DOI: 10.5061/dryad.02v6wwq1p
title: Parasitoid-mediated indirect interactions between unsuitable and suitable hosts can generate apparent predation in microcosm and modeling studies

description: 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.
description: Experiment 1: Parasitoid-mediated indirect interactions between
 milkweed aphid and soybean aphid Experiment 1.1:
 Parasitoid oviposition In the no-choice assay, one
 leaf of soybean or milkweed (leaves of the two plants were chosen to be of
 similar size) was cut at the stem and separately placed into a tube
 (diameter: 2.5 cm, h: 7cm) with 1 cm of watered sand and closed by a
 perforated cap. Each leaf was then infested with 20
 3<sup>rd</sup> instar A. glycines (20Ag) or A. nerii (20An) on
 soybean and milkweed, respectively (n=25-27 replicates).
 In the choice assay, one leaf of soybean and milkweed was cut and
 placed into the same tube in the same conditions as in the no-choice
 assay. The appropriate leaves were then infested with 10 aphids of A.
 glycines and A. nerii (10Ag+10An) (n=25 replicates) or 20 aphids of each
 species (20Ag+ 20An) onto their respective plant (n=25 replicates). The
 two densities enabled us to evaluate the impact of aphid abundance on the
 potential egg sink caused by the presence of the unsuitable host. One
 mated female parasitoid (between 24 and 48 hour-old) was then introduced
 into the tube of both the no-choice and choice assays for 24
 hours. After parasitoid removal in both no-choice and
 choice assays, aphids were frozen and later dissected at 40x magnification
 to record the number of eggs laid by the parasitoid into individuals of
 each aphid species. To determine whether these female parasitoids
 experienced egg limitation during the assays, 12 randomly chosen females
 from each of the four different treatments were frozen after they were
 removed from the arena and later dissected at 40x magnification to count
 the number of remaining eggs. To make sure there was no systematic bias in
 size of adults among the treatment, the right hind tibia length was also
 measured. Experiment 1.2: Parasitoid developmental
 success and offspring fitness In the no-choice assay,
 one plant of soybean (10 days old) or milkweed (21 days old) of
 approximately the same size was potted individually (depth: 13.5 cm, top
 diameter: 14 cm) and covered in a cylindrical plastic cover (diameter: 11
 cm, h: 21cm) with several 3cm wide mesh covered holes cut in the sides and
 on the top. Potting soil was covered with white plaster to prevent the
 development of fungus gnats. The plants were then infested with 50
 3<sup>rd</sup> instar A. glycines (50Ag) or A. nerii (50An) on
 soybean and milkweed, respectively (n = 18 and 33 plants,
 respectively). In the choice assay, single soybean and
 milkweed plants were potted together under the same conditions as in the
 no-choice assay. The appropriate plants were then infested with 25 aphids
 of each species (25Ag+25An) (n=27 replicates) or 50 aphids of each species
 (50Ag+50An) onto their respective plant (n=26 replicates). As in
 Experiment 1.1, the two different densities enabled us to determine the
 impact of aphid abundance on the potential egg sink caused by the presence
 of the unsuitable host. One mated female parasitoid (between 24 and
 48-hours-old) was then introduced into the covered pots for 24
 hours. In both the no-choice and choice assays, plants
 were inspected 10 days later and all mummies were removed and isolated
 into 0.6 mL micro centrifuge tubes. The total number of mummies and
 emerged parasitoids were counted and the adult offspring sex ratios were
 recorded. To determine the effect of the treatments on egg limitation in
 A. certus, 46 (n for each of the 4 treatments was 11 or 12) were frozen
 after the 24 hours of the experiment and later dissected at 40x
 magnification to count the number of remaining eggs and measure the right
 tibia length. Experiment 2: Impact of parasitism on
 milkweed aphid In this experiment we evaluated
 mortality of the unsuitable host <i>A. nerii</i> induced by
 parasitism by <i>Aphelinus certus</i> using a combination of
 direct observation and rearing. To prepare for observations, a milkweed
 leaf was placed upside down within a clear plastic cube
 (1cm<sup>3</sup>) under a magnifying lens (8x). One second or
 third instar milkweed aphid<i> </i>was introduced onto the
 leaf within this cube using a fine brush. Five minutes later, one mated
 female parasitoid (between 24 and 48 hours old) was also introduced and
 observed until oviposition occurred (n=37). The same procedure was
 followed without introducing a parasitoid as a control to quantify
 background aphid mortality (n=33). Aphids parasitized or not were then
 placed separately onto milkweed leaves cut at the stem which were then
 introduced into a tube (diameter: 2.5 cm, h: 7cm) with 1 cm of watered
 sand at the base (as described above) and closed with a perforated plastic
 cap. The aphids were observed daily to record their longevity, fecundity
 and the period of time before the first nymphs were laid. In order to
 record daily fecundity, newly produced nymphs were removed every
 day.