Submitted to: Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 19, 2002
Publication Date: March 1, 2003
Citation: BRANSON, D.H. EFFECTS OF A PARASITE MITE ON LIFE-HISTORY VARIATION IN TWO GRASSHOPPER SPECIES. Evolutionary Ecology Research. 2003. V. 5. P. 397-409. Interpretive Summary: The importance of mites on grasshopper life history variation under natural abiotic and resource availability conditions has not been examined. There is a need for more studies examining how changes in reproductive allocation in response to parasitism fit theoretical predictions. I conducted a field experiment to examine the effects of ectoparasitic grasshopper mites on survival and reproductive allocation of two common grasshopper species. Total and initial reproduction decreased in parasitized individuals. Parasitized females of both species appeared to differentially allocate resources in response to parasitism. Under field conditions, reduced reproduction in parasitized individuals likely resulted from the inability of grasshoppers to increase their intake of food to compensate for the costs of being parasitized. The effect of parasitism on reproduction could have a large impact on grasshopper population dynamics. These results have implications for grasshopper population dynamics and possibly the implementation of biocontrol strategies, as smaller effects of mite parasitism on grasshopper population dynamics are predicted when only considering differences in survival.
Technical Abstract: Parasites often have large effects on the life history patterns of their hosts. There is a need to examine how observed life history patterns of parasitized organisms under field conditions fit theoretical predictions. I conducted a field experiment to examine the effects of an ectoparasitic grasshopper mite on survival and reproductive allocation in two grasshoppers with different life history characteristics, Melanoplus sanguinipes and Ageneotettix deorum. Proportional survival was lower in mite parasitized A. deorum during the period of mite parasitism, but not in M. sanguinipes. As predicted in response to a short lived parasite, females in both species had reduced initial and total reproduction. Egg production declined 39-44% with mite parasitism in the two species of grasshoppers studied. Parasitized females of both species completed development of a lower percentage of ovarioles initiating development. Future reproduction of A. deorum females was unaffected by parasitism. However, future reproduction of parasitized M. sanguinipes remained lower at the end of the experiment indicating parasitism had an effect on reproduction up to 40 days after mite parasites left M. sanguinipes females. There were no interactions between population density and mite parasitism on reproductive allocation or survival in M. sanguinipes. Parasitized females of both species appeared to differentially allocate resources in response to parasitism. The reduced reproduction in parasitized individuals likely resulted from the inability of grasshoppers to increase resource intake to compensate for the direct or indirect costs of parasitism.