Insecticidal and repellent activity of cinnamic acid derivatives: investigating alternative products to control imported fire ants (Formicidae: Hymenoptera)

Trans-cinnamic acid and its derivatives were investigated against Solenopsis invicta Buren (RIFA), Solenopsis richteri Forel (BIFA), and reproductively functional hybrid (HIFA) for their repellency and toxicity. A range of serial concentrations starting from 156 µg/g to the failure of treatments were evaluated. Removal of treated sand in a digging bioassay was used as the criterion for repellency. Treatments were considered active until the removal of treated sand became similar to that in the ethanol control. Based on sand removal, trans-cinnamic acid, benzyl cinnamate and phenethyl cinnamate failed at 156 µg/g, whereas isoamyl cinnamate was active at 19.5, 19.5, and 39 µg/g against RIFA, HIFA, and BIFA workers, respectively. Isopropyl cinnamate and η-butyl cinnamate were active at 19.5, 39, and 39 µg/g against RIFA, HIFA, and BIFA workers, and ethyl cinnamate at 9.8, 19.5, and 39 µg/g against HIFA, RIFA, and BIFA, respectively. DEET failed at 31.25, 62.5, and 15.6 µg/g against RIFA, BIFA, and HIFA, respectively. Ethyl cinnamate, with LC50 value of 54.8, 60.9, and 70.4 µg/g showed toxicity, followed by isopropyl cinnamate (LC50 = 71.1, 69.3, and 74.7 µg/g), η-butyl cinnamate (LC50 = 128.2, 86.1, and 134.2 µg/g) and isoamyl cinnamate (LC50 = 95.8, 142.1, and 139.2 µg/g) against RIFA, BIFA, and HIFA workers, respectively. Fipronil was more toxic with LC50s of 0.43, 0.33, and 0.51 µg/g against RIFA, BIFA, and HIFA workers, respectively. Trans-cinnamic acid, benzyl cinnamate, and phenethyl cinnamate did not cause any mortality at 156 µg/g. In residual bioassays, isoamyl and η-butyl cinnamate at 312 μg/g suppressed the digging activity for more than 3 wk. Different formulations of active cinnamates be further evaluated under field conditions.