The functions of plant volatiles in host-searching behaviors of parasitoid wasps have been noticed by entomologists a long time ago. Since the 1930s, it had been suggested that parasitoid wasps utilize plant volatiles to locate their hosts, and numerous evidences indicated the attraction of parasitoid wasps to odors of great variety of plants (Whiteman 1988). In this seminar, I briefly review the research history of plant volatiles in the field of parasitoid wasp chemical ecology and summarize the functions of intact plant volatiles (IPVs), which have been neglected for a long time compared to herbivore-induced plant volatiles (HIPVs), on host-searching behaviors of parasitoid wasps.
At a very early age, a number of chemical components in IPVs were analyzed and found to be attractive to parasitoid wasps. For instance, Heydenia unica Cook & Davis (Hymenoptera: Pteromalidae), a larval parasitoid wasp of the southern pine beetle Dendroctonus frontalis Zimmerman (Coleoptera: Scolytidae), was attracted to α-pinene, a major terpene component of the host southern pine plants (Camors and Payne 1972).
However, in the early 1990s, it was discovered that many of the volatile compounds produced by plants that can be used as cues by natural enemies of herbivores were inducible and only released by plants after the plants had been damaged by herbivores. These compounds were then termed herbivore-induced plant volatiles (HIPVs) (Hare 2011). The first report of HIPVs that attracts parasitoid wasps was published in 1990 by Turlings et al.. In this study, The corn (Zea mays L., var. “Ioana sweet corn”) seedlings significantly emitted large amounts of several terpenoids after the damage of beet armyworm Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae) larvae. Furthermore, females of the parasitoid wasp Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae) learned to take advantage of those induced volatiles to locate beet armyworms as their hosts.
Since Turlings’ first finding of HIPVs’ attraction to parasitoid wasps, they have caught most of entomologists’ eyes in this field for the past three decades. However, more and more evidence also indicated that other plant volatiles, such as IPVs, also have important functions for parasitoid wasps. Certainly, IPVs were proved to be attractive to parasitoid wasps even before the findings of HIPVs. They have also been proved to have some other functions working together (or against) with HIPVs.
First of all, some studies indicated that HIPVs were not attractive to parasitoid wasps unless they were put together with IPVs. In this case, IPVs were suggested to act as background volatiles for HIPVs (Schröder and Hilker 2008). For instance, a synthetic mixture of volatiles composed of constituents of the headspace of spruce logs (Picea abies) infested by bark beetles Ips typographus L. (Coleoptera: Scolytidae) was only attractive for bark beetle parasitoid Coeloides bostrichorum Giraud (Hymenoptera: Braconidae) when this mixture was offered at the background of odors from intact logs (Pettersson et al. 2001). Another example is that the sesquiterpene (E)-β-farnesene emitted by Scots pine (Pinus sylvestris L.) induced by Egg deposition of the herbivore sawfly Diprion pini L. (Hymenoptera: Diprionidae) was only attractive to the egg parasitoid Chrysonotomyia ruforum Krausse (Hymenoptera: Eulophidae) when it was offered in combination with the volatile blend emitted from intact pine twigs (Mumm and Hilker 2005).
The second category is that IPVs can deter the attractiveness of HIPVs in some tritrophic systems. A recent study showed that the volatiles of intact maize plants has deterrent effects on host-searching behaviors towards HIPVs emitted from maize infested by the oriental armyworm Mythimna separata (Walker) (Lepidoptera: Noctuidae) in the parasitoid wasp Cotesia kariyai (Watanabe) (Hymenoptera: Braconidae) (Isono et 5al., 2020). Parasitoid wasps were attracted to the infested plants and plant models with HIPVs, but were less attracted to the combination of intact plants and HIPVs.
In conclusion, the functions of IPVs seem to be more important for parasitoid wasps than it has been thought. They could be the background to enhance the attractiveness of HIPVs or deterrent against the attractiveness of HIPVs. Therefore, we should pay more attention not only to HIPVs but also to IPVs, especially in their potential applications in biological control programs.
References
D.W. Whitman, Plant natural products as parasitoid cuing agents. H.G. Cuttler (Ed.), Biologically Active Natural Products Potential Use in Agriculture, A.C.S. Publication, Washington, DC, USA (1988), pp. 387-396
Camors Jr, F. B., & Payne, T. L. (1972). Response of Heydenia unica (Hymenoptera: Pteromalidae) to Dendroctonus frontalis (Coleoptera: Scolytidae) pheromones and a host-tree terpene. Annals of the Entomological Society of America, 65(1), 31-33.
Hare, J. D. (2011). Ecological role of volatiles produced by plants in response to damage by herbivorous insects. Annual Review of Entomology, 56, 161-180.
Turlings, T. C., Tumlinson, J. H., & Lewis, W. J. (1990). Exploitation of herbivore-induced plant odors by host-seeking parasitic wasps. Science, 250(4985), 1251-1253.
Schröder, R., & Hilker, M. (2008). The relevance of background odor in resource location by insects: a behavioral approach. Bioscience, 58(4), 308-316.
Pettersson, E. M., Birgersson, G., & Witzgall, P. (2001). Synthetic attractants for the bark beetle parasitoid Coeloides bostrichorum Giraud (Hymenoptera: Braconidae). Naturwissenschaften, 88(2), 88-91.
Mumm, R., & Hilker, M. (2005). The significance of background odour for an egg parasitoid to detect plants with host eggs. Chemical Senses, 30(4), 337-343.
Isono, K., Kuramitsu, K., & Kainoh, Y. (2020). Deterrent effects of intact plants on host-searching behavior of parasitoid wasps. Applied Entomology and Zoology, 55, 199–204.