Testing IPM protocols for Helicoverpa armigera in processing tomato: egg-count- vs. fruit-count-based damage thresholds using Bt or chemical insecticides

A 2-year study (1997-1998) was carried out in Extremadura (southwestern Spain) to compare the effectiveness of IPM strategies based on either fruit counts or egg counts for Helicoverpa armigera Hübner on processing tomato. Five intervention protocols F3, F6, E, C and S were conducted: three IPM protocols with thresholds of 3% and 6% damaged fruits (F3 and F6) and of one egg per 10 leaves (E) as well as two control treatments either unsprayed (C) or systematically sprayed (S). Pest control was executed using either Bt or conventional chemical insecticides. Field trials were arranged following a randomised-complete-block design and fruit quality (percentage total damaged fruits or percentage unmarketable damaged fruits) and yield (undamaged or marketable fruits) were evaluated at harvest. ANOVA results indicated that intervention protocol did not significantly affect yield, although planned comparisons showed significant differences between S and C controls following chemical strategy. This suggested that considerable larval injury was required to significantly affect yield. In contrast, fruit quality (as percentage total damaged fruits at harvest) was largely affected by the intervention protocol, although among-protocol differences were much reduced when just percentage unmarketable fruits were considered. Insecticide type did not significantly affect yield or fruit quality and protocol-by-insecticide interactions were neither significant. However, there was evidence that Bt efficacy was inferior when compared with conventional chemicals, irrespective of intervention protocol implemented. Results overall supported the hypothesis that fruit-count-based damage thresholds were at least as reliable as egg-count-based damage thresholds. In particular, a 3% damage threshold (F3 protocol) following the chemical strategy, proved to be a useful tool for decision making in IPM programmes against H. armigera in processing tomato crops.