ON THE BIONOMICS OF THRIPOCTENUS BRUI VUILLET, A PARASITE OF THRIPS TABACI LIND., IN JAPAN (II)

寄生蜂Thripoctenus brui VUILLETは東京, 京城附近及び琉球3島に分布し, Thrips tabaci LIND.(主寄主)Tacniothrips alliorum PR.及びThrips abdominalis CWFD.に寄生す。以上3地方にてそれへの重寄生又は共寄生を見す。成蟲期間は約20日にして, 雌雄の比は3 : 2なり。被受精不産卵の雌1頭より平均約40の幼卵をその死後摘出せり。例外として多寄生をなせども完寄生の事實を見す。卵∿幼蟲期間は7∿14日, 蛹期間は13∿27日, 前産卵期間は1∿2日にして, 1世代を平均夏期は23.7日秋期は38.8日にて終る。此寄生蜂は體内寄生をなし, 寄主の幼小なる幼蟲に産卵し, 寄主の前蛹期中に蛹化す。寄主の前蛹化する少し前までの期間中に産卵せられたる寄生蟲は完全に蛹化し得れども, それ以後の期間中(前蛹∿蛹期を含む)に産卵せられたるものは, それ自身蛹化し得る以前に寄主を斃死せしむ。但し寄主の羽化後に孵化せる場合は, 寄主の成蟲體内に長らく棲息し得れども, それ自身蛹化し得ず。又かかる寄主はその成蟲期間を短縮させられ, 産卵せず。蛹態にて越冬せる寄生蜂は五月下旬羽化し, 秋までに4世代を經過す。或るものは第5世代を經て越冬に入る。十月下旬又は十一月上旬以後は寄生せられし寄主幼蟲を見ず。但し寄主は冬期中越冬状態に入らず。寄生蜂の活動期中その前半期間は寄生蟲の個體群, 寄主のそれに比し大ならず, 後者のそれを効果的に減少せしめ得ず。後半期間に於ては, 氣象其他の致死條件と共に寄主の個體群を大いに減少せしむ。本寄生蜂はハワイに土着する十分なる可能性を有し, 少くとも寄主の個體群大なる場合それの低減に効果あらん。

Thripoctenus brui VUILLET is found distributed around Tokyo and Keijo, and in three islands of Loochoo within Japan. It parasitizes Thrips tabaci LIND. (major host), Tacniothrips alliorum PR. and Thrips abdominalis CWFD. Epiparasitism or symparasitism is absent in Tokyo, Korea and Loochoo. The length of adult life is about 20 days ; the sex ratio is 2 males to 3 females ; an average of about 40 oocytes are dissected from a single nonlaying mated female ; polyparasitism is sometimes observed but it is hyperparasitic. Egg-larval stage is from 7 to 14 days, pupal stage 13 to 27 days and the preoviposition period is about 1 to 2 days. Average life cycle is 23.7 days in the summer brood and 38.8 in the autumn brood. Thripoctenus brui is an endoparasitic larval parasite ; oviposition takes place on young stages of larvae, and the parasite pupates while the host is in the prepupal stage. Suitable host age for being oviposited in ranges up to little before prepupation. Oviposition after this age brings host mortality and at which time the parasite cannot pupate. However, in cases where parasite eggs hatch after host emergence, the parasite fails to pupate and host adult life is somewhat shortened and sterility is caused. Adults emerge from the over-wintered pupae in late May, and 4 complete broods are present per season. Some of the 4th brood pupae enter hibernation and others have an additional brood before hibernation. Parasitized host larvae disappear in late October or early November. During the first half of the parasite season, the parasite population is not yet high enough to be an efficient mortality factor upon the host population. However, during the latter half of the season the host population is controlled by the parasite operating in association with climatic and other mortality factors. Available data show that the parasite may be established in Hawaii and may reduce the host population at least while it is high.