Mimetic host shifts in a social parasite of ants: Caterpillar survivors in Myrmica nests

Host specificity was measured by comparing the proportion of caterpillars that were adopted into different Myrmica nests with the proportion that survived to adulthood or pupation. Data were obtained from one population for 4 years near Przemy-l, s.e. Poland. The proportion of larvae adopted was estimated by baiting for ants beneath stratified random samples of gentians and by counting the number of eggs on each plant. Previous work had shown there was no difference in egg or larval survival on gentians growing in different ant territories, nor in the ratio of larvae retrieved from beneath plants by different ant species: the first Myrmica worker to encounter a larva retrieved it, and where two species overlapped there was no bias in retrieval towards one species. The distribution of the egg population on gentians is therefore an accurate surrogate for the distribution of the final instar population entering nests of each Myrmica species. Adult estimates of M. rebeli were obtained by recording eclosing individuals along stratified transects across sites, and identifying the nest after confirming that it contained an empty pupal case. Additional data were obtained by excavating all Myrmica nests near gentians along stratified transects that had supported known densities of eggs the previous year, and counting the pupae they contained. No mortality has been recorded in pupae in Myrmica cells before eclosion.

本研究通过对比被不同红蚁属（Myrmica）蚁巢接纳的毛虫比例，与存活至成虫期或化蛹期的毛虫比例，量化了寄主专一性。数据集采集自波兰东南部普热梅希尔（Przemyśl）附近的一个种群，历时4年。接纳幼虫的比例通过两种方式估算：一是在分层随机取样的龙胆（gentians）植株下方布设诱饵诱捕蚂蚁，二是统计每株植株上的虫卵数量。已有研究证实，在不同蚁域内生长的龙胆植株上，虫卵或幼虫的存活率并无差异；不同蚂蚁物种从植株下方取回的幼虫比例也无显著差异——首个遭遇幼虫的红蚁属工蚁会将其带回，当两种红蚁共存时，并不会偏向取回某一物种的幼虫。因此，龙胆植株上的虫卵种群分布，可作为进入各红蚁属蚁巢的末龄幼虫种群分布的精准替代指标。蓝灰蝶（M. rebeli）的成虫种群数量通过以下方式估算：沿各调查点的分层样带记录羽化个体，并在确认蚁巢内存在空蛹壳后确定其所属蚁巢。额外数据则通过如下方式获取：沿前一年已知虫卵密度的分层样带，挖掘龙胆植株附近的所有红蚁属蚁巢，并统计其内所含的蛹的数量。目前尚未记录到红蚁属蚁巢内的蛹在羽化前出现死亡的情况。