Predator efficacy and attraction to herbivore-induced volatiles determine selection of a nutritionally inferior host plant in a moth

Unlike mammals, most invertebrates provide no direct parental care for their progeny, except a limited amount of yolk and a well-selected oviposition site. However, little is understood of how gravid females evaluate opportunities and threats when choosing an oviposition site. Leveraging the wide range of host plant use in a polyphagous pest, Spodoptera littoralis , we investigate oviposition choice and predator activity between two host plants of different nutritional quality. Oviposition experiments and field surveys reveal that females preferred to lay eggs on a host plant, which exhibit an inferior nutritional quality and harbor more natural enemies. However, we found that predation rates of S. littoralis larva and pupae were lower on this plant. Through investigating the predator search and olfactory-guided behaviors, we show that a major predator is more successful in finding S. littoralis and more attracted to herbivore-induced olfactory cues from the superior host plant. Our results show that enemy free-space between host plants is dependent on predator efficacy and odor-gui ded attraction, but not predator abundance, and could contribute to the weak correlation between preference and performance during host plant choice in S. littoralis and in insects in general.

与哺乳类动物不同，绝大多数无脊椎动物并不会对后代提供直接的亲代抚育，仅会为后代提供有限的卵黄资源并挑选适宜的产卵位点。然而，学界对于怀卵雌虫在选择产卵位点时如何评估潜在机遇与威胁的机制仍不甚明晰。本研究以多食性害虫斜纹夜蛾（Spodoptera littoralis）广泛的寄主植物利用谱为研究依托，针对两种营养品质存在差异的寄主植物，探究其产卵选择行为与天敌活动规律。产卵行为实验与野外调查结果显示，斜纹夜蛾雌虫更倾向于在营养品质较低且栖息有更多天敌的寄主植物上产卵。但研究同时发现，该寄主植物上斜纹夜蛾幼虫与蛹的被捕食率反而更低。通过对天敌搜寻行为与嗅觉介导行为的研究，我们发现主要天敌更易定位到斜纹夜蛾种群，且更易被优质寄主植物释放的、经植食者诱导产生的嗅觉信号所吸引。本研究结果表明，寄主植物间的天敌自由空间（enemy free-space）取决于天敌效能与嗅觉介导的吸引作用，而非天敌种群丰度；该发现或可解释斜纹夜蛾乃至多数昆虫在寄主选择过程中，产卵偏好与后代适合度表现之间普遍存在的弱相关性。