| The sweetpotato whitefly,Bemisia tabaci?Gennadius??Hemiptera:Aleyrodidae?is an important agricultural insect pest worldwide.It seriously damages the plant through piercing the leaves and transmitting the plant virus such as Tomato Yellow Leaf Curl Virus?TYLCV?.The B.tabaci is composed of many biotypes?or cryptic species?;B and Q are two invasive and damaging species and Q has been proved to be prominent in many areas in China.Encarsia formosa?Gahan?is a kind of endopasitoid commercially produced and worldwidly used for control of pest whiteflies in the protected cultivation.There are numberous reports on the triple nutrition interactions among plant,insect,parasitoid.However,the parasitis itic characteristics of E.formosa on B.tabaci Q and B and the effect of plant virus vectored by B.tabaci on the interactions among plant,B.tabaci,E.f ormosa still need to be further studied.In this present study,the system of tomato plant,TYLCV,B.tabaci?Q and B?,E.f ormosa was used to investigate the parasitic characteristics of E.formosa commercially produced on diffrent nymphal instars of B.tabaci Q and B respectively firstly.Then,the parasitism and olfactory behavior selection of E.f ormosa between 3rd-instar nymphs of B.tabaci Q/B,and the effect of TYLCV infection on the parasitism and behavior selection of E.f ormosa were carried out.At last,from the perspective of tomato plant volatiles,the meachanism of parasitism and olfactory behavior selection differences of E.f ormosa between 3rd-instar nymphs of B.tabaci Q/B infested in healthy or virus infected tomato plants was preliminarily explored.The findings will not only provide important and scientif ic guidance for the control of B.tabaci whiteflies through the release of E.f ormosa in the field,but also further enrich and deepen our understangings about the complex interactions among plants-plant virus-insect vectors–wasps.1.The parasitical properties of E.formosa on four different nymphal instars of B.tabaci Q and BThe different parasitical properties of E.formosa parasitized four different nymphal instars of B.tabaci Q/B were investigaed using clip-cage method in the laboratory.Encarsia f ormosa can parasitize and successfully develope on all nymphal instars of B.tabaci Q and B.But E.formosa preferably parasitize the 3rd instar of B.tabaci B and the 2nd,3rd,and 4th instars of B.tabaci Q.The host-feeding rate of parasitoid was higher on nymphal instars of B.tabaci Q than on the corresponding nymphal instars of B and was significantly higher on the 2nd and 3rd instars.For both whitefl ies,the parasitoid's immature developmental period was the longest on the 1st instar,intermediate on the 2nd and 3rd instars,and the shortest on the 4th instar.The parasitoid emergence rate was significantly lower on the1st instar than on the other three instars and offspring longevity was shortest on the 1st instar,but host species had no signif icantly effect on emergence rate and longevity of E.formosa.The results indicate that E.formosa commercially produced can parasitize and feed in all instars of B.tabaci Q and B,and parasitism rate and host-feeding rate of E.formosa have differences among different host instars,but still making this parasitoid a promising tool for the management of the two B.tabaci present in China.2.The parasitism preference of E.formosa between the B.tabaci Q and B on TYLCV-free andTYLCV-infected tomato plantsThe parasitism preference of E.formosa between the third nymphy instars of B.tabaci Q and B,and the effect of TYLCV infection on the parasitism selection of E.f ormosa were studied through the vitro and semi-natural experiments.Parasitism of 3rd-instar nymphs of B.tabaci by E.formosa did not differ between Q and B on TYLCV-free tomato plants.On TYLCV-infected tomato plants,however,parasitism was signif icantly greater for 3rd-instar nymphs of B.tabaci Q than B.This result indicates that host plant virus'infection has been found to have effect on parasitism selection of parasitoid when3rd-instar nymphs of B.tabaci Q and B are coexisted.3.The olfactory behavior selection of E.formosa between the B.tabaci Q and B on TYLCV-freeand TYLCV-infected tomato plantsThe olfactory behavior selection of E.f ormosa between 3rd-instar nymphs of B.tabaci Q/B,and the effect of TYLCV infection on the parasitoids'behavior selection were investigated by Y-tube olfactometer assays.Encarsia formosa females didn't show signif icant preference between healthy plants infested with 3rd-instar nymphs of B.tabaci Q and B.When tomato plants were infected by TYLCV,however,E.formosa signif icantly preferred tomato plants infested with 3rd-instar nymphs of Q rather than B.This further validated that different host preference of E.f ormosa on coexisted nymphs of B.tabaci Q and B and TYLCV infection had effect the host selection of E.f ormosa.4.The mechanism of different host preference of E.formosa between the B.tabaci Q and B onTYLCV-free and TYLCV-infected tomato plantsThe plant volatiles of the tomato having been infested with B.tabaci or TYLCV were collected through headspace collecting method.Plant volatile compounds attracted E.formosa were found.The results found that total 32 compounds were collected from tomato plants infested by 3rd-instar nymphs of B.tabaci Q/B and plants simultaneously infected by 3rd-instar nymphs of whiteflies and TYLCV.Differences between plant treatments were quantitative rather than qualitative.Emissions of the monoterpenes?-Myrcene and?-Ocimene,and of the sesquiterpenes?-Caryophyllene,?-Elemene and?-Humulene were greater from TYLCV-infected plants infested with 3rd-instar nymphs of B.tabaci Q than with 3rd-instar nymphs of B.tabaci B.However,the amounts of five compounds emitted from TYLCV-free plants did not significantly differ when the plants were infested with 3rd-instar nymphs of B.tabaci Q vs.B.Results from olfactory test indicated that the mixture of monoterpenes,mixture of sesquiterpenes and mixture of all synthetic volatiles were all strongly attractive for parasitoids.Synthetic?-Myrcene,?-Ocimene,?-Caryophyllene,and?-Humulene signif icantly attracted E.formosa.Results indicated that monoterpenes?-Myrcene and?-Ocimene,and sesquiterpenes?-Caryophyllene,?-Humulene and?-Phellandrene may be the key volatiles which contributed for the different parasitism and behavior choice of E.f ormosa between B.tabaci Q and B on TYLCV-infected tomato plants. |
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