Abstract
BackgroundInsects 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.ResultsAlthough 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.ConclusionsResistance 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.
Highlights
Insects can resist parasites using the costly process of melanotic encapsulation
The factors that cause variation in parasitism and host resistance in natural populations have important implications for the ecology and evolution of both hosts and parasites. Characteristics such as host population size have long been known to affect parasite population dynamics [1], and attention has recently turned to the effects of host sex and age on parasitism and resistance
We examined host resistance to a specialist mite in a damselfly population over seven seasons to determine whether variation in abiotic and biotic factors was correlated with variation in resistance
Summary
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. The factors that cause variation in parasitism and host resistance in natural populations have important implications for the ecology and evolution of both hosts and parasites. The pro-phenoloxidase enzymatic cascade kills the parasite through the synthesis of melanin, which has cytotoxic and antimicrobial properties [10,11] This type of resistance has been shown to vary due to biotic [10,11] and abiotic [12,13,14,15] factors, and can have a heritable component [12,16,17]. Our knowledge of the genetic basis of resistance in animals is increasing [18,19,20,21], but the degree to which resistance can be a plastic response to environmental variation in natural populations is not well understood
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