A Preliminary Investigation Of Population Dynamics Of The Dominant Ladybirds And Their Parasitoids In Nanjing

Coccinellidae,a group of predatory insects with rich species diversity and wide distribution,plays an important role in biological pest control.The predation on aphids by Coccinella septempunctata,Harmonia axyridis and Propylaea japonica have drawn considerable attention by researchers,owing to their dominant position in agroecosystems in China.But there were less research on natural enemies of predatory ladybird beetles.This thesis study addresses two questions:1)How do C.septempunctata and H.axyridis populations fluctuate seasonally in collation with their prey aphid pupulations in Nanjing suburbs?2)Which parasitoids of C.septempunctata and H.axyridis occur in Nanjing,and which environmental factors are responsible for the parasitism.?The main results obtained from this study are summarized as follow.1.Distribution and seasonal dynamics of dominant ladybird beetles and aphids in NanjingOne-week interval investigation using random samples and visual during the occurrence period of Coccinellidae in the spring of 2015.Population size of C.septempunctata and H.axyridis and aphids were investigated and the environmental factors which may have an effect were identified.Both C.septempunctata and H.axyridis emerged at the beginning of April in Nanjing,reached the peak in abundance from late April to early May by 14.4(individuals/m2)and 4.6(individuals/m2),respectively,then diminishedin late May for over-summer.Coccinella septempunctata prefered Vicia gigantea owing its richness in prey resources,while H.axyridis did not show favor for plants.Analysis of the relationship in population size between Coccinellidae and aphids suggest that C.septempunctata and H.axyridi populations follow aphid abundances.2.Parasitoid abundances of ladybirds and factors for parasitism in NanjingField survey and laboratory rearing were carried out in Jiangpu University Experimental Farm,Pukou District and Pailou,Central city in the spring of 2014 and 2015.Results showed that four species of parasitoids were obtained from rearing C.septempunctata,including Tetrastichus coccinellae,Homalotylus flaminus,Dinocampus coccinellae and Phalacrotophora sp..Tetrastichus coccinellae and H.flaminus were common parasitoids at larval and pupal stages and caused 86.0%and 13.7%parasitism rates,respectively.Tetrastichus coccinellae significantly increased in parasitism rate during the late period of ladybird occurrence.Parasitism rates of two parasitoids were higher in the habitat with higher plant diversity.Pupal parasitism rate(28.3%)by T.coccinellae was significantly higher than that by H.flaminus.In addition,plants have no influence on parasitism rates of two parasitoids.Sex ratio of T.coccinellae was invariable,around 20%throughout the season,less impacted by seasonal and habitant factors.Clutch size was greatly affected by seasonal factors,significantly decreasing with increasing hosts in abundance in late April,so did body size of female progeny but not of male progeny.The results suggest that T.coccinellae and H.flaminus have the potential in suppressing ladybird population,with the former having stronger impact on C.septempunctata larvae than the later.