Rand Tatyana A
USDA-ARS Northern Plains Agricultural Research Laboratory, Sidney, MT 59270, USA.
Environ Entomol. 2013 Feb;42(1):116-22. doi: 10.1603/EN12266.
Classical biological control against the alfalfa weevil, Hypera postica (Gyllenhal), a destructive pest of alfalfa (Medicago sativa L.), has resulted in the establishment of nine parasitoid species in the United States. Despite widespread redistribution of a number of species, there remains little postrelease data on their establishment and potential effectiveness in many regions. I surveyed parasitoids associated with alfalfa weevil larvae across 30 or more sites in eastern Montana and western North Dakota over 2 yr. Replicate sites were sampled in two habitat types that differ in their physical characteristics, flood-irrigated and dryland alfalfa fields. Irrigated systems are more productive but also more intensively disturbed habitats because of increased harvest frequency and repeated flooding. Given evidence that both habitat disturbance and herbivore density, which often increases with productivity, can influence parasitoid dynamics, I predicted that parasitism levels, the relative importance of different species, or both, would differ across these two system types. Of four larval parasitoid species released previously or recovered in the region, two were found in this study, Bathyplectes curculionis (Thomson) and Oomyzus incertus (Ratzenberg), with average levels of parasitism across habitat types and years of 37.2 and 3.5%, respectively. Parasitism levels differed between habitat types, but the effect was driven by concomitant differences in host densities that were higher in irrigated than dryland fields. Parasitoid responses to host density varied across years and species. B. curculionis exhibited positive density dependence in parasitism across sites in 2009 and negative density dependence in 2010 when host densities were higher regionally. In contrast, O. incertus exhibited positive density dependence in 2010. Our results suggest that these species may be differentially effective at different host densities. Thus, variation in host density could represent an important axis along which parasitoids exhibit spatial complementarity in function.
针对苜蓿象甲（Hypera postica (Gyllenhal)）的经典生物防治措施已在美国促成了9种寄生蜂的定殖，苜蓿象甲是苜蓿（Medicago sativa L.）的一种毁灭性害虫。尽管多种物种已广泛重新分布，但在许多地区，关于它们定殖情况和潜在效力的释放后数据仍然很少。在两年时间里，我对蒙大拿州东部和北达科他州西部30多个地点的与苜蓿象甲幼虫相关的寄生蜂进行了调查。在两种物理特征不同的栖息地类型（漫灌苜蓿田和旱地苜蓿田）中对重复的地点进行了采样。灌溉系统生产力更高，但由于收割频率增加和反复水淹，也是受干扰更强烈的栖息地。鉴于有证据表明栖息地干扰和草食动物密度（通常随生产力增加）都可能影响寄生蜂动态，我预测这两种系统类型的寄生率、不同物种的相对重要性或两者都会有所不同。在该地区之前释放或发现的4种幼虫寄生蜂物种中，本研究发现了2种，即苜蓿象甲广肩小蜂（Bathyplectes curculionis (Thomson)）和不确定奥米茧蜂（Oomyzus incertus (Ratzenberg)），不同栖息地类型和年份的平均寄生率分别为37.2%和3.5%。不同栖息地类型的寄生率有所不同，但这种影响是由寄主密度的伴随差异驱动的，灌溉田中的寄主密度高于旱地。寄生蜂对寄主密度的反应因年份和物种而异。2009年，苜蓿象甲广肩小蜂在各地点的寄生表现出正密度依赖性，而在2010年，当区域内寄主密度较高时表现出负密度依赖性。相比之下，不确定奥米茧蜂在2010年表现出正密度依赖性。我们的结果表明，这些物种在不同寄主密度下可能具有不同的效力。因此，寄主密度的变化可能代表一个重要轴，寄生蜂沿着这个轴在功能上表现出空间互补性。
