Indirect Chemical Defense Of Brassica Parachinensis Bariley Induced By Damage From Plutella Xylostella Linnaeus

Plants avoid foragers and indirectly chemically defend against herbivores by synthesizing and releasing distinct volatile compounds, which can help parasitic wasps locate their host insects. In recent years, research in this area has attracted considerable attention. This study was based on a tritrophic system involving Brassica parachinensis Bariley, Plutella xylostella Linnaeus (DBM) and Cotesia plutellae Kurdjumov. The behavioral responses of C. plutellae to different varieties of intact B. parachinensis, different varieties of B. parachinensis damaged by third instar DBM larvae and B. parachinensis sijiu plants damaged by different instars of DBM larvae were measured using a Y-shaped olfactometer, and gas chromatography mass spectrometry (GC-MS) was then employed to detect volatile compounds produced in the different treatments. Finally, the behavioral and electroantennogram (EAG) responses of C. plutellae to selected compounds were tested to screen for the active substances. The main results were as follows:The role of volatile cues from B. parachinensis plants subjected to different treatments on the host-selection behavior of C. plutellae was investigated in a Y-shaped olfactometer. The results indicated that the volatile compounds produced by infested B. parachinensis plants were more attractive to C. plutellae compared with those from healthy plants. Among the infested plants, there were large differences in the attractiveness of the volatile compounds released from different varieties of B. parachinensis. The attractiveness of the volatile compounds from infested B. parachinensis plants of the youqin and sijiu varieties was notably stronger compared to those from the shipai variety, whereas no significant difference was found between the youqin and sijiu varieties. In addition, more parasitoids oriented toward the volatile compounds obtained from B. parachinensis sijiu plants infected by third instar DBM larvae compared to those infected by fourth instar DBM larvae, while no differences were found between the other instars.The GC-MS analysis showed that volatile compounds released from the B. parachinensis plants included hydrocarbon alkyls, aldehydes, esters, alcohols, alkenes, ketone, aromatics and other compounds. The volatile constituents and their relative contents varied among the B. parachinensis plants subjected to different treatments. In general, the levels of volatile compounds produced by B. parachinensis plants infested with DBM larvae were notably higher than for healthy plants. This finding indicated that DBM larvae caused a striking induction of volatile compound production B. parachinensis plants. Moreover, the cultivar of B. parachinensis and developmental stage of P. xylostella involved played an important role in the DBM-induced release of volatile compounds from B. parachinensis plants.Analysis of the EAG response of C. plutellae to 13 types of individual standard compounds and mixtures of these compounds showed that C. plutellae displayed a variety of responses to the different substances at different concentrations. The EAG responses were greatest for the treatments involving 10 μl/ml of trans-2-hexenal and benzaldehyde and,100 μl/ml of trans-2-hexenal, benzaldehyde, nonanal and cis-3-hexenol; the relative EAG value obtained for all of these treatments was greater than 100%. Mixtures of these compound also stimulated strong EAG responses in C. plutellae, and all of the resultant relative EAG values were greater than 140% in these treatments.. The EAG responses were enhanced under increasing concentrations of trans-2-hexenal, β-ionone, benzyl alcohol, nonanal, (R)-(+)-limonene, nerolidol, benzyl cyanide, cis-3-hexenol, benzaldehyde and geranyl acetate. The EAG responses to farnesene, eucalyptol and β-caryophyllene increased initially and then decreased under increasing concentrations.The attractiveness of 13 individual synthetic HIPVs and 5 blends of these HIPVs to C. plutellae were tested in a Y-shaped olfactometer. The results showed that different concentrations of the same substances had very different attractive or repellent effects on C. plutellae. Furthermore, at a given concentration of different substances, the attractive or repellent effects on C. plutellae varied widely. Among the individual volatile compounds, the wasps were moderately attracted by trans-2-hexenal and benzaldehyde at a concentration of 1 μl/ml and significantly attracted by β-caryophyllene, cis-3-hexenol and trans-2-hexenal at 10 μl/ml and nonanal, benzyl alcohol, cis-3-hexenol and benzyl cyanide at 100 μl/ml. In contrast, trans-2-hexenal acted as a repellent to C. plutellae. Among the tested blends of compounds, the third and the fourth blends attracted C. plutellae significantly. The results suggest that the DBM-induced volatile compounds produced by B. parachinensis plant may provide chemical information for C. plutellae when searching for a host.