Evaluation of mortality factors and risk analysis for the design of an integrated pest management system

A probabilistic approach of survival analysis based on the theory of competing risks was applied to the potato tuber moth Phthorimaea operculella (Zeller) affecting tubers in Tunisian rustic stores. Exponential curve of death, i.e. constant force of mortality functions were used in a case study on a cohort time basis. The additive property of crude cause-specific forces of mortality is used to combine mortality factors, which permits to judge different control strategies. In an integrated pest management perspective, the effect of basic mortalities, i.e innate and dispersal related mortalities is recombined with native natural enemies. Thereafter, we calculate the force of additional comptatible control factors needed to reduce the proportion of infested tubers below an economically relevant level. Control strategies based on native natural enemies are shown to have variable effects which put farmers at economic risks. The general importance of integrating compatible control factors is recognized in the evaluation of virus applications. Based on risk analyses, Tunisian potato growers were recommended, in the case of standard initial infestations of five eggs per tuber, to use virus preparations at a higher dosage than the 0.0015 larval equivalent/kg as used in this study. Moreover, the study confirms the general importance of entering potatoes with low infestation levels into rustic shelters. This considerably decreases the risk of exceeding the economic threshold and make virus based control efficient even at low dosages. If the initial infestation considerably exceeds five eggs per tuber, the integration of a compatible control factor becomes more difficult. Nevertheless, even in absence of additional control factors the infestation of tubers does not exceed 50%.