Eatough Jones M, Hanlon C C, Paine T D
Department of Entomology, University of California, Riverside, CA 92521.
Environ Entomol. 2015 Aug;44(4):960-5. doi: 10.1093/ee/nvv070. Epub 2015 May 8.
The cerambycid beetle, Phoracantha semipunctata F., was introduced into California in the mid-1980s and killed large numbers of Eucalyptus host trees. The populations of the borer declined to very low levels in the mid-1990s following the establishment of the congener, Phoracantha recurva Newman, and the intentional introduction of the egg parasitoid, Avetianella longoi Siscaro. The distributions of the beetles overlap in the Australian native range, but one species has replaced the other in the adventive range in California. One possible explanation is differential susceptibility to natural enemies introduced for biological control. An alternative explanation for the reduced abundance of P. semipunctata is asymmetric interspecific competition between the two species. To test this hypothesis, equal larval densities of each species were introduced into host logs. In all cases, more P. recurva adults emerged than P. semipunctata adults, but the presence of congeners did not have a different effect than the presence of an equal density of conspecific individuals. Neither the temporal order of introduction or bark thickness altered the outcome of potential competitive interactions. Consequently, it appears that the ecological replacement of one borer with another in the adventive environment in southern California may not be a result of bottom-up intraguild competitive interactions. The top-down effects of natural enemies on P. semipunctata have most likely led to its decline.
1980年代中期，半星斑天牛（Phoracantha semipunctata F.）被引入加利福尼亚，导致大量桉树寄主树死亡。在其同属物种弯角半星天牛（Phoracantha recurva Newman）建立种群以及有意引入卵寄生蜂长管阿韦氏啮小蜂（Avetianella longoi Siscaro）之后，这种蛀干害虫的数量在1990年代中期降至极低水平。这两种甲虫在澳大利亚原生分布范围内的分布有重叠，但在加利福尼亚的外来分布范围内，一个物种已取代了另一个物种。一种可能的解释是对为生物防治而引入的天敌的易感性存在差异。半星斑天牛数量减少的另一种解释是这两个物种之间存在不对称的种间竞争。为了验证这一假设，将每个物种相同的幼虫密度引入寄主原木中。在所有情况下，羽化出的弯角半星天牛成虫都比半星斑天牛成虫多，但同属物种的存在与相同密度的同种个体的存在相比，并没有产生不同的影响。引入的时间顺序或树皮厚度都没有改变潜在竞争相互作用的结果。因此，在南加利福尼亚的外来环境中，一种蛀干害虫被另一种取代的生态现象似乎并非是由自下而上的同业竞争相互作用导致的。天敌自上而下的影响很可能导致了半星斑天牛数量的下降。
