Apparent Acquired Resistance by a Weevil to Its Parasitoid Is Influenced by Host Plant.

Stephen L Goldson,Federico Tomasetto

doi:10.3389/fpls.2016.01259

Abstract

Field parasitism rates of the Argentine stem weevil Listronotus bonariensis (Kuschel; Coleoptera: Curculionidae) by Microctonus hyperodae Loan (Hymenoptera: Braconidae) are known to vary according to different host Lolium species that also differ in ploidy. To further investigate this, a laboratory study was conducted to examine parasitism rates on tetraploid Italian Lolium multiflorum, diploid Lolium perenne and diploid hybrid L. perenne ×L. multiflorum; none of which were infected by Epichloë endophyte. At the same time, the opportunity was taken to compare the results of this study with observations made during extensive laboratory-based research and parasitoid-rearing in the 1990s using the same host plant species. This made it possible to determine whether there has been any change in weevil susceptibility to the parasitoid over a 20 year period when in the presence of the tetraploid Italian, diploid perennial and hybrid host grasses that were commonly in use in the 1990’s. The incidence of parasitism in cages, in the presence of these three grasses mirrored what has recently been observed in the field. When caged, weevil parasitism rates in the presence of a tetraploid Italian ryegrass host were significantly higher (75%) than rates that occurred in the presence of either the diploid perennial (46%) or the diploid hybrid (52%) grass, which were not significantly different from each other. This is very different to laboratory parasitism rates in the 1990s when in the presence of both of the latter grasses high rates of parasitism (c. 75%) were recorded. These high rates are typical of those still found in weevils in the presence of both field and caged tetraploid Italian grasses. In contrast, the abrupt decline in weevil parasitism rates points to the possibility of evolved resistance by the weevil to the parasitoid in the diploid and hybrid grasses, but not so in the tetraploid. The orientation of plants in the laboratory cages had no significant effect on parasitism rates under any treatment conditions suggesting that plant architecture may not be contributing to the underlying mechanism resulting in different rates of parasitism. The evolutionary implications of what appears to be plant-mediated resistance of L. bonariensis to parasitism by M. hyperodae are discussed.

Highlights

Over the last 25 years there has been increasing confidence that the impact of the Argentine stem weevil (Listronotus bonariensis) on Lolium-based pasture grasses has declined (Goldson et al, 2014a,b, 2015). This has largely been based on the use of selected strains of Epichloë endophytes that confer pest resistance in ryegrass (Johnson et al, 2013) combined with the significant impact of the braconid parasitoid biological control agent, Microctonus hyperodae (e.g., Barker and Addison, 2006)
There has been growing field evidence that M. hyperodae may be losing its efficacy as a biological control agent of L. bonariensis
An emphasis of this study was to determine whether there have been significant changes in parasitism rates of Argentine stem weevil by M. hyperodae on typical pasture grasses since the 1990s rather than it being a definitive study of grass type effect on parasitism levels

Summary

Introduction

Over the last 25 years there has been increasing confidence that the impact of the Argentine stem weevil (Listronotus bonariensis) on Lolium-based pasture grasses has declined (Goldson et al, 2014a,b, 2015) This has largely been based on the use of selected strains of Epichloë endophytes that confer pest resistance in ryegrass (Johnson et al, 2013) combined with the significant impact of the braconid parasitoid biological control agent, Microctonus hyperodae (e.g., Barker and Addison, 2006). Such overwintering stability has permitted meta-analyses of historical datasets, which have shown that parasitism rates have declined notably in Loliumbased pastures since the parasitoid’s initial establishment and equilibration in the first 6 years of its release (e.g., Goldson et al, 2014a,b)

Methods

Results

Discussion

Conclusion