Ploidy in Lolium spp. cultivars affects Argentine stem weevil parasitism by Microctonus hyperodae

A laboratory study was conducted to: (1) advance earlier work to determine the susceptibility of Listronotus bonariensis (Argentine stem weevil) to its parasitoid biological control agent, Microctonus hyperodae, in the presence of diploid ryegrasses such as Lolium multiflorum or Lolium perenne; and (2) determine whether or not plant orientation (i.e. horizontal vs. vertical) had any effect on parasitism rates. No significant differences in parasitism rates were found in the Listronotus bonariensis populations in the two grasses. However, combining and analysing these data with those from an earlier experiment conducted in the same way showed that parasitism rates were significantly lower in diploid Lolium multiflorum cultivars than in tetraploid Lolium multiflorum. This is the first clear evidence of a ploidy effect on parasitism rates in the presence of Lolium multiflorum. Tetraploid Lolium multiflorum has fewer, more robust and larger tillers than the diploid Lolium spp. so higher parasitism rates may be related to the lack of hiding places for an evasive genetically-driven behaviourally-based resistance by Listronotus bonariensis.

Effect of Epichlo endophyte strains in Lolium spp cultivars on Argentine stem weevil parasitism by Microctonus hyperodae

This paper reports on an extensive field investigation conducted at Lincoln during the 20142015 summer/early autumn This aimed to assess the effects of a range of novel Epichlo endophytes when present in different cultivars of Lolium spp on parasitism rates by the biological control agent Microctonus hyperodae in Listronotus bonariensis (ie the Argentine stem weevil) Results for the entire summer and including all treatment combinations did not find any significant differences in parasitism in L bonariensis populations However in the early autumn independent of the endophytes present significantly higher levels of parasitism were found in a tetraploid Lolium multiflorum cultivar and a tetraploid L perenne selection compared to the L perenne cultivars Whether this finding has any bearing on a possible mechanism of weevil resistance is discussed

Abundance and parasitism of clover root weevil (Sitona lepidus) and Argentine stem weevil (Listronotus bonariensis) in pastures

The period from 2006 to 2010 in the North Island encompasses the widespread establishment of Irish Microctonus aethiopoides for the biocontrol of clover root weevil (Sitona lepidus) and a severe drought in 2008 that greatly reduced host availability This parasitoid occupies the same pasture habitat as Microctonus hyperodae a parasitoid with a similar life cycle introduced to control Argentine stem weevil (Listronotus bonariensis) A retrospective study was carried out on L bonariensis data and stored samples collected during S lepidus studies It was found that at all four initial M aethiopoides release sites L bonariensis populations were lowest in 2007 and 2008 while S lepidus populations were lowest in 2008 and 2009 Microctonus hyperodae parasitism levels were lowest in 2009 with overall regional means ranging from 5 in Manawatu to 11 in Waikato whereas corresponding Irish M aethiopoides parasitism levels were 43 and 72 respectively

The current status of Argentine stem weevil (Listronotus bonariensis) as a pest in the North Island of New Zealand

Recent sampling in Waikato and Taranaki shows that Argentine stem weevil (Listronotus bonariensis) can still cause major damage to susceptible ryegrass pastures despite the introduction of the parasitoid Microctonus hyperodae in 1991 Percentage of tillers with L bonariensis larval damage in diploid and tetraploid perennial ryegrass and Italian ryegrass in January and February were between 11 and 68 High egg numbers were noted on occasion Although there is evidence that M hyperodae reduces host populations parasitism levels vary considerably between years in Canterbury and are low in Otago and Southland where high populations of L bonariensis have recently been recorded Factors that may be lessening the impact of M hyperodae are considered These include asynchronous generations of the parasitoid and its host host behavioural adaptations compensatory oviposition and inhibition of flight in parasitised individuals leading to low levels of parasitism in new pastures

Microctonus hyperodae in Otago and Southland

In 1990 Microctonus hyperodae was released in Otago and Southland as a biocontrol agent against Argentine stem weevil (ASW) (Listronotus bonariensis) At that time ASW was New Zealands major pasture pest but attained only sporadic importance in the southern South Island Since then possibly due to higher nutritional qualities of modern ryegrass cultivars and/or changes in annual thermal accumulation ASW has increased in significance Farmers once relied on endophytefree ryegrasses but now sow infected grasses While endophyte helps protect against ASW damage augmentation by M hyperodae parasitism is desirable Establishment and localised spread of M hyperodae was measured around the release sites but subsequent distribution over the wider southern South Island was not Between September and November 2011 52 pastures were sampled and ASW was found in all Microctonus hyperodae was reared from 18 of these pastures at levels of 133 parasitism but in 15 pastures parasitism was less than 7 The information gathered covers a limited period but suggests that M hyperodae is not as successful in the far south as in other parts of New Zealand The reasons for this and possible implications warrant further investigation both to understand the situation and to investigate if supplementary releases of M hyperodae can be usefully undertaken

Argentine stem weevil (Listronotus bonariensis, Coleoptera: Curculionidae) population dynamics in Canterbury, New Zealand dryland pasture

The Argentine stem weevil (Listronotus bonariensis) was an economically important pest in New Zealand pastures until the release of the parasitoidMicroctonus hyperodae. This contribution uses historical data to investigate the regulation of the pest populations prior to, and somewhat during, the establishment of this parasitoid in dryland Canterbury, New Zealand. Thus, a significant goal of this study is to provide anL. bonariensispopulation dynamics baseline for any future work that aims to analyse the full effects ofM. hyperodaeon the weevil, now that equilibrium with the weevil host has been reached.The population dynamics ofL. bonariensis, based on a life-table approach, were investigated using data collected regularly for eight years from populations in Canterbury, New Zealand. The key factor affecting end-of-seasonL. bonariensisdensity was found to be variation in second generation fourth instar prepupal and pupal mortality. This may have been caused by arrested development and ongoing mortality resulting from the onset of cooler autumnal conditions.A compensatory response was found in recruitment to the second summer weevil generation, whereby the realised fecundity of the emergent first summer generation of weevils was found to be negatively related to the density of adult weevils per ryegrass tiller. This is the first time that this has been found via long-term population analysis ofL. bonariensis, although indications of this have been found elsewhere in caging, pot and small plot experiments.In this study, the effect of the parasitoid biocontrol agentMicroctonus hyperodaeonL. bonariensispopulation dynamics was unclear, as the analysis covered a period when the parasitoidMicroctonus hyperodaewas introduced and still establishing. It does, however, raise important questions for future analysis in terms of the interaction between parasitism and unrealised fecundity.The results in this contribution also highlighted regional differences. Overwintering mortality of adult weevils in Canterbury was constant between years, whilst earlier studies in the North Island Waikato region indicated this mortality was density dependent. In addition, the availability of tillers in endophyte-free ryegrass pastures in Canterbury had no influence on egg and early-instar larval survival, which contrasts with the finding from endophytic Waikato pastures.

Detection of young Microctonus hyperodae eggs in the Argentine stem weevil (Listronotus bonariensis)

The braconid endoparasitoid Microctonus hyperodae was successfully introduced into New Zealand in 1991 to control the exotic pastoral pest Argentine stem weevil (ASW) The wasp parasitises adult ASW and the weevil normally dies only when the fifthinstar parasitoid larva emerges However during biocontrol research over the last 20 years premature mortality of ASW has been observed under conditions of high parasitoid pressure in the field and laboratory It was hypothesised that this occurs because of injection of venom without an egg To conclude absence of a parasitoid egg in a prematurely dead weevil a sensitive method is needed to detect a newly oviposited parasitoid egg However this cannot be confirmed by visual observation or by dissection because fresh tissue is subsequently needed to detect venom A molecular method was therefore developed Firstly to model oviposition eggs were dissected from the ovaries of an adult parasitoid and one ovarian parasitoid egg was used to spike ASWs that had been dissected to confirm absence of parasitism Secondly multiplexPCR was successful in detecting a single ovarian egg in ASW This result greatly benefits research into the observed premature mortality of ASW after being exposed to M hyperodae

Ryegrass production in Wairarapa, New Zealand: is biological control of Argentine stem weevil important?

To investigate the interaction between Argentine stem weevil (Listronotus bonariensis), its parasitoid Microctonus hyperodae, fungal endophyte (Neotyphodium lolii) and its grass host, four endophyte-ryegrass (Lolium perenne) treatments were established on four farms in Wairarapa in autumn 2003. A diploid ryegrass contained either wild-type or AR1 endophyte or was endophytefree, while a tetraploid ryegrass contained AR1 endophyte. Wild-type, AR1 and endophyte-free ryegrasses average 52%, 91% and 2% endophyte-infected tillers for the 3 years of measurement. Moderate increases over time in the proportion of wild-type infected tillers indicated selection pressures favoured endophyte-infected tillers. Endophyte-free tiller populations were 11% lower than AR1 or wild-type but not significantly so. L. bonariensis populations were highest in endophyte-free and wild-type pastures and 80% lower in AR1 pastures. Mean rates of parasitism in L. bonariensis were lower in spring (9%) than autumn (35%), tended to be higher in wild-type than in AR1 diploid ryegrass, and only had a weak relationship with percent endophyte-infection. Keywords: endophyte, Neotyphodium lolii, Lolium perenne, dry matter yields, plant persistence, Listronotus bonariensis, Microctonus hyperodae

Massemergence devices to improve synchrony between Listronotus bonariensis and Microctonus hyperodae

A novel approach to conserving and augmenting parasitoids involves storing large numbers of parasitised hosts in massemergence devices After eclosing inside the device parasitoid adults which are generally smaller than their hosts are allowed to escape through a mesh screen while any unparasitised hosts remain trapped The goal of this study was to assess the potential of massemergence devices both for augmenting populations of the parasitoid Microctonus hyperodae and for manipulating its synchrony with its host the Argentine stem weevil Listronotus bonariensis The experiments demonstrated that a screen with a 900 micro;m by 900 micro;m mesh is appropriate to separate parasitoid adults from their hosts and that a device of very simple design could be effective However the survival of L bonariensis and M hyperodae was reduced by cool storage and food deprivation and further development is required to overcome these limitations