Brown J M, Abrahamson W G, Packer R A, Way P A
Biology Department, Bucknell University, 17837, Lewisburg, PA, USA.
Oecologia. 1995 Sep;104(1):52-60. doi: 10.1007/BF00365562.
The successful colonization of novel host-plant species by herbivorous insects may be facilitated by a reduction in natural-enemy attack on insect populations associated with the novel (derived) host plant. This is particularly true if natural enemies use host-plant or habitat cues in searching for their herbivore prey. In order to test whether the acquisition of enemy-free space could have influenced the host shift in the goldenrod ball gallmaker, Eurosta solidaginis, we estimated levels of natural-enemy attack in 25 host-race populations associated with Solidago altissima and S. gigantea (Compositae) spanning the zone of sympatry between S. altissima and S. gigantea host races in New England. Mortality due to attack by the parasitoid wasp Eurytoma obtusiventris was significantly higher for the ancestral than for the derived host race (30.5% versus 0.4%) across the transect, which is consistent with the enemy escape hypothesis. Contrary to this hypothesis, mordellid beetles caused significantly higher mortality on the derived than ancestral host race (17.1% versus 2.6%). Mortality by a second parasitoid wasp and birds showed no significant differences between the two host races. Overall, the derived host race had significantly higher survivorship across the transect (36.6% versus 20.8%). An analysis of survivorship and parasitoid mortality levels from sympatric sites in this study and previous studies showed a highly significant correlation between the levels of Eurytoma obtusiventris attack and the survivorship advantage of the derived host race. Observations of this parasitoid's searching behavior confirmed that it preferentially searches the ancestral host for fly larvae. Current patterns of host-race mortality and naturalenemy behavior and abundance are consistent with the facilitation of the host shift by escape from a specialist parasitoid.
食草昆虫成功定殖新的寄主植物物种，可能得益于与新（衍生）寄主植物相关的昆虫种群所遭受的天敌攻击减少。如果天敌在寻找食草动物猎物时利用寄主植物或栖息地线索，情况尤其如此。为了测试获得无天敌空间是否会影响一枝黄花球瘿蜂Eurosta solidaginis的寄主转移，我们估计了与高茎一枝黄花和巨茎一枝黄花（菊科）相关的25个寄主种族种群中的天敌攻击水平，这些种群跨越了新英格兰高茎一枝黄花和巨茎一枝黄花寄主种族的同域分布区域。在整个样带中，寄生蜂钝腹广肩小蜂攻击导致的死亡率，原始寄主种族显著高于衍生寄主种族（30.5%对0.4%），这与天敌逃逸假说一致。与该假说相反，隐翅虫在衍生寄主种族上造成的死亡率显著高于原始寄主种族（17.1%对2.6%）。另一种寄生蜂和鸟类造成的死亡率在两个寄主种族之间没有显著差异。总体而言，衍生寄主种族在整个样带中的存活率显著更高（36.6%对20.8%）。对本研究和以往研究中同域分布地点的存活率和寄生蜂死亡率水平进行分析表明，钝腹广肩小蜂的攻击水平与衍生寄主种族的存活优势之间存在高度显著的相关性。对这种寄生蜂搜索行为的观察证实，它优先在原始寄主上搜索蝇幼虫。当前寄主种族死亡率以及天敌行为和丰度的模式，与通过逃离专一寄生蜂促进寄主转移相一致。
