Abstract
To determine if host plant abundance determined the size of clover root weevil (CRW) Sitona obsoletus larval populations, a study was conducted over 4 years in plots sown in ryegrass (Lolium perenne) (cv. Nui) sown at either 6 or 30 kg/ha and white clover (Trifolium repens) sown at a uniform rate of 8 kg/ha. This provided a range of % white clover content to investigate CRW population establishment and impacts on white clover survival. Larval sampling was carried out in spring (October) when larval densities are near their spring peak at Lincoln (Canterbury, New Zealand) with % clover measured in autumn (April) and spring (September) of each year. Overall, mean larval densities measured in spring 2012–2015 were 310, 38, 59, and 31 larvae m-2, respectively. There was a significant decline in larval populations between 2012 and 2013, but spring populations were relatively uniform thereafter. The mean % white clover measured in autumns of 2012 to 2015 was 17, 10, 3, and 11%, respectively. In comparison, mean spring % white clover from 2012 to 2015, averaged c. 5% each year. Analysis relating spring (October) larval populations to % white clover measured in each plot in autumn (April) found the 2012 larval population to be statistically significantly larger in the ryegrass 6 kg/ha plots than 30 kg/ha plots. Thereafter, sowing rate had no significant effect on larval populations. From 2013 to 2015, spring larval populations had a negative relationship with the previous autumn % white clover with the relationship highly significant for the 2014 data. When CRW larval populations in spring 2013 to 2015 were predicted from the 2013 to 2015 autumn % white clover, respectively, based on their positive relationship in 2012, the predicted densities were substantially larger than those observed. Conversely, when 2015 spring larval data and % clover was regressed against 2012–2014 larval populations, observed densities tended to be higher than predicted, but the numbers came closer to predicted for the 2013 and 2014 populations. These differences are attributed to a CRW population decline that was not 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%.
Highlights
Weevils belonging to the genus Sitona include a number of species that are recognized pests including Sitona lineatus L. (Cantot, 1989; Lohaus and Vidal, 2010), S. discoideus Gyllenhal (Aeschlimann, 1978; Goldson et al, 1988), S. hispidulus F. (Quinn and Hower, 1986; Dintenfass and Brown, 1988) and S. obsoletus Gmelin (Murray and Clements, 1998; Gerard et al, 2007)
There were no clover root weevil (CRW) larvae found in the cores taken on 13 December 2011, which is consistent with our understanding that colonization of the newly established plots by spring emerged CRW adults would have only just begun and the lack of reproductively mature females would have meant an absence of larvae
There was a significant decline in larval populations between 2012 and 2013 (P < 0.001), thereafter larval densities between the October 2013–2015 samples were not significantly different (P > 0.05)
Summary
Weevils belonging to the genus Sitona include a number of species that are recognized pests including Sitona lineatus L. (Cantot, 1989; Lohaus and Vidal, 2010), S. discoideus Gyllenhal (Aeschlimann, 1978; Goldson et al, 1988), S. hispidulus F. (Quinn and Hower, 1986; Dintenfass and Brown, 1988) and S. obsoletus Gmelin (Murray and Clements, 1998; Gerard et al, 2007). S. obsoletus (formerly described as S. lepidus and S. flavescens), is a Palaearctic species first detected in the North Island of New Zealand in 1996 (Barratt et al, 1996). It was first discovered in the South Island of New Zealand in 2006, with discrete populations located near Richmond, Rai Valley and Christchurch (Phillips et al, 2007). The weevil shows a strong preference for white clover (Trifolium repens L.), overseas it has been considered a pest of red clover (T. pratense L.) (Brudea, 1982; Murray and Clements, 1994). L.) and white clover are the predominant plant species in improved grasslands (Woodward et al, 2003; Tozer et al, 2014), so the potential impact from S. obsoletus was considerable
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