Studies On The Susceptibilities Of Two Species Of Poplar Trees To Wood Boring Insect Pests And Associated Impact Factors

In order to explore the reasons induced insect population dynamics,the native poplar species Populus davidiana Dode and exotic Populus nigra var.italica(Moench)were selected as host plants.Infestation status and differences between the trees were investigated,as well as the biological and ecological characteristics of main wood borers,and their major natural control factors.The results were as follows:1.The susceptibility of non-native poplar to wood boring pests was much higher than native tree species.Species structure of wood borers and their infestation rates were compared and calculated by dissecting sample logs in the two stands.Results indicated that 4 kinds of wood borers were found on P.nigra var.italica trees,including Agrilus fleischeri Obenberger,Xylotrechus rusticus L.,Mesosa myops(Dalman),and Saperda alberti Plavilstshikov,with natural infestation rates of A.fleischeri and X.rusticus were 11.76 %,5.88 %.However,the infestation rates on artificially stressed trees were 90.00 %,65.00 %,5.00 % and 5.00 %,respectively.Agrilus fleischeri,M.myops were discovered on P.davidiana with a damage percentage of 65.00 % and 5.00 % on artificially stressed trees,while they did little harm on healthy trees.Results showed that damage caused by A.fleischeri were more seriously on exotic trees than native species no matter under the natural or artificially stressed states.2.Infestations of A.fleischeri were more seriously on exotic trees P.nigra var.italica than native species P.davidiana.Survey results in the first part showed that A.fleischeri was the main wood borers on the two species of host trees.Thus population density and niche widths of A.fleischeri were investigated on artificially stressed trees.Results revealed that the average population densities of this beetle were 61.95 larvae per tree on P.nigra var.italica and 34.89 larvae per tree on P.davidiana.Furthermore,the niche widths of A.fleischeri larvae were 0.53 and 0.32 on P.nigra var.italia and P.davidiana respectively.These findings suggest that exotic poplar species P.nigra var.italica is more vulnerable than native poplar P.davidiana.3.Determination of biological parameters and larval instars of A.fleischeri.Significant differences were found in the life history of A.fleischeri on the two species of poplar trees.We observed that A.fleischeri were univoltine on exotic P.nigra var.italica and semivoltine on native P.davidiana through regular field surveys.Adult could be collected from late May to middle August.Eggs were first observed in the field in early June,and peaked in July,declining in late August.The early-instar larvae were first seen in the field in middle June,and continued to feed on the cambium and xylem until the middle of September.Pupation began in mid-August in the cell where they overwintered as mature larvae,and continued to development in early May of the following year.Generation overlap occurred on P.davidiana and larvae overwintered as early instars,fed during the second summer,and then emerged next(the third)spring.Results showed that the average longevity of adults was 34.61 ± 1.33 days after emergence,with males 34.33 ± 1.91 days and females 34.87 ± 1.80 days at average temperature of 23.4°C and RH 69 %.The sex ratio of males to females was 0.913:1,which significantly conformed to 1:1 based on chi-square test.The eggs needed 12.83 days to hatch at average temperature of 23.4°C and RH 67 %.It took around 26.4 days for pupal development to adults at average temperature of 17.5°C and RH 49 %.Larvae were collected by sampling every three days in the forests.Based on the Dyar's law,larvae could be separated into 4 instars according to the frequency distribution of width of the peristoma.4.The species of natural enemies of Agrilus fleischeri have been investigated in the two poplar stands.Species and parasitism rates of natural enemies were different between the two forests.Oobius sp.,Paramblynotus sp.,Polystenus rugosus Foerster were discovered on P.nigra var.italica trees and their parasitism rates were 48.00 %,43.79 %,1.97 %.In addition,one clerid beetle was found in the pupal chamber of A.fleischeri.Oobius sp.,E.fleischerii,P.rugosus,Spathius sp.were found on P.davidiana trees and their parasitism rates were 6.48 %,7.63 %,4.83 %,0.25 % respectively.Oobius sp.and Paramblynotus sp.were the main parasitiods in the P.nigra var.italica stand.Oobius sp.and E.fleischerii were the dominant parasitiods in the P.davidiana stand.5.Natural enemies of other wood borers and their roles in natural control were surveyed.Results indicated that one species of parasitoid and a predator were found attacking larvae of longhorned beetle X.rusticus on Populus nigra var.italica,with the parasitism rates 3.73 %and 3.73 %.This study compared the species of wood borers and their infestation rates on host trees,and provided suggestions on rational afforestation.The biology of key wood borer was studied in detail for its better management.Species of natural enemies and their contributions to the suppression of host pests were analysed for their further biological control.