Pureswaran Deepa S, Sullivan Brian T, Ayres Matthew P
Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA.
Oecologia. 2006 Jul;148(4):720-8. doi: 10.1007/s00442-006-0400-9. Epub 2006 Apr 12.
Timing of arrival at a resource often determines an individual's reproductive success. Tree-killing bark beetles can reproduce in healthy trees by attacking in adequate numbers to overcome host defences that could otherwise be lethal. This process is mediated by aggregation and antiaggregation pheromones. Beetles that arrive early in such a "mass attack" must contend with undiminished tree defences, and produce enough pheromones to attract more beetles, but have a head start on gallery construction and egg-laying. Beetles that arrive late may be impeded by competition and diminishing availability of phloem, but should experience fewer costs associated with pheromone production and battling tree defences. We investigated relationships between timing of arrival, body size, pheromone production and fitness in the southern pine beetle, Dendroctonus frontalis. In field experiments, we captured beetles that arrived early (pioneers) and late on slash pine trees, Pinus elliottii, and measured pheromone amounts in their hindguts. We marked gallery entrances of beetles as they landed on a tree and measured their reproductive success after the attack terminated. We found no difference in body size or pheromone amounts between early and late arrivers. Most beetles arrived at the middle of the attack sequence, and excavated longer galleries per day than early arrivers. The number of offspring produced per day by beetles that established galleries midway through mass attack was higher than those that arrived early or very late in the sequence. Our results suggest that beetles do not exhibit adaptive phenotypic plasticity in pre-landing pheromone production, depending on the extent of previous colonisation of a host. Rather, it appears that stabilising selection favours beetles that attack in the middle of the sequence, and contributes to attack synchrony. Synchronous attack on trees is essential before population booms characteristic of tree-killing bark beetles can occur in nature.
到达资源的时间往往决定个体的繁殖成功率。杀死树木的树皮甲虫能够通过大量攻击来克服原本可能致命的宿主防御机制，从而在健康树木中繁殖。这个过程由聚集和抗聚集信息素介导。在这样的“大规模攻击”中早早到达的甲虫必须应对未减弱的树木防御，产生足够的信息素以吸引更多甲虫，但在蛀道建造和产卵方面有先机。晚到的甲虫可能会受到竞争和韧皮部可用性下降的阻碍，但与信息素产生和对抗树木防御相关的成本应该更低。我们研究了南方松甲虫（Dendroctonus frontalis）到达时间、体型、信息素产生与适合度之间的关系。在野外实验中，我们捕获了在湿地松（Pinus elliottii）上早早到达（先驱者）和晚到的甲虫，并测量它们后肠中的信息素含量。当甲虫落在树上时，我们标记它们的蛀道入口，并在攻击结束后测量它们的繁殖成功率。我们发现早到者和晚到者在体型或信息素含量上没有差异。大多数甲虫在攻击序列的中间到达，并且每天挖掘的蛀道比早到者更长。在大规模攻击中途建立蛀道的甲虫每天产生的后代数量高于在序列中早早到达或非常晚到达的甲虫。我们的结果表明，甲虫在着陆前的信息素产生方面没有表现出适应性表型可塑性，这取决于宿主先前的被定殖程度。相反，似乎稳定选择有利于在序列中间进行攻击的甲虫，并有助于攻击同步性。在自然界中出现杀死树木的树皮甲虫特有的种群激增之前，对树木的同步攻击至关重要。
