Use of the coccinellid, Stethorus pinctillum Weise (Coleoptera: Coccinellidae) to control the glasshouse red spider mite, Tetranychus urticae Koch (Acari: Tetranychidae)

Stethorus punctillum Weise (Coleóptera: Coccinellidae) is a predator of tetranychid mites. The aim of the study was to evaluate the coccinellid as a predator of two-spotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae) in glasshouses. Development of S. punctillum was evaluated under a range of constant temperatures from 18 to 34 °C and a cycling temperature regime at 10/25 °C. Results were analysed using polynomial regression analysis and a subsequent experiment evaluated this and the accumulated heat method as predictors of lifecycle duration.

Two temperatures typically tolerated by natural enemies in glasshouses were evaluated for effect on pre-oviposition period and fecundity in S. punctillum. Statistical analysis indicated that temperature did not have a significant effect on pre-oviposition period although there was a tendency for decreased pre-oviposition period with an increase in temperature from 20 to 26 °C. Beetles reared at 26 °C tended to oviposit at a significantly higher rate (eggs day _1) compared with beetles reared at 20 °C. Adult longevity was not determined. However, the experiment was terminated on day 85 when the majority of adults remained alive.

Cold storage of S. punctillum adults for five, 10 and 15 days at 6 °C was evaluated for effect on the number of days to resume oviposition (end of storage to first egg), oviposition rate (eggs day _1) and egg viability post storage. Storage was possible with no significant effect on mean oviposition rate when compared with the mean oviposition rate of beetles that were not stored. There was an indication that the physiological state of 5. punctillum was an important factor affecting its ability to withstand cold storage. However, further research is indicated to evaluate this.

Effect of short day-length on juvenile development, pre-oviposition period and oviposition rate in S. punctillum was determined and compared with equivalent parameters for beetles incubated under long day-lengths. Analysis indicated that short day-length did not significantly effect juvenile development period in S. punctillum. Pre-oviposition period was significantly effected by photoperiod. A reduction in photoperiod resulted in a decrease in mean oviposition rate. However, results were variable and data for individual beetles were considered. Results are discussed with reference to age of beetles and overall variability in oviposition rate of the test populations.

Olfactory responses of S. punctilium to odours from Vicia faba L. and V. faba plus T. urticae were evaluated in a four-arm olfactometer. Beetles showed directed movement towards host odours. The behaviour of S. puncltllum was significantly modified in odour fields containing odours from V faba plus T. urticae. The implications of these results to glasshouse pest management of T. urticae are discussed.

Semi-field trials were conducted to evaluate S. punctillum as a control agent of T. urticae in a glasshouse cucumber crop. Destructive leaf samples from the upper and lower crop canopies were taken on five assessment dates. While statistical analysis indicated no significant effect of crop level on densities of two-spotted spider mites, significant effects of crop level were recorded for densities of S. punctillum. Mean densities of two-spotted spider mites leaf 1 were significantly lower on the final assessment date compared with the first. This contrasts with data for S. punctillum densities, where no significant differences were detected between assessment dates. Details of the inadvertent establishment of Feltiella acarisuga Vallot (Diptera: Cecidomyiidae) are also included in the results.

All experimental results are discussed with a view to the inclusion of S. punctillum as a commercially viable biological control agent in the management of T. urticae in glasshouses.