Investigation of populations of parasitic wasps parasitizingCarposina sasakiiMatsumura (Lepidoptera: Carposinidae) in jujube orchards in China, with respect to the wasp-host relationship

The peach fruit borer Carposina sasakii Matsumura(Lepidoptera: Carposinidae)(= Carposina niponensis(Walsingham)) is a harmful and widespread pest insect that attacks many types of fruit,including apple,peach and jujube,in northern China.Among these hosts,jujube has been most severely affected,with about 90% of Chinese jujube orchards being damaged.The larvae drill into the fruit and eat the pulp and kernel,producing bore dust and frass that make the fruits inedible by humans,and causing the fruit to fall from the trees.The quality and quantity of fruits affected by the borer are markedly reduced,as are the yields of infested orchards,causing large economic losses.The peach fruit borer is predominantly found in Asian countries(China,Japan,Korea) and the far east of Russia.Because of the risk to fruit orchards,in 1988 the European and Mediterranean Plant Protection Organization placed C.sasakii on the A1 list of pests recommended for control by quarantine.However,the peach fruit borer is difficult to control because it is concealed within the fruit,and general methods of pest treatment fail to achieve sustainable control.It has therefore become necessary to identify and utilize natural enemies of this pest to develop a bio-control strategy.Many researchers have worked on this problem,and have obtained valuable data and results.However,there are many factors that remain to be elucidated;for example,precisely which species are parasitoids of the peach fruit borers that attack the jujube tree,how are they restricted to their host,and what is the host-parasitoid relationship? In this project,we investigated populations of parasitic wasps attacking C.sasakii in jujube orchards in Shaanxi,Shanxi and Henan provinces and in Ningxia Hui Autonomous Region in 2009—2010.Three species of parasitic wasp were identified(Pristomerus chinensis Ashmead,Chelonus(Microchelonus) zhangi Zhang and a species of Pteromalidae) and all were reared from their host cocoons.P.chinensis Ashmead and C.(M.) zhangi Zhang are both preponderant agents,but their population size varied markedly among the above four regions;P.chinensis was found predominantly in Shaanxi and Shanxi provinces,whereas C.(M.) zhangi was observed principally in Henan.The relationship between emergence,parasitism rate and host were studied for P.chinensis and C.(M.) zhangi.The results indicate that P.chinensis emerged half a month later than its host.Adult hosts emerged from over-winter cocoons in the middle of June,with a parasitism rate of 8.86%;the summer generation emerged from the last week of August to the first week of September,with a 25% parasitism rate.For C.(M.) zhangi,the summer generation emerged from the middle of August to the first week of September.In general,fruit drop occurred in the first(in 2010) to the second(in 2009) week of August in jujube orchards in Puyang city,Henan province;C.(M.) zhangi began to emerge one week later,with the peak emergence at 10—12 days.The graph of parasitism rate for C.(M.) zhangi revealed three time periods: from 16 to 26 August,from 27 to 31 August and from 1 to 8 September.As a whole,the parasitism rate declined from the first period to the third,which could be related to host behavior.No insecticide was used in the experimental areas during the years 2009—2010;as a consequence,the parasitism rate for C.(M.) zhangi increased markedly to 64.52% in 2010.Thus,it can be seen that local natural enemies have been playing an important role in controlling the peach fruit borer naturally,and should be protected and utilized appropriately.