Biology Of The Peristenus Relictusand Its Potential As A Biological Control Agent Ofthe Alfalfa Plant Bug, Adelphocoris Lineolatus

Peristenus relictus Ruthe (Hymenoptera:Braconidae) is a solitary, koinobiont, generalist (polyphagous), indigenous endoprasitoid identified from Yangtze and Yellow river regions of China. This synovigenic, nymphal, arrhenotokus parasitoid is a dominant member of the parasitoid guild along with P. spretus which parasitizes readily a wide range of economically important mirid bug (Hemiptera:Miridae) pests in its native range. Despite the huge gains from Bt cotton in China during the last decade, a large-scale adoption of transgenic Bt cotton has led to the outbreak of secondary pests, especially different mirid bugs due to a drastic reduction of broad-spectrum foliar insecticides on this crop. Therefore, P. relictus is being taking into consideration to use for augmentative releases as a potential candidate of biological control agent against rapidly growing population of different mirid species in Bt cotton habitats of China. It has been widely distributed across the world and much is known of its ecology, developmental and reproductive biology. However, some attributes to elucidate its biological potential for augmentative releases are yet to be exploited.With the aim of designing an effective and profitable mass-rearing system, understanding the population dynamics and developing an effective field releases program of this parasitoid with the phenology of mirids, laboratory experiments were conducted to evaluate its biological potential by using alfalfa plant bug, Adelphocoris lineolatus Goeze as host. This doctoral dissertation examined the potential of P. relictus as a biological control agent of mirid bugs through studies on three following aspects of P. relictus biology: 1) Functional response, mutual interference and sex allocation pattern of P. relictus to varying host and parasitoid densitiesP. relictus exhibited a Holling’s disc Type Ⅱ functional responses, meaning that the parasitism rate decreased exponentially with the increase in A. lineolatus density. In other words, an inverse relationship was established between host density and proportion of host parasitized. It was indicated that parasitism increased with the increment of host density until parasitoid reached its upper asymptote of reproductive potential. In terms of mutual interference, the decreased nymph-wasp ratio (by offering constant80hosts) significantly reduced the number of host parasitized and the number of progeny produced per parasitoid by20and30folds, respectively, at16parasitoids group as compared to single parasitoid. The interference constant (m) was estimated at1.20indicating a strong mutual interference. The scarcity of proportion of hosts and increased co-presence of conspecifics led to the significant increase of self-and conspecific superparasitism and male-biased sex ratio in the offspring population which explains the strong intraspecific competition amongst the foraging parasitoids. The interactive effects of host and parasitoid density reflected to the observed sex ratios to be in broad agreement with the predictions of local mate competition (LMC) theory.2) The preference and suitability of different host instars of A. lineolatus for the development and reproduction of P. relictusThe relative preference indices from host preference and suitability studies revealed that preference was mostly concentrated towards first to third instar nymphs of A. lineolatus. The highest parasitization was occurred in the early instar hosts (first and second) ranging from89.26to91.09%in no-choice tests; and93.21to93.08%in choice tests. However, first instar was the most susceptible to oviposition-induced mortality than all later-instar nymphs. Superparasitism decreased with the progress to later host-instar. Although first to fourth instar hosts were able to reproduce, second instar was the most suitable host stage in terms of highest progeny production and more female-biased sex allocation. Third instar was the second line of options for host suitability. Fifth instar did not yield any progeny. The later the host instar at parasitism, the faster the parasitoids emerged from host instar.3) Effect of varying constant temperatures on reproductive biology of P. relictus attacking A. lineolatus.Temperature is one of the key abiotic factors on the parasitoid’s reproductive potential. Oviposition period, longevity, daily oviposition and lifetime fecundity were significantly affected by temperature. Longevity shortened as the temperature increased from15to30℃. The average longevity of adult female parasitoids ranged from28.40±1.29d at15℃to7.20±0.66d at30℃while the daily mean oviposition was highest (41.51±6.05) at25℃and lowest (11.47±3.31) at15℃. Reproductive longevity was maximal at20℃with18.60±1.86days of oviposition. The highest potential lifetime fecundity was achieved at20℃with675.4±56.78eggs, which is followed by530.60±34.50eggs at25℃. Similarly, the total parasitism over lifetime followed the same trend as potential lifetime fecundity and ranged from591.20±52.87parasitized nymph at20℃to217.80±3.61nymphs at15℃.The outcomes of this dissertation research suggested that P. relictus has the potential to be an effective biological control agent of mirid bugs in Bt cotton fields of China. For the purpose of providing large number of wasps for augmentative releases into Bt cotton, the optimal parasitoid-nymph ratio for efficient mass-production system of P. relictus is1:80and the parasitoid should be reared on second instar of A. lineolatus at20℃. Second instar nymph is the best stage to target for effective control in field releases of P. relictus.