Determination of the sampling size for the reliable identification of organic crops by inducing sublethal effects in beneficial insects

Abstract

Among the main innovations in olive growing, the conversion of cultivation practices towards organic agriculture stands out. Since the organic crops are subject to the granting of economic subsidies by the Spanish Ministry of Agriculture, this has given rise to fraudulent situations, masking the use of conventional pesticides in “fake organic” crops. To investigate alternative methods, complementary to analytical chemists that can provide reliable information to discerning organic crops from those subjects to synthetic pesticides is the aim of this research. A new technique is being developed, based on the induction of sublethal effects on the beneficial insects, through the experimental application of a commercial insecticide on a small scale, in the target crop. The effect triggered by the insecticide can be monitored by means of a yellow sticky trap device, installed for each olive tree, which allows to observe deviations in its capture rate from what occurs in the absence of insecticide (control) and finally verifying if the populations of beneficial insects affected in the olive grove, whether or not they are accustomed to the toxic action of the insecticide. Obviously, in order to maintain environmental integrity in the supposed ecological target olive groves, this induction test should be applied on the smallest scale as possible, thus reducing the application area of the pesticide, which implies adjusting the sample size to a minimum, maintaining an acceptable reliability in the estimates. During the autumn of 2017, six plots of 7x7 configuration were selected in an olive grove in the province of Jaén (southern Spain), three of these were treated with Dimethoate, while the remaining were considered as control. After the treatment application, sticky yellow traps were installed (one in each of the olive trees), thus allowing a maximum of 49 repetitions per plot. Among the identified beneficial insects, the most abundant species was Aeolothrips intermedius. Taking this species as a reference, the results indicate that the differences between treated and control parcel were statistically significant from a minimum sample size of 17 replications onwards. In coincidence with the above, the results have allowed determining that the minimum size of the experimental target plot, to obtain acceptable population estimates (relative error of up to 10%), would require a minimum area of at least 100 m2, representing a minimum number of 17 olive trees. These results provide a reasonable statistical basis for suitably adapting the methodology that allows acceptable estimates, therefore, the application of this methodology of detection of organic crops would require a very small area, equivalent to a practically insignificant number of olive trees, so it would not compromise the ecological quality of the target plantation.