fpls-07-01397 September 14, 2016 Time: 19:20 # 1 View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Frontiers - Publisher Connector ORIGINAL RESEARCH published: 16 September 2016 doi: 10.3389/fpls.2016.01397 Does White Clover (Trifolium repens) Abundance in Temperate Pastures Determine Sitona obsoletus (Coleoptera: Curculionidae) Larval Populations? Mark R. McNeill1*, Chikako van Koten1, Vanessa M. Cave2, David Chapman3 and Hamish Hodgson3 1 AgResearch, Christchurch, New Zealand, 2 AgResearch, Hamilton, New Zealand, 3 DairyNZ, Lincoln University, Lincoln, New Zealand To determine if host plant abundance determined the size of clover root weevil (CRW) Edited by: Sitona obsoletus larval populations, a study was conducted over 4 years in plots sown Ivan Hiltpold, Western Sydney University, Australia in ryegrass (Lolium perenne) (cv. Nui) sown at either 6 or 30 kg/ha and white clover Reviewed by: (Trifolium repens) sown at a uniform rate of 8 kg/ha. This provided a range of % white James Michael William Ryalls, clover content to investigate CRW population establishment and impacts on white clover Hawkesbury Institute for the Environment, Australia survival. Larval sampling was carried out in spring (October) when larval densities are Phil Murray, near their spring peak at Lincoln (Canterbury, New Zealand) with % clover measured Rothamsted Research – in autumn (April) and spring (September) of each year. Overall, mean larval densities Biotechnology and Biological −2 Sciences Research Council, UK measured in spring 2012–2015 were 310, 38, 59, and 31 larvae m , respectively. *Correspondence: There was a significant decline in larval populations between 2012 and 2013, but Mark R. McNeill spring populations were relatively uniform thereafter. The mean % white clover measured [email protected] in autumns of 2012 to 2015 was 17, 10, 3, and 11%, respectively. In comparison, Specialty section: mean spring % white clover from 2012 to 2015, averaged c. 5% each year. Analysis This article was submitted to relating spring (October) larval populations to % white clover measured in each plot Agroecology and Land Use Systems, a section of the journal in autumn (April) found the 2012 larval population to be statistically significantly larger Frontiers in Plant Science in the ryegrass 6 kg/ha plots than 30 kg/ha plots. Thereafter, sowing rate had no Received: 30 June 2016 significant effect on larval populations. From 2013 to 2015, spring larval populations had Accepted: 01 September 2016 a negative relationship with the previous autumn % white clover with the relationship Published: 16 September 2016 highly significant for the 2014 data. When CRW larval populations in spring 2013 to Citation: McNeill MR, van Koten C, Cave VM, 2015 were predicted from the 2013 to 2015 autumn % white clover, respectively, Chapman D and Hodgson H (2016) based on their positive relationship in 2012, the predicted densities were substantially Does White Clover (Trifolium repens) Abundance in Temperate Pastures larger than those observed. Conversely, when 2015 spring larval data and % clover Determine Sitona obsoletus was regressed against 2012–2014 larval populations, observed densities tended to be (Coleoptera: Curculionidae) Larval higher than predicted, but the numbers came closer to predicted for the 2013 and 2014 Populations? Front. Plant Sci. 7:1397. doi: 10.3389/fpls.2016.01397 populations. These differences are attributed to a CRW population decline that was not Frontiers in Plant Science | www.frontiersin.org 1 September 2016 | Volume 7 | Article 1397 fpls-07-01397 September 14, 2016 Time: 19:20 # 2 McNeill et al. CRW Larvae and White Clover accounted by % white clover changes, the CRW decline most likely due to biological control by the Braconid endoparasitoid Microctonus aethiopoides, which showed incremental increases in parasitism between 2012 and 2015, which in 2015 averaged 93%. Keywords: insect pest management, biological control, dairy pasture, pasture persistence, Microctonus aethiopoides INTRODUCTION burrow down into the soil to locate the root system. There are five larval instars, with the first instar dependent on root nodules Weevils belonging to the genus Sitona include a number of for establishment and survival (Gerard, 2001) and inoculated species that are recognized pests including Sitona lineatus L. nodules with viable rhizobia preferred to uninoculated nodules (Cantot, 1989; Lohaus and Vidal, 2010), S. discoideus Gyllenhal (Hackell and Gerard, 2004). Later instars feed on the general root (Aeschlimann, 1978; Goldson et al., 1988), S. hispidulus F. (Quinn system including the stolons. There are two larval generations per and Hower, 1986; Dintenfass and Brown, 1988) and S. obsoletus year in the Canterbury region of New Zealand, with first major Gmelin (Murray and Clements, 1998; Gerard et al., 2007). peak occurring in October–November (spring), and a second S. obsoletus (formerly described as S. lepidus and S. flavescens), smaller peak occurring in April–May (autumn) (M. R. McNeill, is a Palaearctic species first detected in the North Island of New unpublished data). Zealand in 1996 (Barratt et al., 1996). It was first discovered The arrival of clover root weevil (CRW) and its natural and in the South Island of New Zealand in 2006, with discrete anthropogenic-assisted spread across New Zealand, coincided populations located near Richmond, Rai Valley and Christchurch with farmer reports of white clover production and consequent (Phillips et al., 2007). The weevil shows a strong preference animal production declines as population numbers built up in for white clover (Trifolium repens L.), although overseas it newly colonized regions (e.g., McNeill et al., 2012). This was has also been considered a pest of red clover (T. pratense L.) also confirmed by small plot study that showed up to a 35% (Brudea, 1982; Murray and Clements, 1994). Worldwide, white decline in DM production under a modest population of c. 300 clover is recognized as a valuable component of pastures larvae m−2 (Gerard et al., 2007). Management recommendations because of its ability to fix nitrogen and provide quality feed to minimize impacts included increased applications of artificial for improved animal production. In New Zealand, ryegrass nitrogen, especially in spring, pasture management to encourage (Lolium spp. L.) and white clover are the predominant plant white clover production, and a prolonged fallow or crop to species in improved grasslands (Woodward et al., 2003; Tozer eliminate larval populations prior to resowing of new pastures. et al., 2014), so the potential impact from S. obsoletus was As part of the management program to control S. obsoletus, considerable. the Braconid endoparasitoid Microctonus aethiopoides, was Sitona obsoletus is found almost all year round and depending introduced in 2006 (Gerard et al., 2006; Phillips et al., 2007). on season, both adults and larvae can be present in pasture Originally from Ireland, the parasitoid attacks the adult weevil, where white clover is growing. Under severe larval infestations, with parasitism resulting in sterilization and the eventual death a decrease in percent foliar N levels in spring can occur, with of the weevil upon emergence of the parasitoid larvae. For female subsequent reductions in herbage dry matter (DM) yield levels CRW, parasitism prevents further egg laying, with a subsequent (Gerard, 2002). Under severe infestations, and in combination impact on the number of larvae that establish. The parasitoid with livestock herbivory, plant stress or poor fertility, white clover is able to complete 3–4 generations per year, attacks multiple plants can disappear from the sward. Although this is often weevils and has a winter diapause, occurring as a first instar temporary, it does have implications for pasture productivity larva inside the adult weevil. As of June 2016, CRW is widely and cost to the farmer through a reliance on artificial N distributed across New Zealand pasture containing white clover, to compensate for the loss of white clover. New generation and depending on season, either adults or larvae or both stages adults commence emergence in spring (November), with peak occur concurrently. emergence occurring in the following 4–6 weeks. Following The importance of root nodules to Sitona spp. larval emergence, adults feed extensively on the foliage, and can be development has been shown in several contributions (e.g., damaging to both seedling clover and plants in mature stands, Danthanarayana, 1967; Quinn and Hower, 1986; Wolfson, 1987; particularly when populations are high in early-mid-summer. Murray and Clements, 1998; Gerard, 2001; Lohaus and Vidal, During this period, flight muscles and the reproductive system 2010), although the relationship was less clear for S. discoideus are maturing. Following mating, females exhibit reproductive first instar survival (Aeschlimann, 1986). The result of nodule maturation with egg laying commencing in mid-summer, and feeding and destruction of the root system, is that the plant can continuing through winter into spring. The percentage of become stressed causing overcompensation in nodule production reproductively mature females is highest over winter–spring with (Quinn and Hall, 1992), reduction in leaf and root N (Murray eggs laid indiscriminately in the foliage. Eggs accumulate in the et al., 2002), modification to C:N ratios (Murray et al., 1996) and litter, with larval development occurring at temperatures above