| A non-host plant species, Ricinus communis, exhibited physiological toxicity effect on both the adults and the larvae of various scarab species, but the adults often show significant approaching response to this plant, and this phenomenon can not be explained by current evolution-related theories. Therefore, the author conducted a serial of laboratory behavioural bioassays, chemical analysis, and field cage bioassay, intended to elucidate the biological mechanism of this phenomenon. The main conclusions are shown as follows:(1) The living plant of R. communis showed significant attractiveness to both the female andmale scarab beetles in the laboratory bioassay, the Ulmus pumila branch with leaves had theequivelent attractiveness as that of R. communis plant, and both of them were stronger than thecommon host of scarab beetles, Arachis hypogaea. Volatiles emitted from R. communis plantplayed a primary role in its attraction, and the clipped leaves were more attractive than that of theplant.(2)We applied solvent cold-extraction and Soxlet extraction to collect theactive compoundscontained in R. communis leaves, and found that all the solvent extracts at room temperaturewere less attractive than that of the plant, however, the attractiveness of the Soxlet products hadthe equal attractiveness as that of the living plant.(3) The haed-space of R. communis leaves contained four major components: 2-furanc arboxaldehyde, 2, 5-furandicarboxaldehyde, benzoic acid, and 5-(hydroxymethyl)-2-furancarb oxaldehyde, and six components were shared in our two GC-MS analysis of the Soxlet ex tracts, which were 2-methoxypropyl acetate,(Z3)-hexenol acetate, 3, 7, 11, 15-tetramethyl-(Z2)- hexadecenol, 3, 7, 11, 15-tetramethyl-(Z2)-hexadecenol, methyl hexadecanoate, and ric inine, respectively.(4) We prepared a serial of ternary blends from the major volatiles in the head space of R. communis leaves, and conducted a laboratory bioassay on the selective response of scarab beetles, and found that only one blend, benzeneacetaldehyde + 2, 5-furandicarboxaldehyde + 5-(hydroxymethyl)-2-furancarboxaldehyde, showed significant attractiveness to the males, and the blends contained both 2, 5-furandicarboxaldehyde and 2-furancarboxaldehyde showed fairly strong repellent effect on both the females and the males.(5) When R. communis plant was combined with a sex, and tested the choice response of theother sex, and the result shows that all the treatments exhibited strong attractiveness as long as R.communis plant was present, but the scarab beetles of the opposite sex did not show significanteffect on the choice response of the tested subjects.(6)We planted A. hypogaea adjacent to R. communis in the field, and investigated the numbers and total body weight of the grubs in the peanut plots with different distances from R. communis, the results show that both the number and the total body weight in the plots with 1m distance were signigicantly lower than that in the 6 m distance.(7)The field cage bioassay showed that the mixed sexes of scarab beetles were significantlyattracted by R. communis plant, but when sexes were released seperately, only females wereattracted.(8)Finally, we measured the body weight, fore-wing length, and fore-wing width of the females showing different approaching response to R. communis plant, and found that the individuals with heavier body weight and smaller forewing exhibted stronger response to R. communis plant.All together, we speculated that the females may be the pioneer to R. communis plant in the field, and the non-host plant may provide a mating site for the sexes, and may be helpful to select out high productive female adults naturally. Therefore, the approaching response of scarab beetles to the non-host plant R. communis may present a trade-off between toxic risk and population reproductivity. |
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