FACTORS THAT OPTIMIZE THE EFFECTIVENESS OF THE POWDER OBTAINED FROM Senecio salignus ROOTS AGAINST THE MEXICAN BEAN WEEVIL

Abstract

Chilca (Senecio salignus DC., Asteraceae) root has shown effectiveness for the control of the Mexican bean weevil (Zabrotes subfasciatus Boheman, Coleoptera: Bruchidae); however, several other factors should be evaluated in order to reach a rational use of this natural resource and to obtain the best insecticidal effect. Several bioassays were carried out in order to know: a) The effecttiveness of the root of S. salignus collected at several dates and different storage times, against Z. subfasciatus; b) The period that chilca roots can protect against this insect pest; c) The effect of chilca root powder on the oviposition of Z. subfasciatus in bean (Phaseolus vulgaris L., Fabaceae) grains cv. ‘Flor de mayo’; and d) The effect of this powder on a native black bean from Chiapas, México, also regarding the control of this weevil. Ten pairs of the Mexican bean weevil adults were placed in a glass jar which contained from 0.05 to 0.5 g of ground chilca root and 100 g of bean grains. At 4 and 50 d after infestation, insect mortality and number of eggs per grain were registered, along with emergence of new adults and level of damage to the bean grains. The powder of chilca root at 0.5 % (5 g of powder per kg of bean grains), collected at any time of the year and stored up to seven months, was able to kill the insect adults of Z. subfasciatus and thus to protect the stored bean up to 30 d after impregnation. The chilca powder also inhibited the insect oviposition, and was effective against the pest on a native black bean.