The economic injury level of Mahanarva fimbriolata (Stål) to sugarcane was experimentally evaluated in a field area harvested at November. The inseticide thiamethoxam was applied: 150, 200 e 250 g i.a.ha-1. Applications started when the pest populations reached of 1.8 spittlebugs.m-1 (12/04/02), 7.6 spittlebugs.m-1 (12/17/02) or 12.4 spittlebugs.m-1 (12/26/02). Untreated plots were maintained as check. After the insecticide applications, infestations of the sugarcane root froghopper were monthly evaluated till 9/26/03 when the experiment was harvested. Compared with check plots, the pest control resulted in significant increment of sugar productivity. When applied under infestation of 1.8 (12/04/02) or 7.6 (12/17/02) the productivity was resulted higher than control made with populations of 12,4 spittlebugs.m-1. Thiamethoxam was efficient when applied in all of the three doses. It was observed interaction between dose and time of insecticide application. When the applications had been made in 12/04/02, under infestation of 1,8 spittlebugs.m-1, the treatment with thiamethoxam 250 g i.a.ha-1 resulted in stalks and sugar productivities higher than the treatment with 150g i.a.ha-1. This fact was attributed to the greater amount of residue and better initial control of the pest, obtained with the highest dose. The economic analysis of the data showed that thiamethoxam at 250g i.a.ha-1 applied with the infestation of 1.8 spittlebugs.m-1, provided the greatest profit, whereas when applied at the dose of 250g i.a.ha-1, under infestation of 12.4 spittlebugs.m-1, the smallest net income was obtained. Regression analysis allowed the estimation of the economic injury level for the pest, in the present experimental conditions, as ranging from 3 to 5 spittlebugs.m-1.
Insecta; sugarcane root froghopper; Saccharum
