Examination of the interaction between the black vine weevil (Coleoptera: Curculionidae) and an entomopathogenic fungus reveals a new tritrophic interaction

The purpose of this study was to characterize the behavior of black vine weevil larvae, Otiorhynchus sulcatus (F.), in the presence of two possible control options: the synthetic pyrethroid bifenthrin and the entomopathogenic fungus Metarhizium anisopliae (Metch.) Sorokin. Five third-instar black vine weevil were placed in two-choice soil olfactometers that allowed larvae to infest one of two pots. Larvae were allowed to choose between M. anisophae (1 X 10(6) spores/g dry media) and untreated media, bifenthrin (25 ppm) and untreated media, as well as M. anisopliae- and bifenthrin-treated media. For all comparisons, experiments were conducted without plants in the system to test for innate responses, as well as with plants to test host-plant influence. Larvae were significantly deterred by bifenthrin without plants present in the system. No significant effect on larval preference was observed when M. anisopliae was present in media for trials without plants. M. anisopliae-treated media was preferred by black vine weevil larvae over bifenthrin without plants present in the two-choice soil olfactometer. When plants were included, a significant attraction to M. anisopliae-treated media was observed over untreated media. Unlike comparisons without plants, larvae were not repelled by bifenthrin when plants were included in the two-choice soil olfactometer. The attraction of black vine weevil larvae to pots containing plants and fungus indicates the operation of a previously undescribed tritrophic interaction. This behavior may be useful in the development of more effective biological control programs.