Phenology and damage potential of Whitefringed weevil (Naupactus leucoloma boheman) in the Northern North Island of New Zealand

Whitefringed weevil (Naupactus leucoloma Boheman) populations were monitored over a 36 month period at two sites in dairy pasture in Waikato and Bay of Plenty, northern New Zealand. In each year, at both study sites, larval densities declined from 400-600 m⁻² in April to 30-60 m⁻² in December, by which time most larvae were 8-11th instars. Eggs placed in the field in January 1995 suffered high mortality from desiccation, suggesting that most 1st instar larvae observed in March-May arose from eggs laid from late January onwards. Adult N. leucoloma were present from November to late April. The presence of late-instar larvae all year round indicated that some individuals did not complete their development in one year. No parasites or diseases were isolated from any life stage during this population study. Feeding studies showed that N. leucoloma larvae have the ability actively to select white clover roots over ryegrass roots. The feeding of 2nd -11th instar larvae consisted of scarring, detachment and destruction of roots. Larvae had a higher rate of survival and growth when feeding on white clover compared with ryegrass. Feeding trials demonstrated that larvae could significantly reduce white clover root dry matter, nitrogen fixation, and productivity. Evidence from a small plot trial suggested that field densities of N. leucoloma larvae of approximately 100 m⁻² in June could lead to reduced white clover persistence and fixation rates. Reduction in nitrogen fixation measured in the small plot trial ranged from 13-27% when larval populations were 90-220 m⁻²in June. Reductions of this magnitude may be costing between $18-37 ha ⁻¹ year ⁻¹ or the equivalent of 19-40 kg N ha ⁻¹ year ⁻¹. N. leucoloma larval densities of the magnitude measured at the two field sites in the population study may therefore be having a significant impact on the nitrogen fixing ability and sustainability of white clover in ryegrass/white clover pastures. A RAPD-PCR technique was developed to compare the genetic material of three species (N. leucoloma, N. peregrinus (Buchanan) and N. tucumanensis (Hustache)) of whitefringed weevil and 23 populations of N. leucoloma. Banding profiles from the RAPD-PCR study consistently distinguished between these three Naupactus species and suggested that N. leucoloma is genetically more similar to N. peregrinus than to N. tucumanensis. The technique distinguished between six 'genotypes' of N. leucoloma in the 23 populations sampled from Australia, New Zealand, North America, South Africa and South America. Due to the damage potential and the current lack of convenient and effective control measures for N. leucoloma, a biocontrol agent may be useful in reducing the effect of this insect on white clover persistence and nitrogen fixation. The results of the RAPD-PCR studied indicated that the New Zealand populations of N. leucoloma may have originated from areas on both the east and west coasts of South America. These areas may be possible sources for successful biocontrol agents.