Potential causes and consequences of decreased body size in Potential causes and consequences of decreased body size in field populations of field populations of Coccinella novemnotata Coccinella novemnotata
Insects use endogenous mechanisms and infection with protective symbionts to thwart attacks from natural enemies. Defenses that target specific enemies, however, potentially mediate competition between rivals and thereby impact community composition. Following its introduction to North America to control pea aphids (Acyrthosiphon pisum), the parasitoid Aphidius ervi competitively displaced other parasitoids, except for the native Praon pequodorum. The pea aphid exhibits tremendous clonal variation in resistance to A. ervi, primarily through infection with the heritable bacterial symbiont Hamiltonella defensa, although some symbiont-free aphid genotypes encode endogenous resistance. Interestingly, H. defensa strains and aphid genotypes that protect against A. ervi, provide no protection against the closely related, P. pequodorum. Given the specificity of aphid defenses, we hypothesized that aphid resistance traits may contribute to the continued persistence of P. pequodorum. We conducted multiparasitism assays to determine whether aphid resistance traits mediate internal competition between these two solitary parasitoid species, but found this was not the case; P. pequodorum was the successful internal competitor across lines varying in susceptibility to A. ervi. Next, to determine whether resistance traits influence competitive interactions resulting in the stable persistence of P. pequodorum, we established replicated cages varying in the proportion of resistant aphids and recorded successful parasitism for each wasp species over time. As expected, A. ervi outcompeted P. pequodorum in cages containing only susceptible aphids. However, P. pequodorum not only persisted, but was the superior competitor in populations containing any proportion (20–100%) of resistant aphids (20–100%). Smaller scale, better replicated competition cage studies corroborated this finding, and no-competition and behavioral assays provide insight into the processes mediating competition. Genetic variation, including that acquired via infection with protective symbionts, may provide a supply of hosts susceptible only to particular enemies, mediating competition with effects on community richness and stability.
European frogbit (Hydrocharis morsus-ranae L.) is an invasive floating plant that negatively affects native plants in freshwater ecosystems. We conducted a field mesocosm experiment in Oneida Lake, New York (USA), to study the effect of frogbit cover on macroinvertebrates and to test the effects of 2 physical control methods, hand pulling and shading, on macroinvertebrate assemblages. We used Hester-Dendy samplers and Ekman grabs to collect surface/phytophilous and benthic macroinvertebrates. Metrics including density of different types of organisms, total abundance, taxon richness, and Simpson's diversity index were compared between control methods. Frogbit cover had significant positive effects on mollusk density, amphipod density, taxon richness, and Simpson's diversity in the Hester-Dendy surface samples. Sites with frogbit had significantly fewer benthic worms and more chironomids (Hester-Dendy bottom samples) and possibly higher diversity of benthic macroinvertebrates (Ekman samples). These data suggested that frogbit had positive effects on surface and phytophilous macroinvertebrates and may change population density and increase diversity of benthic macroinvertebrates. Hand pulling and shading did not affect macroinvertebrates except that amphipod density in surface water was significantly lower in shading sites. Therefore, from the perspective of minimizing effects on the macroinvertebrate communities in the lakes, hand pulling is preferable over shading for controlling frogbit.
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