Ginsberg Howard S, Albert Marisa, Acevedo Lixis, Dyer Megan C, Arsnoe Isis M, Tsao Jean I, Mather Thomas N, LeBrun Roger A
USGS Patuxent Wildlife Research Center, Woodward Hall-PSE, University of Rhode Island, Kingston, RI, United States of America.
Department of Plant Sciences and Entomology, Woodward Hall, University of Rhode Island, Kingston, RI, United States of America.
PLoS One. 2017 Jan 11;12(1):e0168723. doi: 10.1371/journal.pone.0168723. eCollection 2017.
Recent reports suggest that host-seeking nymphs in southern populations of Ixodes scapularis remain below the leaf litter surface, while northern nymphs seek hosts on leaves and twigs above the litter surface. This behavioral difference potentially results in decreased tick contact with humans in the south, and fewer cases of Lyme disease. We studied whether north-south differences in tick survival patterns might contribute to this phenomenon. Four month old larvae resulting from a cross between Wisconsin males and South Carolina females died faster under southern than under northern conditions in the lab, as has previously been reported for ticks from both northern and southern populations. However, newly-emerged larvae from Rhode Island parents did not differ consistently in mortality under northern and southern conditions, possibly because of their younger age. Survival is lower, and so the north-south survival difference might be greater in older ticks. Larval survival was positively related to larval size (as measured by scutal area), while survival was positively related to larval fat content in some, but not all, trials. The difference in larval survival under northern vs. southern conditions might simply result from faster metabolism under warmer southern conditions leading to shorter life spans. However, ticks consistently died faster under southern than under northern conditions in the laboratory when relative humidity was low (75%), but not under moderate (85%) or high (95%) RH. Therefore, mortality due to desiccation stress is greater under southern than under northern conditions. We hypothesize that mortality resulting from the greater desiccation stress under southern conditions acts as a selective pressure resulting in the evolution of host-seeking behavior in which immatures remain below the leaf litter surface in southern I. scapularis populations, so as to avoid the desiccating conditions at the surface. If this hypothesis is correct, it has implications for the effect of climate change on the future distribution of Lyme disease.
近期报告表明，肩突硬蜱南方种群中寻找宿主的若虫停留在落叶层表面以下，而北方若虫则在落叶层表面以上的树叶和嫩枝上寻找宿主。这种行为差异可能导致南方蜱与人类的接触减少，莱姆病病例也较少。我们研究了蜱生存模式的南北差异是否可能导致这种现象。如之前对北方和南方种群的蜱的报道一样，在实验室中，由威斯康星州雄性和南卡罗来纳州雌性杂交产生的4个月大的幼虫在南方条件下比在北方条件下死亡更快。然而，来自罗德岛亲本的新孵化幼虫在北方和南方条件下的死亡率并没有持续差异，可能是因为它们年龄较小。蜱的存活率较低，因此南北存活率差异在年龄较大的蜱中可能更大。幼虫存活率与幼虫大小（以盾板面积衡量）呈正相关，而在一些但并非所有试验中，存活率与幼虫脂肪含量呈正相关。北方与南方条件下幼虫存活率的差异可能仅仅是由于南方温暖条件下新陈代谢更快导致寿命缩短。然而，当相对湿度较低（75%）时，实验室中蜱在南方条件下始终比在北方条件下死亡更快，但在中等（85%）或高（95%）相对湿度下则不然。因此，南方条件下因干燥压力导致的死亡率高于北方条件。我们推测，南方条件下更大的干燥压力导致的死亡率作为一种选择压力，导致了寻找宿主行为的进化，即在南方肩突硬蜱种群中，未成熟个体停留在落叶层表面以下，以避免表面的干燥条件。如果这个假设正确，那么它对气候变化对莱姆病未来分布的影响具有启示意义。
