Olfactory Attraction of the Larval Parasitoid,Hyposoter horticola, to Plants Infested with Eggs of the Host Butterfly,Melitaea cinxia

Abstract

Parasitoids locate inconspicuous hosts in a heterogeneous habitat using plant volatiles, some of which are induced by the hosts. Hyposoter horticola Gravenhost (Hymenoptera: Ichneumonidae) is a parasitoid of the Glanville fritillary butterfly Melitaea cinxia L. (Lepidoptera: Nymphalidae). Melitaea cinxia lays eggs in clusters on leaves of Plantago lanceolata L. (Lamiales: Plantaginaceae) and Veronica spicata L. (Lamiales: Plantaginaceae). The parasitoid oviposits into host larvae that have not yet hatched from the egg. Thus, though H. horticola is a parasitoid of Melitaea cinxia larvae, it must find host eggs on plants that have not been fed on by the larvae. Using a Y-tube olfactometer, the response of H. horticola to odors of Melitaea cinxia and extracts of the attacked plant species were tested. Three week-old eggs (near hatching) were attractive to young H. horticola, but one week-old eggs were attractive only to old or experienced H. horticola. Melitaea cinxia larvae were not attractive. A water extract of P. lanceolata was attractive, but ethanol or hexane extracts were not. None of the extracts of V. spicata were attractive. Leaves of V. spicata were attractive only if harboring eggs, but P. lanceolata leaves with eggs were not. Free flying H. horticola in a large outdoor enclosure were presented with host and plant cues. As in the olfactometer, V. spicata was attractive only when eggs were on it, and P. lanceolata was somewhat attractive with or without eggs. This study shows for the first time that a parasitoid of larvae uses egg volatiles or oviposition-induced plant volatiles, to find host larvae, and that Melitaea cinxia eggs or traces of oviposition induce the production of these volatiles by the plant. Based on the results, and given the natural distribution of the plants and M. cinxia eggs, parasitism of Melitaea cinxia eggs on P. lanceolata would be expected to be low. Instead, under natural conditions, a fraction of the eggs in virtually all egg clusters are parasitized on both plant species. The mismatch between the experimental results and the natural pattern of host-parasitoid interactions is discussed in terms of the expected coupling foraging cues with foraging success.

The roles of foraging environment, host species, and host diet for a generalist pupal parasitoid

“…We found that they retained IGs as pupae, which has been shown in other herbivores adapted to IGs (Bowers & Puttick, 1986). ), and the two specialist parasitoids of M. cinxia larvae are more attracted to hosts on V. spicata than to hosts on P. lanceolata (van Nouhuys & Hanski, 2004;Castelo et al, 2010). Catalpol is the more toxic and (Puttick & Bowers, 1988;Bowers & Puttick, 1989;Bowers, 1991), and catalpol has been shown to have a negative effect on some parasitoids (Nieminen et al, 2003;Singer & Stireman, 2003;Laurentz et al, 2012).…”

“…Plantago lanceolata and V. spicata have different volatile emissions (JH Reudler, D Mofikoya, JK Holopainen & S van Nouhuys, unpubl. ), and the two specialist parasitoids of M. cinxia larvae are more attracted to hosts on V. spicata than to hosts on P. lanceolata (van Nouhuys & Hanski, 2004;Castelo et al, 2010). We found no direct effect of plant species on whether a pupa became parasitised by P. apum in the field, where the attractiveness of the plant and the accessibility of the host were part of the experiment.…”

Wolbachiaincreases the susceptibility of a parasitoid wasp to hyperparasitism

“…A foraging hyperparasitoid must first of all find M. cinxia caterpillar nests parasitized by H. horticola. Herbivory by M. cinxia causes the host plant to release volatile odors that lead H. horticola to their hosts (Castelo et al, 2010;Pinto-Zevallos et al, 2013). Such volatiles can also be attractive to hyperparasitoids (Zhu et al, 2014).…”

Attraction of Melitaea cinxia Butterflies to Previously-Attacked Hosts: A Likely Complement to Known Allee Effects?

“…However, because of the difference in experimental design between the experiments with P. lanceolata and V. spicata that gave significantly different results, we cannot conclude with significance that effects on the two hosts were different, although this is likely. Correspondingly, and potentially through the same mechanism of induced plant response, V. spicata becomes attractive to the specialist parasitoid wasp Hyposoter horticola after M. cinxia eggs are laid on it, but P. lanceolata does not (Castelo et al 2010).…”