Fatal attraction: The influence of prey sensory modality on capture rates in insectivorous plants

Background/Question/Methods

Carnivorous plants are adapted to growing in areas of poor soil nutrition and are able to derive some of their nutrients by trapping animals. Unlike most other predators, carnivorous plants lack mobility thus must attract their prey through a combination of visual and olfactory cues. In this study, the relative importance of visual versus olfactory cues was tested by comparing capture rates of wild type Drosophila melanogaster and a visually impaired mutant (w¹¹¹⁸). The study compared prey capture rates in plants with adhesive traps (Drosera capensis, sundews) and plants with a pitcher trap (Nepenthus ventricosa). This study addressed three different questions regarding the predator-prey relationships between D. capensis and N. ventricos, and D. melanogaster. 1) Are visual or olfactory cues more important for prey capture in carnivorous plants? (2) Is there a difference in capture rates between the plant species? (3) How does predator vision affect insect capture rates in the carnivorous plant species examination in this study?

Results/Conclusions

Both D. capensis, and N. ventricosa were equally effective at capturing prey; capture rates did not differ between the two species (P<0.8).  There were however, differences between individual plants in terms of their ability to capture flies (P<0.002 ). In comparing wild type and mutant flies, we found that the visually-impaired mutants were captured at nearly double the rate of wild type flies (P<0.03). This suggested that either mutant flies had heightened olfactory sense, leading to increased attraction to the plants, or that visual cues were exploited for predator avoidance in wild-type flies. To test whether mutant flies had heightened olfactory senses, fruit traps (using ripened bananas) were used to test for chemotactic differences between mutants and wild type flies. We found that wild type flies were caught in the fruit traps at a higher rate than mutant flies (P<0.01 ). This supports the hypothesis that wild type fruit flies may be using available visual cues in avoiding the traps of the carnivorous plants. These findings highlight a potential predator-prey arms race dynamic, where carnivorous plants preying primarily on Drosophila species may be under selection to produce stronger olfactory cues and less conspicuous coloration, while the fruit fly prey species may be under reciprocal selection for heightened sensitivity to visual cues in the predatorial plant.