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

The endosymbiont Serratia symbiotica improves aphid fitness by disrupting the predation strategy of ladybeetle larvae

Aphids, the important global agricultural pests, harbor abundant resources of symbionts that can improve the host adaptability to environmental conditions, also control the interactions between host aphid and natural enemy, resulting in a significant decrease in efficiency of biological control. The facultative symbiont Serratia symbiotica has a strong symbiotic association with its aphid hosts, a relationship that is known to interfere with host-parasitoid interactions. We hypothesized that Serratia may also influence other trophic interactions by interfering with the physiology and behavior of major predators to provide host aphid defense. To test this hypothesis, we investigated the effects of Serratia on the host aphid Acyrthosiphon pisum and its predator, the ladybeetle Propylaea japonica. First, the prevalence of Serratia in different A. pisum colonies was confirmed by amplicon sequencing. We then showed that harboring Serratia improved host aphid growth and fecundity but reduced longevity. Finally, our research demonstrated that Serratia defends aphids against P. japonica by impeding the predator's development and predation capacity, and modulating its foraging behavior. Our findings reveal that facultative symbiont Serratia improves aphid fitness by disrupting the predation strategy of ladybeetle larvae, offering new insight into the interactions between aphids and their predators, and providing the basis of a new biological control strategy for aphid pests involving the targeting of endosymbionts.

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

Invasive plants from their native and introduced ranges differ in their interactions with herbivores but it is not known whether they also vary in their interactions with herbivore natural enemies. Here, we used olfactometer bioassays and cage experiments to investigate how foraging behaviors of 2 parasitoid and 1 hyperparasitoid species depended on plant population origin. Triadica sebifera (Euphorbiaceae) is native to China but invasive in the United States. In China, it is fed on by a specialist noctuid Gadirtha fusca (Lepidoptera: Nolidae), which hosts a parasitoid Apanteles sp. (Hymenoptera: Microgastinae) and hyperparasitoid (Hymenoptera: Eurytomidae) plus a generalist aphid Toxoptera odinae (Homoptera: Aphidiidae) parasitized by Lysiphlebus confusus (Hymenoptera: Aphidiinae). Both parasitoids preferred plants infested by their host over herbivore-free plants in olfactometer bioassays. Apanteles sp. and Eurytomid wasps preferred G. fusca infested plants from China populations over those from US populations in olfactometer bioassays but L. confusus wasps did not discriminate between T. odinae infested plants from China vs. US populations. Similarly, G. fusca caterpillars on China population plants were more likely to be parasitized than ones on US population plants when they were in the same cage but odds of parasitism for T. odinae did not differ for those on China vs. US population plants. These results suggest that populations from the native and introduced ranges may differ in traits that impact higher trophic levels. This may have implications for successful control of invasive plants as biocontrol agents are introduced or herbivores begin to feed on them in their introduced ranges.

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

In pioneering studies on plant-aphid interactions, we have observed that Vicia faba plants infested by aphids can transmit signals via the rhizosphere that induce aboveground defence in intact, neighbouring plants. The aphid parasitoid Aphidius ervi is significantly attracted towards intact broad bean plants grown in a hydroponic solution previously harbouring Acyrtosiphon pisum-infested plants. To identify the rhizosphere signal(s) possibly mediating this belowground plant-plant communication, root exudates were collected using Solid-Phase Extraction (SPE) from 10-day old A. pisum-infested and un-infested Vicia faba plants hydroponically grown. To verify the ability of these root exudates to trigger defence mechanisms against the aphids we added them to V. fabae plants grown in hydroponic solution, and tested these plants in the wind-tunnel bioassay to assess their attractiveness towards the aphids' parasitoids A. ervi. We identified three small volatile lipophilic molecules as plant defence elicitors: 1-octen-3-ol, sulcatone and sulcatol, in SPE extracts of A. pisum-infested broad bean plants. In wind tunnel assays, we recorded a significant increase in the attractiveness towards A. ervi of V. faba plants grown in hydroponic solution treated with these compounds, compared to plants grown in hydroponic treated with ethanol (control). Both 1-octen-3-ol and sulcatol have asymmetrically substituted carbon atoms at positions 3 and 2, respectively. Hence, we tested both their enantiomers alone or in mixture. We highlighted a synergistic effect on the level of attractiveness towards the parasitoid when testing the three compounds together in respect to the response recorded against them singly tested. These behavioural responses were supported by the characterization of headspace volatiles released by tested plants. These results shed new light on the mechanisms underlying plant-plant communication belowground and prompt the use of bio-derived semiochemicals for a sustainable protection of agricultural crops.

Control of tea aphids via attracting the parasitic wasp, Aphelinus sp. with synthetic semiochemicals

The tea aphid (Toxoptera aurantii Boyer de Fonscolombe) is an important tea plant pest insect worldwide. The parasitoid wasp, Aphelinus sp., is one of the most important natural enemies of the tea aphid in China. Unfortunately, Aphelinus sp. alone cannot effectively control the outbreaks of the aphid under natural conditions. In this study, 27 volatile compounds from tea aphid-injured tea shoots, tea flowers, aphid sex pheromones, or body rinses were selected and tested in Y-tube olfactometer assays to find potential attractants of the parasitoid wasp, Aphelinus sp. Based on the Y-tube assay results, the following three attractant mixtures were formulated and further tested in the field. Attractant-1 (HIPV-based) included trans-2-hexenal (10−6 g/ml), β-ionone (10−6 g/ml), allyl isothiocyanate (10−4 g/ml), trans-2-pentenal (10−2 g/ml), and jasmone (10−2 g/ml) at equal loading volume of their solutions. Attractant-2 (with aphid sex-pheromone and body rinse compounds) included nepetalactone (10−6 g/ml), 2,5-hexanedione (10−4 g/ml), benzaldehyde (10−2 g/ml), eicosane (10−2 g/ml), and heptadecane (10−2 g/ml) at equal loading volume of their solutions. Attractant-3 (partial combination of Attractant-1 and Attractant-2) included nepetalactone (10−4 g/ml), benzaldehyde (10−2 g/ml), jasmone (10−2 g/ml), trans-2-hexenal (10−6 g/ml), eicosane (10−2 g/ml), and heptadecane (10−2 g/ml) at equal loading volume of their solutions. Field trials showed that Attractant-3 was much more attractive to the parasitic wasps than Attractant-1 and Attractant-2. From late August to late September the controlled release of Attractant-3 effectively attracted Aphelinus sp. to parasitize and colonize the aphid populations in the treated tea plantations, resulting in a progressive decrease of the tea aphid abundances/populations in the fall. The continued enhanced parasitism of overwintered aphids by Aphelinus sp. further reduced this population during the next spring tea harvest season. This approach may present an environmentally sound, non-insecticidal control tactic against tea aphids using synthetic semiochemicals.