Response of the alligator weed flea beetle,Agasicles hygrophila( Coleoptera:Chrysomelidae) to overwintering protection and its controlling effect on alligator weed Alternanthera philoxeroides( Amaranthacese: Alternanthera)

The alligator weed flea beetle,Agasicles hygrophila( Coleoptera: Chrysomelidae) is a biological control agent of the alligator weed Alternanthera philoxeroides( Amaranthacese: Alternanthera),a worldwide invading weed. Low temperature and frost stress in winter-spring seasons are the key factors that affect A. hygrophila. To explore whether the protections used in habitat could increase the population of overwintered A. hygrophila,we investigated and compared the populationnumber,structure,distributing characteristic in spatial niches of A. hygrophila and the controlling efficiency of A.hygrophila on A. philoxeroides in protected habitat( PH) with those in natural habitat( NH). The investigation was phased with the variation of air temperature and the elapse of time. Low temperature stage was from 21th,January to 9th,April,2009,followed by temperature rise period from 15th,April to 1st,May,and the third investigated period which was the first emergence time of beetle adults in NH from 1stto 29th,June. The protected habitat was constructed by covering plastic film on net room before low temperature and freezing came. The results showed that A. hygrophila could overwinter smoothly and no dormancy was observed in protected habitat at low temperature in winter-spring time. Adults gadded and fed on up-right stems of A. philoxeroides both in sunny and air temperature rising day in winter-spring time,but were inactive and stayed creep layer of weed or surface or crevice of soil at overcast,rainy or low temperature time. Beetle adults in protected habitat laid eggs in early April. There was a rapid increasing of the populationwhich played an important role in controlling the above ground parts of A. philoxeroides. Only a few beetle adults can be found in natural habitat even at the beginning of June. Consequently,the weed grew very well due to the low density of the beetle population which was difficult to play the role of a biocontrol factor. Comparing the results obtained in the first day with that in 15th,June,the increasing rate of stem height of A. philoxeroides in this interval of time were-13.91% in the protected habitat and-2.94% in the natural habitat,respectively. The stem height of A. philoxeroides in the natural habitat was significantly higher than that in the protected habitat. The control rate of a plant biomass of A. philoxeroides by A. hygrophila in the protected habitat was 47.56% which was significantly stronger comparing with the control rate of 4.89% in the natural habitat. The control effect to leaves of A.philoxeroides in the protected habitat was extremely better as compared with that in the natural habitat in the interval of time from 15thto 29th,June. There were no leaves left on stems of A. philoxeroides plants as leaves being either eaten up by A.hygrophila or withered after attacked by A. hygrophila in the protected habitat. The rate of damage stem,rate of stem bore hole and the number of stem bore hole per plant in the protected habitat were significantly higher than that in the natural habitat. These results suggested that the overwintered A. hygrophila population could be significantly increased by covering plastic film to protect the habitat. The beetle adults in protected habitat could lay eggs ahead of time compared to that in the natural one,thus could effectively control the growing of weed.