Four different laboratory and/or field experiments as followings have been done to understand the relative strength of biotic and abiotic factors on the abundance of phytophagous insects on soybeans in the field.

(1) Effects of prey species and its density on larval performance of two species of hoverfly larvae
　 The impacts of prey quality and quantity on the larval performance of two predatory hoverflies were examined by laboratory experiment. 　 The results indicated a better performance of predatory insects when obtained better prey species, 　 although several variations in terms of compensation to feed on prey species with low quality were also revealed, 　 suggesting the ability of the generalist predators to switch consuming on prey species. Furthermore, 　 the ability of predators to suppress the population of phytophagous insects might rely on the biological traits, 　 such as body size which influences the voracity of such predators on prey. 　 In the present result, the bigger size of the generalist hoverfly, Episyrphus balteatus, 　 impacted in the higher rate of consumption on aphids compared with the smaller species, Eupeodes corollae.

(2) Oviposition preference of two hoverfly species in response to risk of intraguild predation by ladybird larvae
　 The detrimental impacts of ladybirds on the larval survivorship and adult oviposition behaviour of the predatory 　 hoverflies were revealed. In general, 　 ladybird larvae tended to have superiority to hoverfly larvae in terms of larval interactions between the two species, 　 suggesting that biological traits, such as mobility, body size, and defence mechanism are important as determinant impact. 　 The less activity of hoverfly larvae resulted in the higher vulnerability against the more active ladybird larvae, 　 although when confronted with the smallest species of the ladybird, Scymnus posticalis , the hoverfly larvae tended to be 　 superior due to the bigger body size. The subsequent effect of such predation influenced the oviposition behaviour of adult hoverflies, 　 for example, the females of hoverfly tended to change laying number of eggs on aphid colonies in relation to species 　 and developmental stages of the ladybird larvae. 　 These results suggest that the oviposition behaviour of adult hoverflies might coevolve under the interactions 　 with ladybird larvae due to intraguild predation.

(3) Interactive impacts of soil and nutrient to influence the abundance of phytophagous and carnivorous arthropods via soybean growth
　 As the important impact of abiotic factors, i.e. soil texture and nutrient, 　 to determine the ultimate role of soybeans on the abundance of both phytophagous and carnivorous arthropods via bottom-up mechanism, 　 the soil texture functioned on the nutrient absorption by soybeans, which in turn gave positive influence on 　 biomass and N content of the plants. For example, the response of soybeans to fertilization was stronger in loam soil than 　 in sandy soil. As a consequence, the abundance of phytophagous and predatory insects, sucking insects in particular, 　 also increased more rapidly in fertilized soybeans in loam soil than other treatments.

(4) Temporal variation of bottom-up and top-down strength in a soybean arthropod community
　 The variation of the relative strength of bottom-up and top-down forces on soybeans was revealed 　 between summer and early autumn seasons and the different requirement of nutrient resulted in the variation 　 which is attributable on the different developmental stages of the plants. In summer season, for example, 　 when soybeans were in vegetative stage, the fertilization impacted on the raising content of foliar nitrogen, 　 while fertilization in reproductive stage in early autumn season did not work significantly. 　 As a consequence, the significant impact of nutrient addition on the foliar nitrogen in summer season 　 increased the abundance of sucking insects like aphids. 　 However, suppression force of carnivores on phytophagous insects was relatively smaller than bottom-up 　 forces due to nutrient through seasons.

　These results suggest that biotic and abiotic factors influenced differently on abundance of phytophagous and carnivorous insects, 　 and bottom-up forces tended to give stronger impacts on the phytophagous insects than top-down forces. 　 As the possible mechanisms causing these patterns, three mechanisms in terms of the traits of predatory insects 　 were suggested as follows: 1) larval performance of predatory insects is influenced by prey quality and quantity, 　 2) predatory species tend to have intraguild predation and its negative influence as decreased number of eggs 　 laid after decrease of prey abundance, and 3) predatory species have delayed numerical response to prey abundance.

（日本語訳）

本論文は、ダイズを寄主とする植食性昆虫の個体数決定に及ぼす生物的及び非生物的要因の相対的な重要性を解明することを目的に、 　 以下に述べる４つの実験を実施した。

（１）ヒラタアブ幼虫の生存と発育に及ぼす餌種と密度の影響
　 　餌密度の増加により幼虫の生存と発育及び成虫の体重は増加する傾向があった。 　 一方、餌種の影響は種により異なり、ホソヒラタアブではエンドウヒゲナガアブラムシの方がマメアブラムシより幼虫の生存と 　 発育には好適であったが、フタホシヒラタアブでは逆の傾向が示された。 　 これは、２種の食性と体サイズの違いに起因していると考えられた。

（２）ヒラタアブとテントウムシ幼虫の種間相互作用とそれがヒラタアブの産卵に及ぼす影響
　 　２種ヒラタアブと３種テントウムシ幼虫の種間相互作用は、それぞれの幼虫の体サイズにより決定されている可能性が示唆された。 　 ヒラタアブの産卵に及ぼすテントウムシ幼虫の影響は、ホソヒラタアブでは、ナミの４令幼虫がいた植物上では他の区より有意に 　 産卵数が減少する傾向があったが、フタホシヒラタアブではこのような傾向はなかった。 　 これは、ホソヒラタアブはテントウムシと生息場所及び時期が重複しているが、フタホシヒラタアブでの重複は少なく、 　 このような共進化の程度に起因すると考えられた。

（３）土壌の質と施肥がダイズの成育を通じ植食性及び捕食性昆虫個体数へ及ぼす影響
　 　土壌が粘土質と砂質では施肥の効果が異なり、粘土質でダイズのバイオマス及び葉の窒素量が多い傾向があり、植物の生育、 　 植食性及び捕食性昆虫へのボトムアップ効果は粘土質で施肥をした場合が他の処理区より高い傾向があった。

（４）施肥と天敵がダイズの植食性昆虫群集構成種の個体数に及ぼす影響
　 　施肥のボトムアップと天敵のトップダウン効果がダイズの植食性昆虫群集構成種の個体数に及ぼす影響の相対的な重要性と 　 その季節変化を明らかにした。その結果、ボトムアップ効果がトップダウン効果より重要であり、この効果は初秋より夏で大きいことが明らかとなった。

これらの結果から、植食性昆虫群集構成種の個体数に及ぼす影響は、生物的及び非生物的要因により変化し、 　 天敵による生物的要因のトップダウン効果より土壌の質や施肥による非生物的要因のボトムアップ効果が重要であることが明らかになった。 　 これは、天敵の特性である、１）餌種の質と量により生存及び発育が影響を受ける、 　 ２）餌の減少によるギルド内捕食とそれにより産卵数が減少する、３）害虫の増加に対して遅れて天敵が発生する、 　 などによりトップダウン効果が低下することに起因すると考えられた。