| The Asian corn borer,Ostrinia furnacalis(Guenée),is the most devastating pest in maize growing regions of the East and Southeast Asain countries and Western Pacific Islands including Myanmar.Although the biological control program using egg parasitoids,Trichogramma has been established and the effective biological control agents have been studied intensively to manage maize borers,European corn borer,Ostrinia nubilalis(Hu?bner)in the Europe,and the US and the Asia corn borer,Ostrinia furnacalis in some Asian countries since 1960 s,any biological control program has not initated for this pest in Myanmar.Identifcation and utilization the correct species are fundamental steps for the successful biological control programs.This research project was implemented to explore the existing Trichogramma speceis and select the most promising Trichogramma strains for the establishment of biological control program in Myanmar.We identified the endemic Trichogramma species from the different geographical regions of Shan State,major maize growing areas of Myanmar.A series of selection assessments were applied in the laboratory and in semi-field conditions to investigate the behavioral performance of the collected Trichogramma strains.Finallly,the selected potential Trichogramma strains were evaluated under field condition to select the most effective Trichogramma strains for mass production and field releases on the target pest,O.furnacalis.As a first step for biological control of Asian corn borer in Myanmar,sampling of egg parasitoids,Trichogramma were conducted in Southern and Northern Shan State of Myanmar during the two years survey of 2017 and 2018.Naturally occurring egg parasitoids were characterized as Trichogramma ostriniae Pang et Chen,T.chilonis Ishii,and T.dendrolimi Matsumura.The distinguished characters for male antennae and genitalia were documented for the three Trichogramma species.The distribution of the Trichogramma was varied in different geographical regions.Trichogramma ostriniae was recorded as the dominant species on O.furnacalis,contributing accounting for > 70% of total population,followed by T.chilonis(24%)and the remaining was T.dendrolimi.Species diversity was attained in Southern Shan State whereas only T.ostriniae was dominated from all populations from Northern Shan State.The natural parasitism was varied greatly among the sampling sites,locations and years.The field parasitism was dropped by 21% in 2018 when compared in 2017 with 71%.It is suggested that natural egg parasitoids population is not sufficient to control O.furnacalis,inoculative or inundative release is needed.This study is the first report on the composition of Trichogramma species,and an important step towards the selection of effective biological control agents against O.furnacalis in Myanmar.Selecting the potential Trichogramma species or strains is a fundamental step in developing effective biological control programs.Ostrinia furnacalis or Corcyra cephalonica egg preferences were compared among six strains of T.ostriniae,three of T.chilonis,and four of T.dendrolimi using a choice-test assay design.Significant differences were observed at the inter-and intraspecific level.Three strains of T.ostriniae and one of T.dendrolimi showed a strong preference for O.furnacalis eggs,while two strains of T.chilonis and one T.dendrolimi strain preferred C.cephalonica eggs.Eleven strains were examined on O.furnacalis eggs only,wherein egg parasitism,sex ratio,and total progeny per female were highest for two strains of T.ostriniae and one strain of T.dendrolimi with(40.8±1.9,40.6±2.2,39.8±2.2),(82.7±2.3,90.0±2.4,84.7±2.6)% and(49.6±2.8,42.7±2.6,46.3±2.8),respectively.Parasitism and searching capacity were assessed for the best three performing strains in cages containing maize plants.This small field-cage method revealed that T.ostriniae strain from Yatsawk,To YS showed consistently the highest egg mass parasitism rate at all release ratio,reached 81%,followed by T.dendrolimi strain from Yatsawk,Td YS(66%)and T.ostriniae from Taungyi,To TG(53%)at high release ratio.These three Trichogramma strains should be considered for additional selection experiments for the sustainable management of O.furnacalis in Myanmar.Host age is one of the limiting biotic-factors affecting the capacity of the egg parasitoids,Trichogramma in the field condition.We investigated three strains of T.ostriniae and two strains of T.dendrolimi at the four different age regimes of O.furnacalis eggs(6-12,12-24,24-36 and 36-48)hr under the choice and non-choice conditions.Both T.dendrolimi strains from Southern Shan State and China exhibited the stronger preference on the younger host eggs 6-12 hr old at 27 ?C showing 45% mean parasitism than three other T.ostriniae strains with 19%,29% and 35%,respectively.The host age has a significant effect on parasitism,emergence rate,developmental time and number of progenies per female under non-choice condition.All the strains exhibited the higher parasitism rate(53%)on 6-12 hr eggs than on the other older age regimes(45% for 12-24 hr,40% for 24-36 hr and 30% for 36-48 hr,respectively).Wasp emergence rate was also higher(94%)from parasitized 12-24 hr old eggs while immature developmental time was significantly prolonged on the 6-12 hr old eggs.The great difference in parasitism was observed among the tested Trichogramma strains.Two T.ostriniae strains from Southern Shan State exhibited higher parasitism(? 50%),followed by T.ostriniae from China(44%),T.dendrolimi from Southern Shan State(38%)and the miniumum T.dendrolimi from China(30%).The best effective Trichogramma strains with high egg parasitism and wide-host age preference would be selected for the field release assessment to control O.furnacalis.Two strains of T.ostriniae(Pang et Chen)and one T.dendrolimi(Matsumura)originating from Myanmar,were assessed under field-cage conditions against Asian corn borer,O.furnacalis.Three release rates,180?/36 m2,360?/36 m2 and 720?/36 m2 with corresponding to 50,000/ha,100,000/ha,and 200,000/ha were applied 3 times at weekly intervals during tasseling stage of maize.Parasitism rates of both,egg masses and eggs,differed among the three release dates with highest rates found at the highest release rate.Egg mass and egg parasitism also differed significantly among the three tested Trichogramma strains with highest egg parasitism rate achieved by T.ostriniae from Yatsawk,Southern Shan State(overall 89%),followed by T.dendrolimi from Yatsawk and T.ostriniae(67%)from Taungyi.Plant damage was not significantly differed among the tested Trichogrammastrains.Yield measured as fresh and dry kernel weight was significantly higher(30-60%)in the release plots compared to control.Trichogramma ostriniae from Yatsawk overall showed the best performance not only in terms of egg parasitism,but also regarding plant damage and yield.At the high release rate this strain achieved 60% increase of kernel weight compared to control.Thus,this strain is the most promising for biological control of O.furnacalis in Myanmar where farmers currently face high yield losses.Our results facilitate the initiation of a biological control program to improve maize production;details on ideal timing ideally would be studied under field conditions in Myanmar. |
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