Development of Action Thresholds for Management of Bactericera cockerelli and Zebra Chip Disease in Potatoes at Pukekohe, New Zealand

Bactericera cockerelli (tomato potato psyllid, TPP) is a serious pest of potato crops, causing feeding damage and also vectoring Candidatus Liberibacter solanacearum, the causal agent of the Zebra Chip (ZC) disease in potatoes. The results of 6 years of early and main crop potato trials at Pukekohe, New Zealand, are summarised and damage caused by TPP is reported. Results show that spring-sown (early) potato crops do not require insecticides. In main crop summer trials we tested action thresholds based on 10 and 20 TPP nymphs per 100 middle leaves but the incidence of ZC damage was commercially unacceptable, ranging from 4 to 9 %. Subsequently we tested an action threshold of >3 TPP adults per yellow sticky trap per week that led to ZC damage ranging from 0.9 to 1.6 %. We also compared monitoring of TPP using sticky trap catches with a degree day model started in mid winter (1 July) for forecasting generation times of TPP. On the basis of the field trial results, we recommend that from early summer onwards, the timing of the first foliar application of insecticide needs to be applied early enough to protect main crop potatoes from the first generation of TPP that occurs after potato tubers have emerged (at Pukekohe, this is the third TPP generation from 1 July when using degree day modelling). Three years of main crop trials indicate that an action threshold of >3 TPP per trap per week provides effective TPP/ZC management in the Pukekohe region when used in conjunction with natural enemies and an insecticide programme that features the use of selective insecticides.