Fig. 4

Heptanal does not participate in the direct interaction between Plutella xylostella and its host plant. Preference index of P. xylostella wild-type (WT) and PxylOR16-knockout (PxylOR16^−/−) adults and larvae to heptanal. A Preference index of WT and PxylOR16^−/− mutant female and male moths for blank (one side blank vs an intact healthy Brassica parachinensis plant), for control (an intact healthy B. parachinensis plant vs an intact healthy B. parachinensis plant), and for an intact healthy B. parachinensis plant + paraffin oil vs an intact healthy B. parachinensis plant + heptanal in a Y-tube olfactometer (n = 16). WT and PxylOR16^−/− mutant female and male moths were significantly attracted to healthy B. parachinensis plant, compared with blank. Three doses (100 ng, 1 μg, and 10 μg) and two doses (1 μg and 10 μg) of heptanal elicited avoidance behavior in WT female and male P. xylostella adults, (Welch’s t-test; NS, no significant difference, P > 0.05; *** P < 0.001). The numbers of PxylOR16^−/− adults on each side of the Y-tube olfactometer were not significantly different at all heptanal doses tested, when comparing B. parachinensis + heptanal with B. parachinensis plant + paraffin oil (Welch’s t-test; NS, no significant difference, P > 0.05). B Preference indexes of 10 WT and mutant female and male third instar larvae for blank (one side blank vs an intact healthy B. parachinensis plant), for control (an intact healthy B. parachinensis plant vs an intact healthy B. parachinensis plant), and for an intact healthy B. parachinensis plant + paraffin oil vs an intact healthy B. parachinensis plant + heptanal in a screening device (n = 10). Both sexes of WT and mutant larvae were attracted to the B. parachinensis plant. WT female and male larvae (third instar larvae) preferred the healthy plant with paraffin oil to a healthy plant with heptanal at three doses (100 ng, 1 μg, and 10 μg), unlike PxylOR16^−/− larvae (Welch’s t-test; NS, no significant difference, P > 0.05; *** P < 0.001). C GC–MS analysis of the volatiles produced by larval-infested plants and healthy plants; heptanal was not detected. Larval-infested B. parachinensis (top), healthy B. parachinensis (middle), and synthetic heptanal (bottom). D Preference index of WT and PxylOR16^−/− mutant adults for control (intact healthy B. parachinensis plant vs intact healthy B. parachinensis plant) and for an intact healthy B. parachinensis plant vs a larval-infected B. parachinensis plant in a Y-tube olfactometer (n = 16). There was no difference in selection between the WT and PxylOR16^−/− mutant; both preferred the larval-infected plant (Welch’s t-test; NS, no significant difference, P > 0.05; *** P < 0.001). E Preference index of WT and PxylOR16^−/− mutant larvae for control (intact healthy B. parachinensis plant vs intact healthy B. parachinensis plant) and for an intact healthy B. parachinensis plant vs a larval-infected B. parachinensis plant in a screening device (n = 10). Like adults, WT and PxylOR16^−/− mutant larvae both preferred the larval-infected plant (Welch’s t-test; NS, no significant difference, P > 0.05; *** P < 0.001)