Effects Of Castor Bean On Taxis,Mating,and Detoxification Of Key Scarab Species In North China

The larvae of scarabs(so-called grubs)are a group of below-ground pests damaging most seriously in crop fields in North China,especially three species as Holotrichia parallela,Anomala corpulenta,and Holotrichia oblita.Castor bean,Ricinus communis,is a dead-end attractive non-host plant species of these scarab beetle species.In this disertation,the author compared the attractiveness of castor bean plant when paired with several common host/oviposition host plants.After that,I recorded flight parameters and crawling parameters of these scarabs in different plant environments including castor bean using flight mill and tracksphere technologies,respectively.I also analyzed the effects of castor bean on sexual communication and copulation behaviors of these scarabs.Finally,I tested feeding response of these scarabs to castor bean as well as the activities of six in vivo enzymes after intaking castor bean.The main results are summarized as follows:1.Approaching response of scarabs to castor bean plant paired with other plant species.The results show that the attractiveness of castor bean plant to H.parallela was significantly stronger than all the test plant species but Siberian elm shoots with leaves.Inter-specific comparison indicates that the approaching response of H.parallela was strongest,followed by A.corpulenta.In contrast,H.oblita could be olfactorily repelled by castor bean odors because that.2.Field trapping.The sex pheromone of H.parallela could trap both conspecific males and the males of another species,A.corpulenta.The optimal dose of H.parallela was 500 ?L sex pheromone embedded in agaropectin.However,even the sexual lure was applied at this optimal dose,the effectiveness could not reach up to the trap catches of castor bean leaf extracts applied at the same dose.Additionally,the mixture of these two different types of semiochemicals exhibited no significant synergistic effect.3.The effects of castor bean plant on sexual communication and copulation behaviors of H.parallela and A.corpulenta.Both scarab species achieved intra-specific communication via sex pheromone releasing by females and attacted males.The result of the approaching response of H.parallela males indicated that castor bean plant showed fairly strong synergistic effect on three blend ratios of H.parallela sex pheromone(R-(-)-linalool and L-isoleucine methyl ester mixed at 1 : 4?1 : 5 ? 1 : 7),suggesting that the synergistic effect of castor bean on sexual communication of H.parallela only achieved at some special ecological context.In contrast,castor bean could not enhance the approaching response of A.corpulenta males to the sex pheromone of H.parallela.I further investigated the effect of different complexes of castor bean plants and females(placed together,air flew from castor bean plant to female,and air flew from female to castor bean plant)on the approaching response of conspecific males using serial-chamber bioassay.Th results show that the attractiveness of a castor bean plant to H.parallela males was significantly stronger than that of a sexually mature female.However,its attractiveness to A.corpulenta males did not differ significantly to a sexually mature female.Regardless of the combination approaches of a H.parallela female with a castor bean plant,the complex always showed stronger attractiveness than a male,and always did not show significant difference with a castor bean plant.The differences between A.corpulenta males and H.parallela males were that the complex of a A.corpulenta female and a castor bean plant was more attractive when placed together in a chamber,and the attractiveness of the mixed air flow from a female to a castor bean plant to males did not show significant difference with that from a female.Therefore,although the sexual communication of both scarab species could be enhanced by castor bean plants,but the mechanism was different.The responsiveness of H.parallela males to female sex pheromone could be improved when the odor of castor bean was present,while the sex pheromone production of A.corpulenta females could be stimulated when the odor of castor bean was present.Additionally,both in the laboratory bioassay and field cage bioassay,the two scarab species showed the highest copulation rate in castor bean environment compared with the other common host plant species.4.Flight activities of H.parallela and A.corpulenta in different plant environments.In this experiment,I found a remarkable differentiation between the sexual choice or mating advantages between H.parallela and A.corpulenta.Heavier individuals and stonger fliers of A.corpulenta have more chance in mating,while the advantageous characteristics of H.parallela for mating were that the female parterners were heavier,or the male parterners were stronger fliers,or both.As for flight parameters in the flight mill,the flying duration of test scarabs in castor bean environment was longest,and the mean values of another three flight parameters(flight distance,maximal duration of a single flight,maximal distance of a single flight)were also highest in castor bean environment,being equal to those in Siberian elm(branches with leaves)environment.5.Crawling track parameters of H.parallela and A.corpulenta in different plant environments.I investigated the crawling behaviors of H.parallela and A.corpulenta beetles in different environments using a recording system of insect crawling tracks,and specially analyzed two parameters being the most relevant to reflect the odor preference of test subject insect to an odor source: oritentation index and oritentation rate.The oritentation indices of the two scarab species were positive unless test insects were placed in the environments with the presence of Siberian elm branches with leaves or a castor bean plant,i.e.,test insects moving the most distant along the way toward to the direction of these two odor sources at the end of the test period.Additionally,the oritentation precision of H.parallela beetles was higher than that of A.corpulenta beetles,i.e.,more tracking points deposited from-30 degrees to 30 degrees along the direction from moving start to odor source.6.Feeding response of H.parallela and A.corpulenta to castor bean crudes coated on the leaf discs of Siberian elm and the enzyme activities after intake of castor bean toxins.Every coating dose of castor bean crude on Siberian elm leaf disc did not show any feeding deterent effect on test scarabs.On the contrary,most doses showed feeding stimulating effect.However,most individuals exhibited temporary paralysis after intake of castor bean leaf crudes,and recovered to normal state after some time.The enzymatic activities of three protective enzymes(superoxide dismutase,peroxidase,and catalase)and three detoxication enzymes(glutathione S-transferase,caboxylesterase,and acetylcholinesterase)of scarabs were higher after intake of castor bean crudes compared with the individuals intaking only Siberian elm leaf disc.The response curve of caboxylesterase to castor bean toxins was parabolic shape.The activities of all the three protective enzymes increased linerally at low doses of castor bean toxins,and achieved to a saturated level at high doses.The activities of acetylcholinesterase and glutathione S-transferase increased gradually at the range of test doses of castor bean toxins.I speculate that in the concentration range of toxins contained in natural castor bean leaves,caboxylesterase in the midgut of scarabs can be induced after they fed on castor bean leaves,but this enzyme activity will be inactivated after more toxins were intaken.Both acetylcholinesterase and glutathione S-transferase have fairly broad response range to castor bean toxin concentrations,so they can be sustainably induced.Therefore,the recovering mechanism of paralyzed scarabs after intake of castor bean toxins may be related to the recovering of acetylcholinesterase activity.In summary,it was found that the crude of castor bean leaves could stimulate feeding of scarabs rather than feeding deterrence,and potential toxins contained in these leaves could induce paralysis of these beetles.However,these scarabs could not recognize the dis-matching relationships among the attractive odors,the appetitive taste,and the in vivo toxins.I put forward a novel mechanism which can explain this evolutionary dead-end.Some advantageous offset may present in A.corpulenta and H.parallela against to the riskful aggregation of scarabs in castor bean plant environment: odors emitted from castor bean leaves could enhance mate-searching of the males and then improve the copulation rate of leks.Holotrichia oblita may avoid this potential toxication of this non-host plant via olfactory avoidance.