不利個體表現的同類相食與同功群間捕食行為有益於秋行軍蟲族群存續

Abstract

秋行軍蟲（Spodoptera frugiperda）是近年入侵全球的廣食性作物害蟲，了解其族群快速成長背後的機制是至關重要的研究主題。與其他鱗翅目昆蟲相比，秋行軍蟲幼蟲對其他競爭個體表現出更高頻率的無差別攻擊行為（non-selective aggression）。儘管秋行軍蟲為植食性昆蟲，其幼蟲除了以玉米等作物為主食外，也會進行同類相食（cannibalism），並將其競爭對手玉米螟（maize stemborer）幼蟲視為獵物，展現出同功群間捕食（intraguild predation，IGP）的行為。過往研究多認為同類相食與同功群間捕食可增進捕食者個體的生長表現，進而對捕食者族群產生益處。然而，對秋行軍蟲幼蟲而言，同類相食卻會降低捕食者個體的發育速度與存活率，也因此這種攻擊行為對秋行軍蟲族群的影響仍有待研究。為了探討這樣類有害於捕食者個體的無差別攻擊行為如何影響秋行軍蟲的族群動態，我們建立了一個具有年齡結構的消費者－資源模型（stage-structured consumer–resource model），以討論秋行軍蟲同類相食、同功群間捕食與資源競爭對其族群量的影響。在只有秋行軍蟲的情況下，不論同類相食對個體生長表現是正面或負面的影響，攻擊行為都會導致秋行軍蟲族群數量減少。然而，在有玉米螟競爭玉米資源的情況下，即便同類相食對捕食者個體有害，秋行軍蟲的無差別攻擊行為仍有助於其競爭排除玉米螟。這是因為秋行軍蟲透過捕食玉米螟減少資源競爭的壓力，其所釋出的資源效益足以彌補同類相食的負面影響。除此之外，我們發現當面對具有更強競爭性狀的玉米螟，或是在食物資源短缺的環境中時，秋行軍蟲必須展現更高頻率的無差別攻擊行為才得以避免滅絕。本研究凸顯了群落脈絡對研究攻擊行為的重要性，並指出其對其他玉米害蟲的競爭優勢可能是秋行軍蟲近幾年快速擴張的原因之一，為秋行軍蟲的入侵機制提供了全新的視角。

Spodoptera frugiperda (fall armyworm) is a globally invasive pest of maize, and understanding the mechanisms behind its outbreak is essential for managing its impact. Compared to other lepidopterans, fall armyworm larvae show higher levels of non-selective aggression toward competing individuals, engaging in cannibalism by feeding on conspecific larvae and acting as an intraguild predator by consuming the larvae of maize stemborer species. While typically advantageous for consumer performance, cannibalism in fall armyworm has been shown to reduce individual fitness by lowering the development efficiency and survival rates, making its population-level consequences unclear. To study how this self-harming aggression shapes the dynamics of the fall armyworm population, we developed a stage-structured consumer–resource model involving the fall armyworm and maize stemborer, where the fall armyworm simultaneously engages in cannibalism, IGP, and resource competition. In a single-species scenario, cannibalism reduces the population size of the fall armyworm across varying levels of non-positive cannibalism effect. However, in a two-species system, increasing non-selective aggression (i.e., both cannibalism and IGP) leads to an increase in the fall armyworm population size, despite the self-harming cannibalism effect. We further show that such non-selective aggression is necessary for the persistence of the fall armyworm when competing against stemborer species with superior competitive traits and when resources are scarce. Further analyses suggest that the ecological advantage of non-selective aggression arises not from the nutritional gain of predation, but from reduced competition and freed resources following competitor mortality. Our results emphasize the importance of the community context in shaping the ecological consequences of aggressive behaviors, offering new insight into behavioral strategies that may underlie the invasive success of fall armyworm.