Spatial population biology of parasitoids

Our long-term monitoring of hosts and parasitoids in the Åland islands has led to advances in the general understanding of spatial aspects of host-parasitoid population dynamics, beyond what has been found in any other natural host-parasitoid system. For instance, how parasitoid metapopulation dynamics are influenced by behavior, habitat connectivity, plant distribution, hyperparasitism, temperature mediated phenology, and more recently a plant pathogenic fungus that influences parasitoid sex ratio and large scale population dynamics.

 

van Nouhuys, S. & R. Kaartinen 2008. A parasitoid wasp uses landmarks while monitoring potential resources. Proceedings of the Royal Society B, 275: 377-385 PDF van Nouhuys, S. and Laine, A-L. 2008 Population dynamics and sex ratio of a parasitoid altered by fungal infected diet of host butterfly. Proceedings of the Royal Society B, 275: 787-795 PDF Elzinga, J. A., van Nouhuys, S., van Leeuwen, D. J., Biere, A. 2007. Distribution and colonization ability of three parasitoids and their herbivorous host in a fragmented landscape. Basic and Applied Ecology, 8: 75-88 PDF Kankare, M., van Nouhuys, S., Gaggiotti, O., Hanski, I. 2005. Metapopulation genetic structure of two coexisting parasitoids of the Glanville fritillary butterfly. Oecologia, 143: 77-84 PDF van Nouhuys, S. 2005. Effects of habitat fragmentation at different trophic levels in insect communities. Annales Zoologici Fennici, 42: 433–447 PDF van Nouhuys, S., & G. C. Lei. 2004. Parasitoid and host metapopulation dynamics: the influence of temperature mediated phenological asynchrony. Journal of Animal Ecology, 73: 526-535 PDF van Nouhuys, S., & I. Hanski. 2002. Colonization rates and distances of a host butterfly and two specific parasitoids in a fragmented landscape. Journal of Animal Ecology, 71: 639-650 PDF van Nouhuys, S. & W.T. Tay. 2001. Causes and consequences of small population size for a specialist parasitoid wasp. Oecologia, 128: 126-133 PDF van Nouhuys, S. & I. Hanski. 1999. Host diet affects extinctions and colonizations in a parasitoid metapopulation. Journal of Animal Ecology, 68: 1248-1258 PDF

Following are brief descriptions of three of our current projects:

Host-parasitoid dynamics of the Glanville fritillary and Cotesia melitaearum
In order to follow the population dynamics of the parasitoid C. melitaearum we conduct a survey each spring. Previously we have analyzed the association of parasitoid population dynamics with explanatory factors such as connectivity, population size, and plant species using linear regression statistical models. Over the next few years we would like to take other analytical approaches, and address other factors, such as climate change.

Sex ratio variation in parasitoid populations
Parasitoid populations can differ in sex ratio (proportion of individuals male) because Hymenoptera are haplodiploid, and the mother wasps can control the sex of each egg that is laid. Local mate competition predicts that if individuals mate locally then a mother should produce as few males as possible to assure that her daughters are mated. We predict that population size and connectivity affect parasitoid sex ratio such that the fraction male will decrease with decreasing host population size and connectivity. This is because in small and isolated populations most progeny will mate with close relatives. We expect the effect of habitat isolation to be weak for H. horticola because it is mobile and mating is delayed, and stronger for C. melitaearum which is less mobile and has been observed to sib mate, and M. stigmaticus because it is at a high trophic level so experiences a more fragmented landscape. We are using field survey and a large scale field experiment to test these predictions.

Related papers:

van Nouhuys, S. and Laine, A-L. 2008 Population dynamics and sex ratio of a parasitoid altered by fungal infected diet of host butterfly. Proceedings of the Royal Society B 275: 377-385 PDF

Fox L. R., D. K. Letourneau, J. Eisenbach & S. van Nouhuys. 1990 Parasitism rate and sex ratios of a parasitoid wasp: Effects of herbivore and plant quality Oecologia, 83: 414-419 PDF

Joint host-parasitoid-hyperparasitoid interaction
One generally thinks of rate of parasitism as an outcome of the interaction between a host and parasitoid. But the parasitoid is actually sandwiched between the host and its own secondary parasitoids. Thus, how a parasitoid distributes progeny among hosts may be explained in part by the parasitoids relationship with the secondary parasitoid. About 30% of the host eggs in almost every Glanville fritillary egg cluster in Åland are parasitized by H. horticola. Each egg cluster is parasitized by an single female. Preliminary data suggests that secondary parasitism by M. stigmaticus is positively density dependent at the scale of butterfly larval group. Perhaps the rate of primary parasitism must be be low enough to prevent high secondary parasitism, but high enough to allow persistence given the host metapopulation dynamics. To test this idea we are experimentally measuring the degree and spatial scale of density dependence of hyperparasitism, and constructing quantitative models of the interaction.