Parasitoid-mediated indirect interactions between unsuitable and suitable hosts can generate apparent predation in microcosm and modeling studies

Parasitoids used as biological control agents often parasitize more than a
single host species and these hosts tend to vary in suitability for
offspring development. The population dynamics of parasitoids and hosts
may be altered by these interactions, with outcomes dependent on the
levels of suitability and acceptance of both host species. Parasitism of
individuals of an unsuitable host species may indirectly increase
populations of a suitable host species if eggs laid into unsuitable hosts
do not develop into adult parasitoids. In this case the unsuitable host is
acting as an egg sink for parasitoids and this can reduce parasitism of
suitable hosts under conditions of egg limitation. We studied
parasitoid-mediated indirect interactions between two aphid hosts, Aphis
glycines (the soybean aphid) and A. nerii (the milkweed, or oleander
aphid), sharing the parasitoid Aphelinus certus. While both of these aphid
species are accepted by A. certus, soybean aphid is a much more suitable
host than milkweed aphid is. We observed a drastic reduction of parasitoid
offspring production (45%) on the suitable host in the presence of the
unsuitable host in microcosm assays. Aphelinus certus females laid eggs
into the unsuitable hosts (Aphis nerii) in the presence of the suitable
host leading to egg and/or time limitation and reduced fitness. The impact
of these interactions on the equilibrium population sizes of the three
interacting species was analyzed using a consumer-resource modeling
approach. Both the results from the laboratory experiment and the modeling
approaches identified apparent predation between soybean aphid and
milkweed aphid, in which milkweed aphid acts as a sink for parasitoid eggs
leading to an increase in the soybean aphid population. The presence of
soybean aphids had the opposite effect on milkweed aphid populations as it
supported increases in parasitoid abundance and thus reduced the fitness
and abundance of this aphid species.