Acaricidal effect of an isolate from Hoslundia opposita vahl against Amblyomma variegatum (Acari: Ixodidae)

Background: Hoslundia opposita Vahl. (Lamiaceae), a common local shrub in Ghana, is traditionally known not only for its pharmacological benefits but also for its insecticidal properties. Its acaricidal property, however, has not been investigated. Objective: To test the acaricidal effects of the crude extract and fractions of H. opposita leaves as well as to isolate and characterize the acaricidal principles. Materials and Methods: The crude methanolic extract, pet. ether, ethyl acetate and aqueous fractions of the leaves of H. opposita were tested against the larvae of the cattle tick, Amblyomma variegatum, using the Larval Packet Test. A bioassay-guided isolation was carried out to identify the acaricidal principle obtained from the ethyl acetate fraction. Results: The active principle was characterised as ursolic acid, a triterpene previously isolated from the leaves of the same plant. The extract and fractions were less potent than the control, malathion (LC₅₀ 1.14 Χ 10 -4 mg/ml). Among the plant samples however the crude methanolic extract exhibited the highest effect against the larvae (LC 50 5.74 Χ 10 -2 mg/ml), followed by the ethyl acetate fraction (LC₅₀ 8.10 Χ 10 -2 mg/ml). Ursolic acid, pet. ether and aqueous fractions however showed weak acaricidal effects with LC₅₀ values of 1.13 mg/ml, 8.96 Χ 10 -1 mg/ml and 1.44 mg/ml, respectively. Conclusion: Ursolic acid was not as potent as the crude methanolic extract and the ethyl acetate fraction from which it was isolated. The overall acaricidal effect of H. opposita may have been due to synergy with other principles having acaricidal properties.