Egg Parasitoids of Phytophagous Bugs in Soybean: Mobile Natural Enemies as Naturally Occurring Biological Control Agents of Mobile Pests Y OSHIMI HIROSE,KEIJI TAKASU, 1 AND MASAMI TAKAGI Institute of Biological Control, Faculty of Agriculture, Kyushu University, Fukuoka 812-81, Japan Received July 10, 1995; accepted January 26, 1996 Egg parasitism of mobile phytophagous bugs in two soybean fields was analyzed to address the question: what characteristics do their egg parasitoids have as naturally occurring biologically effective control agents? Eggs of four bug species, Piezodorus hybneri (Gmelin), Eysarcoris guttiger (Thunberg), Riptortus clavatus (Thunberg), and Megacopta punctatissimum (Montandon) were found. Ooencyrtus nezarae Ishii parasitized eggs of all four bug species, while Teleno- mus triptus Nixon parasitized eggs of only P. hybneri and E. guttiger. Gryon nigricorne (Dodd) and Gryon japonicum (Ashmead) were specific to R. clavatus. Only Paratelenomus minor (Watanabe) emerged from M. punctatissimum eggs. Percentage parasitism by each of the two nonspecific parasitoids often was higher than percentage parasitism by each of the three host-specific parasitoids. O. nezarae was a good colo- nizer in soybean fields, synchronizing female coloniza- tion with the appearance of its host. Although a high level of parasitism by O. nezarae occurred during the first half of a host infestation period, part of the emerging O. nezarae females dispersed from fields while their hosts continued to oviposit. This behavior resulted in a failure of O. nezarae to increase parasit- ism of its host during the latter half of its reproductive period. Female dispersal of this polyphagous parasi- toid is a strategy for exploiting its mobile hosts, and the degree of its dispersal is correlated with its coloniz- ing ability. It is suggested that polyphagy and high dispersal ability are the main characteristics of effec- tive natural enemies of naturally occurring mobile pests. r 1996 Academic Press, Inc. KEY WORDS: phytophagous bugs; egg parasitoids; soy- bean; Ooencyrtus nezarae; Telenomus triptus. INTRODUCTION Many arthropod pests of agriculture move seasonally over wide areas and among crops in diversified agroeco- systems. Kennedy and Margolies (1985) called these species ‘‘mobile pests.’’ Such pests also are character- ized by polyphagy and multivoltinism, and their habi- tats including crops often are ephemeral. Examples of mobile pests are phytophagous mites, such as Tetrany- chus urticae Koch; aphids, such as Myzus persicae (Sulzer); moths, such as Helicoverpa zea (Boddie); and thrips, such as Thrips palmi Karny. Most mobile pests are major pests (Kennedy and Margolies, 1985). This may be largely because natural enemies of these pests have difficulty in shifting their habitats to follow the seasonal movement of the pests. However, little attention has been paid to relationships between mobile pests and their natural enemies. Thus, naturally occurring biological control of mobile pests by their natural enemies rarely has been investigated from the viewpoint of colonization, reproduction, and dispersal of natural enemies in various habitats. Certain stink bugs and squash bugs are mobile pests that attack many crops, by moving seasonally from habitat to habitat within agroecosystems. Many of these phytophagous bugs belonging to Pentatomidae, Alydidae, and Plataspidae are pests of soybean (Ku- wayama, 1953; Turnipseed and Kogan, 1976); some species of Alydidae are referred to as Coreidae in these references. These phytophagous bugs have a wide range of host plants, exhibit multivoltinism, and shift their habitats seasonally. Thus, adults of the phytopha- gous bugs colonize soybean from various habitats out- side the fields in summer or autumn in Japan to lay their eggs on soybean. The resulting adults and nymphs hatching from eggs laid on soybean mainly attack seeds and pods, frequently causing serious damage to a crop. Egg parasitoids are important natural enemies of phytophagous bugs in soybean (Takasu and Hirose, 1985). We expect that effective egg parasitoids of these bugs colonize soybean as early as ovipositing host bugs 1 Present address: Laboratory of Insect Science, Department of Biological and Environmental Science, Kobe University, Kobe 657, Japan. BIOLOGICAL CONTROL 7, 84–94 (1996) ARTICLE NO. 0069 84 1049-9644/96 $18.00 Copyright r 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.