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Density-dependent reductions in grasshopper fecundity in response to nematode parasitism

Published online by Cambridge University Press:  02 April 2012

Angela Nardoni Laws
Angela Nardoni Laws
Affiliation:
Department of Biology, University of Notre Dame, Notre Dame, IN 46556, United States of America (e-mail: alaws@ksu.edu)
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Abstract

Grasshoppers (Orthoptera: Acrididae) are hosts to many internal parasites, including nematodes. However, the effects of nematode parasitism on grasshopper fecundity and how these effects vary with population density are unclear. I report on the fecundity of Melanoplus dawsoni (Scudder) infected and uninfected with mermithid nematodes in northern Wisconsin from 2002 to 2005. Each year grasshoppers were stocked over a range of five densities into field enclosures. Fecundity, body size, and nematode prevalence were measured for female grasshoppers. Nematode prevalence was moderately high, ranging from 15% in 2003 to 37% in 2004. Fecundity was measured by examining grasshopper reproductive tracts. On average, past reproductive activity (number of eggs laid) and current reproductive activity (number of eggs forming) of parasitized grasshoppers were reduced by 40% and 48%, respectively. Interestingly, the reduction in fecundity was less for parasitized females in the low-density treatments (25%-50% of field density) than for those in the moderate- and high-density treatments, suggesting that grasshoppers can compensate somewhat for negative effects of parasites on fecundity when per-capita resources are high. No difference in hind-femur length between parasitized and unparasitized females was observed, indicating that nematode infection did not affect grasshopper body size.

Résumé

Les criquets (Orthoptera: Acrididae) servent d'hôtes à de nombreux parasites internes et, en particulier à des nématodes. Il n'est cependant pas clair comment le parasitisme par les nématodes affecte la fécondité des criquets, ni comment ces effets varient en fonction de la densité de population. On trouvera ici des informations sur la fécondité de Melanoplus dawsoni (Scudder) infectés et non infectés par les nématodes dans le nord du Wisconsin de 2002 à 2005. À chaque année pendant cinq ans, des criquets ont été gardés dans des enclos de terrain présentant cinq densités différentes. La fécondité, la taille corporelle et la fréquence des nématodes ont été déterminées chez les femelles adultes. La fréquence des nématodes était modérée, variant de 15 % en 2003 à 37 % en 2004. La fécondité a été déterminée à l'examen du système reproducteur. En moyenne, l'activité reproductive passée (œufs pondus) et l'activité présente (œufs en formation) sont réduites de respectivement 40 % et 48 % chez les criquets parasités. Fait intéressant, la réduction de la fécondité des femelles parasitées est moins importante chez les criquets dans les conditions expérimentales de faible densité (25 % – 50 % de la densité en nature) que chez les criquets dans les conditions expérimentales de moyenne ou de forte densité; cela laisse croire que les criquets peuvent compenser dans une certaine mesure les effets négatifs des parasites sur la fécondité lorsque les ressources par individu sont abondantes. Il n'existe pas de différence de longueur du fémur entre les femelles parasitées et non parasitées, ce qui indique que l'infection à nématodes n'affecte pas la taille corporelle des criquets.

[Traduit par la Rédaction]

Type
Articles
Information
The Canadian Entomologist , Volume 141 , Issue 4 , August 2009 , pp. 415 - 421
Copyright
Copyright © Entomological Society of Canada 2009

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