Department of Biology, Carleton University, Ottawa, Canada.
BMC Ecol. 2010 Feb 14;10:5. doi: 10.1186/1472-6785-10-5.
Insects can resist parasites using the costly process of melanotic encapsulation. This form of physiological resistance has been studied under laboratory conditions, but the abiotic and biotic factors affecting resistance in natural insect populations are not well understood. Mite parasitism of damselflies was studied in a temperate damselfly population over seven seasons to determine if melanotic encapsulation of mite feeding tubes was related to degree of parasitism, host sex, host size, emergence timing, duration of the emergence period, and average daily air temperature.
Although parasite prevalence in newly emerged damselflies was > 77% each year, hosts did not resist mites in the early years of study. Resistance began the year that there was a dramatic increase in the number of mites on newly emerged damselflies. Resistance continued to be correlated with mite prevalence and intensity throughout the seven-year study. However, the percentage of hosts resisting only ranged from 0-13% among years and resistance was not sex-biased and was not correlated with host size. Resistance also was not correlated with air temperature or with timing or duration of damselfly emergence.
Resistance in host damselflies was weakly and variably expressed over the study period. Factors such as temperature, which have been identified in laboratory studies as contributing to resistance by similar hosts, can be irrelevant in natural populations. This lack of temperature effect may be due to the narrow range in temperatures observed at host emergence among years. Degree of mite parasitism predicted both the appearance and continued expression of resistance among parasitized damselflies.
昆虫可以通过代价高昂的黑色素包裹过程来抵抗寄生虫。这种形式的生理抗性已经在实验室条件下进行了研究，但在自然昆虫种群中影响抗性的非生物和生物因素还不是很清楚。在七个季节中，对温带蜻蜓种群中的螨虫寄生进行了研究，以确定黑色素包裹螨虫取食管是否与寄生程度、宿主性别、宿主大小、羽化时间、羽化期持续时间和平均日气温有关。
尽管每年新羽化的蜻蜓寄生虫的患病率都超过 77%，但在研究的早期，宿主并没有抵抗螨虫。抵抗始于当年新羽化的蜻蜓上螨虫数量急剧增加的那一年。在七年的研究中，抗性一直与螨虫的流行程度和强度相关。然而，抵抗的宿主百分比在不同年份之间仅在 0-13%之间波动，且这种抵抗没有表现出性别偏向，也与宿主大小无关。抗性也与空气温度或蜻蜓羽化的时间或持续时间无关。
在研究期间，宿主蜻蜓的抗性表现较弱且不稳定。在类似的宿主的实验室研究中确定的温度等因素在自然种群中可能并不相关。这种缺乏温度影响的现象可能是由于每年宿主羽化时观察到的温度范围较窄所致。螨虫寄生程度预测了受寄生的蜻蜓中抗性的出现和持续表达。
