Blacklegged ticks, Ixodes scapularis, reduce predation risk by eavesdropping on communication signals of Formica oreas thatching ants

Ticks spend most of their life inhabiting leaf litter and detritus where they are protected from sun but preyed upon by ants. Ants secrete chemical communication signals to coordinate group tasks such as nest defense. Ticks that avoid ant semiochemicals – as indicators of ant presence – would reduce predation risk by ants. We tested the hypotheses that (1) chemical deposits from the thatching ant Formica oreas deter blacklegged ticks, Ixodes scapularis, (2) deterrent semiochemicals originate from the ants' poison and/or Dufour's gland(s), and (3) tick-deterrent semiochemicals serve as alarm-recruitment pheromone components in F. oreas. In two-choice olfactometer bioassays, filter paper soiled with ant chemical deposits significantly deterred female and male ticks. Poison and Dufour's gland extracts deterred ticks in combination but not alone. Gas chromatographic-mass spectrometric analyses of gland extracts revealed formic acid as the major constituent in the poison gland and 8 hydrocarbons as constituents in the Dufour's gland. Synthetic formic acid and hydrocarbons deterred ticks only when combined. F. oreas workers sprayed both formic acid and hydrocarbons when distressed. A synthetic blend of these compounds elicited alarm-recruitment responses by F. oreas in behavioural bioassays. All results combined indicate that ticks eavesdrop on the ants' communication system.

蜱虫（tick）的一生大多栖息于落叶凋落物与腐殖质中，此处既可躲避阳光直射，却也会遭受蚂蚁的捕食。蚂蚁会分泌化学通讯信号以协调群体任务，例如巢穴防御。能够规避指示蚂蚁存在的蚂蚁信息化合物（semiochemicals）的蜱虫，可降低被蚂蚁捕食的风险。
本研究验证了三项假说：其一，茅草蚁（thatching ant）*Formica oreas*的化学沉积物可驱避黑腿蜱（blacklegged tick）肩突硬蜱（*Ixodes scapularis*）；其二，具有驱避活性的信息化合物源自蚂蚁的毒腺（poison gland）和/或杜氏腺（Dufour's gland）；其三，蜱虫驱避型信息化合物是*F. oreas*警报招募信息素（alarm-recruitment pheromone）的组分。
在双选择嗅觉仪生物测定实验中，沾染了蚂蚁化学沉积物的滤纸片可显著驱避雌雄两性蜱虫。毒腺与杜氏腺提取物单独使用时无法驱避蜱虫，但二者联合施用时则具备显著驱避效果。
通过气相色谱-质谱联用分析腺体提取物，研究发现毒腺的主要成分为甲酸（formic acid），杜氏腺则含有8种烃类化合物。人工合成的甲酸与烃类化合物仅在联合使用时，才能对蜱虫产生驱避作用。
当*F. oreas*工蚁受到惊扰时，会同时释放甲酸与烃类化合物。行为生测实验显示，由这两类化合物组成的人工合成混合物，可引发*F. oreas*的警报招募反应。
综合所有实验结果表明，蜱虫会窃听蚂蚁的通讯系统。