits of each couple were analyzed(r2♀1♂=0.57,P=0.0261;r1♀2♂=0.69,P=0.0001).Therefore,the males preferred to choose females with shorter body length to mate was caused by the difference of allometric growth between sexes,that is,a couple with a longer male and a shorter female would result in the matched abdominal width.(3)Synergistic effect of corn volatiles on sex lure of S.frugiperdaI determined the potential synergistic effect of different corn volatiles on a sex lure of S.frugiperda in a dual-choice trap.The result shows that captures of phenylacetaldehyde,linalool,and β-pinene(each 15 μL combined with a sex lure)were significantly higher than those caught by a sex lure used alone(χ2 phenylacetaldehyde=6.72,P=0.0095;χ2linalool=10.14,P=0.0015;χ3-pinene=6.72,P=0.0095).All together,the olfactory receptor range of M.separata larvae was much wider than that of S.frugiperda larvae.(E)-β-caryophyllene and carvacrol could activate S.frugiperda larvae,and β-pinene and nonyl acetate could attract the larvae to move in a fixed direction.The males of S.frugiperda preferred to choose females with shorter body length to mate in a close range,which could result in assortive mating with matched abdominal width.Phenylacetaldehyde,linalool,and β-pinene could enhance the attractive activity of the sex lure of S.frugiperda.