Small‐scale farmers in northern Ecuador, at an elevation of 2500 m, place leaves of castor beans (Ricinus communis) in germinating corn fields to reduce two Coleoptera pest species (Pseudothinobatis rufinasus(Kirsch), Tenebrionidae, and an unknown genus, subfamily Naupactini, Curculionidae). The objective of this study was to determine howP. rufinasusis affected by castor leaves. In a preference study under laboratory conditions,P. rufinasuspreferred castor leaves to leaves of corn, cabbage or paper, yet they did not appear to consume the castor. Approximately 80% of the beetles associated with castor. leaves for 12 h or more exhibited paralysis. In the laboratory, castor leaves were not directly lethal toP. rufinasus.Similar levels of mortality occurred when beetles were kept in vials with the test materials for 10 days. In the field, castor leaves were placed between rows of recently planted corn, as local farmers do. Mortality ofP. rufinasuscollected from exposed leaves was compared toP. rufinasusmortality from similar leaves protected from the sun by small shelters. The development of paralysis prevented beetles from leaving the castor leaves to hide during the day and the resulting exposure to direct solar radiation significantly increased their mortalaity.