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potential impact of Sitona lepidusadults on white clover seed production. .... TABLE 3: Mean clover seed weight (± SE) in presence of varying densities of.
Pasture Weeds and Pests

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PRELIMINARY INVESTIGATION INTO IMPACT OF SITONA LEPIDUS ADULTS ON WHITE CLOVER SEED PRODUCTION P.J. GERARD, T. EDEN and W. TOZER Biocontrol and Biodiversity Group, AgResearch, Private Bag 3123, Hamilton ABSTRACT A small plot trial was undertaken in the Waikato to determine the potential impact of Sitona lepidus adults on white clover seed production. Five densities of adult weevil were caged on 1 m2 plots from the time of peak adult emergence in November 1998 until harvest in late January 1999. In the laboratory, weevil adults were found to severely damage clover inflorescences when no leaves were present. Although feeding damage on leaves was highly visible in the field plots, with over 80% of the leaflets damaged at the highest density of 600 adults/m2, weevil adults were found to have no effect on flowering or seed production. Keywords: Sitona lepidus, white clover, seed production. INTRODUCTION Clover root weevil, Sitona lepidus Gyllenhal (Coleoptera: Curculionidae), poses a significant threat to the clover seed industry, either directly through seed production losses, or through reductions in demand for seed should farmers turn to fertiliser nitrogen to maintain animal production. Current paddock hygiene strategies used to produce certified clover seed crops (Clifford et al. 1996) will ensure that clover crops are sown and established in paddocks free of the weevil. However, infestation through immigration of adults during the period of maximum flight in early summer will probably occur (P. Addison pers. comm.). Residual soil pesticides are used to control grass grub in clover seed production and should control any S. lepidus larvae arising from eggs laid by these incoming adults during the summer. Thus, the main threat to first year clover crops is likely to be the impact of adult feeding. This report summarises experimental work undertaken to quantify the impact of S. lepidus adult feeding on white clover seed production. METHOD The trial was carried out in three 10 x 1 m cold-frame beds at Ruakura Research Centre, Hamilton. White clover seed (cv. Prestige) was sown in early April 1998 in rows 45 cm apart. In November 1998, thirty 1 x 1 m plots (each containing two 1 m long rows of clover) were set up within the beds and 25 of these were enclosed in mesh cages. A nucleus beehive was connected to each cage to ensure clover pollination. S. lepidus adults were then collected from clover-dominant pasture using a suction machine and introduced into the cages at four rates to give five replicates of six treatments (Table 1). The number of clover inflorescences/m2 was assessed at the time of introduction of S. lepidus adults and then at one or two weekly intervals until harvest. Adult damage was scored on leaves in early January. Plots were harvested on 27 January 1999. Total seed weight from 50 seed heads was determined, and a sub-sample of 200 seeds weighed. An estimation of seed yield/ m2 was obtained by multiplying weight of seed /seed head by combined inflorescence densities at peak flowering and seven days later. Ten seed heads from each plot were inspected under a microscope for evidence of adult feeding. Foliage from the plots was removed, sorted to capture as many remaining weevils as possible, then oven dried to obtain a plot dry matter assessment. The plot surface was suction-sampled to collect remaining weevils. Proc. 52nd N.Z. Plant Protection Conf. 1999: 250-253 © 1999 New Zealand Plant Protection Society (Inc.) www.nzpps.org

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As feeding damage by S. lepidus adults on clover florets was not detected prior to harvest, a laboratory experiment was undertaken to determine if feeding on florets occurred and so that the visual appearance of the damage could be recognised. Twenty batches of 10 starved S. lepidus adults were placed in small cages (80 mm diameter x 50 mm high). Ten cages were provided with one freshly picked white clover (cv. Prestige) inflorescence. The remaining 10 cages were provided with a clover inflorescence combined with two clover leaves. The cages were held at ambient temperatures in the laboratory and damage on inflorescences and leaves was assessed at 24 h. Data obtained from cold-frames was analysed by regression analysis, and where appropriate, standard errors for the predictions from the regression model have been presented in tables. Log transformation of the S. lepidus adult data was undertaken to stabilise the variance, and means were back-transformed for data presentation. The laboratory data were subjected to ANOVA. RESULTS Effect of S. lepidus adults on flowering 2 S. lepidus adults, even at 600/m , did not impact on flowering as measured at the time of peak inflorescence density (3 weeks after weevil introduction), at final flowering assessment, or on the total inflorescence production (Table 1). Inflorescence density was reduced by the cages as shown by the difference between the no cage and caged controls (Table 1). This effect increased over time but was only statistically significant at the final assessment at harvest (P