The effect of dispersal on the population dynamics of a gall‐forming midge and its parasitoids

Abstract

Summary1. Theoretical studies predict that limited amounts of dispersal of individuals in host-parasitoid systems can both enhance the stability of the subpopulations and promote the co-existence of competing parasitoid species. We investigated the eects of dispersal on the population dynamics and parasitoid community structure of a natural host-multi-parasitoid system consisting of the midge Rhopalomyia californica that forms galls on the shrub Baccharis pilularis and the parasitoids that attack the midge.2. An experiment involving the release of midges into a ®eld with a low background density of galls demonstrated that the midges, on average, travelled approximately 1´7m in their lifetime. This suggests that the appropriate spatial scale at which to look at the eects of dispersal is relatively small. 3. Dispersal of midges and parasitoids between individual bushes was experimentally eliminated in a caging experiment. The midge populations in all of the uncaged replicates displayed dynamics that were similar to each other, while the dynamics of the midge populations in the caged replicates diverged. The midge dynamics on the uncaged bushes were not signi®cantly more stable than those on the caged bushes. 4. Dispersal among bushes was found to play a major role in co-existence of the competing parasitoid species. There was a dramatic drop in the parasitoid species diversity on the caged bushes, with only a single parasitoid species, Platygaster californica, persisting at high numbers in the caged populations. In accordance with theoretical models, P. californica is the parasitoid species in the community that has the highest attack rate and is most eective at searching for hosts in a restricted area. Alternative explanations for this pattern are discussed.

Complexity of leaf miner–parasitoid food webs declines with canopy height in Patagonian beech forests

“…Secondly, parasitoids may exhibit a reduced aggregative response at the scale of study (Heads & Lawton, ), perhaps due to limited dispersal from overwintering sites in the forest floor. Indeed, low dispersal capacity often leads to spatial aggregation of parasitoids irrespective of host densities (Godfray, ; Briggs & Latto, ). Lastly, vertical changes in canopy microclimate could affect the intensity of miner–parasitoid interactions (Basset et al ., ; Ulyshen, ).…”

Evolutionary radiation of Asteromyia carbonifera (Diptera: Cecidomyiidae) gall morphotypes on the goldenrod Solidago altissima (Asteraceae)

“…An allopatric scenario of divergence would be supported by significant geographic isolation of populations which would suggest limited dispersal of A. carbonifera . Cecidomyiid gall midges are generally considered to have limited capacities of dispersal associated with directed flight (Briggs & Latto, 2000), but passive dispersal with wind currents may be an important means of dispersal (Pendleton & Teetes, 1994) and long distance (e.g. > 1 km) dispersal of cecidomyiids is an established fact (Yukawa & Rohfritsch, 2005).…”

Gall size determines the structure of theRabdophaga strobiloideshost–parasitoid community

“…Gall‐forming insects and their natural enemies have played an important role in testing general theories in insect population dynamics such as the regulation of host abundance, and the effect of dispersal on host–parasitoid dynamics (Varley, 1947; Ehler & Kinsey, 1991; Dempster et al ., 1995a, b; Latto & Briggs, 1995; Price et al ., 1995; Briggs & Latto, 2000; Briggs & Latto, 2001). In the models used to describe these communities, variation in susceptibility of individual galls to parasitism or predation has largely been omitted despite the fact that this factor is related to gall size (Weis et al ., 1985; Plakidas & Weis, 1994; Redfearn & Cameron, 1994), morphology (Plantard & Hochberg, 1998), or toughness (Craig et al ., 1990).…”