Aphids Facing Their Parasitoids: A First Look at How Chemical Signals May Make Higher Densities of the Pea Aphid Acyrthosiphon pisum Less Attractive to the Parasitoid Aphidius ervi

Abstract

Herbivore-induced plant volatiles constitute the first indicators of insect host presence, and these can affect the foraging behavior of their natural enemies. The density of insect hosts may affect the nature and concentration of these plant-induced volatiles. We tested the impact of infestation density (low, intermediate, and high) of the pea aphid, Acyrthosiphon pisum (Homoptera: Aphididae), feeding on the broad bean Vicia faba, on the attractiveness of the parasitoid Aphidius ervi (Hymenoptera: Braconidae), using a Y-tube olfactometer (infested vs. non-infested plants). The emitted volatile compounds from both infested and non-infested plants were collected and identified. In addition, two series of experiments were carried out to test the impact of the presence of a conspecific female parasitoid within the aphid/plant complex on the attractiveness to other females. Parasitoids were significantly more attracted to the plants with low and intermediate aphid infestation levels. The volatile blend composition of the infested plants changed in relation to aphid density and may explain the low attraction of parasitoids toward high aphid density. The presence of conspecific females on the aphid patch had no apparent impact on the behavioral choices of other parasitoid females. Our study adds a new aspect to understanding plant–aphid–parasitoid interactions, including the possibility that aphids may manipulate chemical cues of host plants affecting the orientation of parasitoids.

Small volatile lipophilic molecules induced belowground by aphid attack elicit a defensive response in neighbouring un-infested plants

“…These three SVLMs are released upon aphid infestation in different plant-aphid complexes including V. faba. (Quiroz et al, 1997;Liu et al, 2009;Markovic et al, 2014;Dong et al, 2015;Takemoto and Takabayashi, 2015;Lin et al, 2017;Ismail et al, 2021;Higashida et al, 2022). Three results of the behavioural bioassay can be considered important: the indication of the active enantiomers of two identified SVLM (sulcatol, octen-ol); each SLVM elicited a significant higher response in terms of attraction of the natural enemy in respect of control (and non-active enantiomers); a synergistic effect was recorded when adding all 3 SVLMs.…”

Behavior of higher trophic levels associated with an invasive plant varies among populations

“…The odors of infested plants strongly affect foraging behavior of parasitoids, and consequently, the decisions on which plants they choose to visit. It is evident that the odors released from infested plants are used as key cues and play a pivotal role in foraging behavior by parasitoids to find their insect hosts (Turlings et al 1990, War et al 2012, de Rijk et al 2013, Ismail et al 2021). Here, we show that populations of invasive populations differ from native populations in their attractiveness to parasitoids and, for the first time ever, their attractiveness to a hyperparasitoid as well.…”

“…In insects, individuals of each trophic level, ranging from herbivorous insects to parasitoids and hyperparasitoids, forage for their hosts in the same environment. For all these foragers, the increase in quantity of chemical cues emitted from the infested plants (the first trophic level) is the first cue used to localize their hosts over long distances (Bruinsma et al 2009, Aartsma et al 2019, Ismail et al 2021). The interactions among multi-trophic levels have been studied largely to understand the foraging behavior and evolution of fitness traits of parasitoids and hyperparasitoids (Holopainen and Blande 2012, Poelman and Kos 2016).…”

Scents and sensibility: Best practice in insect olfactometer bioassays

“…It is possible to estimate the appropriate number of replicates needed for an experiment through power analysis (Ismail et al, 2021). This is a method for determining the number of replicates to detect an effect of a given size with a certain degree of assurance.…”