An adjustable action threshold using larval parasitism of Helicoverpa armigera (Lepidoptera: Noctuidae) in IPM for processing tomatoes

An adjustable action threshold that uses estimates of larval parasitism of Helicoverpa armigera (Hübner) in individual fields was assessed over three consecutive years in processing tomatoes in Hawke’s Bay, New Zealand. Tomato fields were monitored weekly for levels of infestation by H. armigera larvae. When infestation levels became of concern, either approaching or exceeding the standard action threshold of one larva per plant, collections of 30+ representative larvae were made. These larvae were measured and reared individually at 30 °C, and after 4 days rates of parasitism were estimated visually. From these data, spray recommendations were made using a formula that adjusted the action threshold to allow for mortality from parasitism. At harvest, damage assessments were made to validate these recommendations. Results showed that estimates of parasitism after 4 days were accurate predictors of final assessments of parasitism. Overall parasitism during the three seasons was 71%, confirming that the original threshold, which relies on about 50% parasitism, needed revision. The dominant parasitoid was Cotesia kazak, reared from 91% of all parasitized larvae over the 3 years. Microplitis croceipes and the self introduced polyphagous parasitoid Meteorus pulchricornis were reared from 4% and 5% of the parasitized larvae respectively. In 16 of 17 fields examined, the adjustable threshold kept fruit damage at harvest below the tolerated level of 5%. The single crop with excessive damage had only 0.5% fruit damage above this level. This adjustable threshold, which varied in this study from 1–8.3 larvae per plant, has been incorporated into an updated IPM programme and contributed to a 95% reduction in insecticide use.