Tropical legumes of the genus Stylosanthes immobilize and kill cattle ticks

Abstract

Ticks affect 800 million cattle and a similar number of sheep worldwide¹ and their control by chemical ‘dipping’ is expensive and threatened by the widespread development of resistance². Resistant varieties of crops have been used widely to control phytophagous insects³, but larvae of all species of hard ticks (Ixodidae) ascend plants only in order to transfer to a passing host. Contacts with hosts are infrequent thus the ticks must often wait in the pasture for several weeks⁴. A substantial reduction in the life expectancy of such larvae would lower populations of the damaging parasitic stages. Molasses grass Melinis minutiflora, has been shown to reduce tick survival but the effect is small and slow⁵. Some highly productive, nutritious varieties of the tropical pasture legume Stylosanthes^6,7, are covered in glandular trichomes or hairs which secrete a viscous fluid. Such trichomes are well known for their role in defence against a variety of plant-feeding insects^3,8. We report here that two South American species of Stylosanthes produce sticky secretions which immediately immobilize larvae of the cattle tick, Boophilus microplus. The larvae are poisoned within 24 hours by an unidentified vapour from the secretions. The legumes appear to have the potential to substantially reduce populations of all species of ticks in the extensive tropical and subtropical areas in which Stylosanthes sp. can be grown in pasture.