The interaction of Typhlodromus pyri (Scheuten) and Panonychus ulmi (Koch) (Acari) on apple trees in Nelson, New Zealand

Abstract

The dispersion statistic, k, of the negative binomial distribution was calculated for Panonychus ulmi and Typhlodromus pyri; mite species present in Nelson apple orchards. Distribution data taken from 2300 leaves, taken at intervals of 7-14 days during the 1976-77 season, found P. ulmi to be contagiously distributed. Contagion became more severe as the season progressed and by later summer most sample means could not be adequately described by this distribution series. Sample data for T. pyri was well described by the negative binomial distribution in mid and late summer only. Distribution of T. pyri during early summer was random with many sample means departing from the negative binomial series. The parameter ‘b’ of Taylor’s power law was determined for both species. Using this, leaf sample size estimates for both species at varying densities and error margins (expressed as ratios of standard error to mean) were determined.

The Henderson-McBurnie mite brushing machine, used in a commercial mite-monitoring service provided for Nelson orchardists, was investigated for efficiency of removal of P. ulmi and T. pyri. For all leaf samples this machine had an acceptable efficiency of removal for P. ulmi, however removal of T. pyri from leaves was low and variable. Modifying the brushing procedure to expose the leaf midrib increased removal of T. pyri but did not significantly reduce the variability of numbers removed.

Estimation of mite population density using the relationship between the observed proportion of mithe infested leaves and the sample mean density was investigated. Estimation of P. Ulmi density for three leaf infestation classes was determined. A mite density of five P. Ulmi active stages per leaf was unlikely to be exceeded if less than half of the leaves in a sample were infested with mites. The results showed that the reliability of this technique decreased with increasing mite density.

Examination of leaf distributions of both species throughout the season found them unlikely to occur on leaves concurrently in November and December. This suggested that T. pyri was unlikely to influence population development of P. ulmi significantly during this period. In mid and late summer T. pyri failed to disperse from leaves where P. ulmi had been effectively suppressed, to those where P. ulmi was increasing in the absence of predators, before some leaf damage occurred.

The predation efficiency of an azinphosmethyl-resistant strain was investigated. It was found that azinphosmethyl-resistant T. pyri were not as effective in the control of P. ulmi as a susceptible strain not receiving azinphosmethyl. It was considered that sublethal effects of this chemical may exist which disrupted T. pyri distribution patterns in the resistant strain thereby permitting damage by European red mite.