Effect Of Triterpenoid Saponins In Alternanthera Philoxeroides(Amaranthaceae: Alternanthera)on Excretion Of Agasicles Hygrophila(Coleoptera: Chrysomelidae)

Alternanthera philoxeroides is a highly adaptable and vicious invasive weed which is difficult to control.It contains plant secondary metabolites triterpenoid saponins,causing animals diarrhea.Agasicles hygrophila is a specialized natural enemy insect of A.philoxeroides.However,in our previous study,we observed that A.hygrophila also suffered diarrhea in part after feeding A.philoxeroides.In this research,we explored the number of excretion,100-grain weight of feces,the amount of solid excretion and the yellow area of excretion of A.hygrophila after feeding A.philoxeroides to uncover the effect of A.philoxeroides triterpenoid saponins on the excretion of A.hygrophila and to preliminary clarify the physiological basis underlying the adaptation of A.hygrophila to A.philoxeroides secondary metabolites.The results showed as follows:(1)The annual changes of the triterpenoid saponin content in A.philoxeroides presented that the content increased from January to June,and decreased from July to December.The maximum content was in July with 82.69 mg/g,and the minimum was in January with34.58 mg/g.(2)Taken July as an example,A.hygrophila adults that normally feed on A.philoxeroides have the highest total excretion,followed by the 3rd,2nd and 1st instar larva.There was no difference in total excretion between the 1st instar and 2nd instar.(3)The additional triterpenoid saponins on A.philoxeroides test showed methanol has no effect on the excretion of A.hygrophila,methanol can be used as extraction solvent.Feeding on A.philoxeroides supplemented with triterpene saponin extract,the 1st and 2nd instar larvae excreted greatly increasingly.Feeding on A.philoxeroides supplemented with triterpene saponin extract in July and September in the same volume,the solid excretion of the 1st instar larvae increased 3.53 and 1.75 times respectively,and the liquid excretion increased 5.72 and 2.15 times;the solid excretion of the 2nd instar larvae increased 2.58 and 1.9 times respectively,and the liquid excretion increased 2.96 and1.36 times;the solid excretion of the 3rd instar larvae increased 1.64 and 1.36 times respectively,and the liquid excretion increased 1.36 and 1.96 times;the solid excretion of the adult increased 2.08 and 2.23 times respectively,and the liquid excretion increased 1.06 and 1.09 times.It was found that A.hygrophila overcomes the adversity of triterpene saponins by increasing excretion,and the diarrhea of 1st instar larvae is obvious.It can be seen that A.hygrophila adults have stronger detoxification ability to triterpene saponins.It is verified again that the lower instar larvae of adults have a strong ability to overcome triterpene saponins.(4)Triterpene saponin content change test,compared with July,the content of triterpene saponins in September A.philoxeroides was lower.The liquid excretion of the 3rd instar larvae of A.hygrophila reduced significantly to 99.56 mm2,and the solid excretion of the adult reduced significantly to14.27 mg in summer.It suggests that A.hygrophila adapts to the triterpene saponins in A.philoxeroides by its excretion."Excretion" is an important indicator for A.hygrophila to cope with the toxicity of triterpene saponins.To sum up,triterpenoid saponins in A.philoxeroides are toxic secondary metabolites,the content is related to the season and promote the excretion of A.hygrophila.With the development of A.hygrophila,the excretion gradually elevates.Therefore,"Excretion" can be used as one of the observation indicators to measure the ability of insects to adapt to the host and respond to plant toxic secondary metabolites.