Exposure to sugarcane borer-induced plant volatile (E)-caryophyllene enhances parasitoid recruitment

Natural enemy parasitoids locate herbivore-infested plants via detection of herbivore-induced plant volatiles (HIPVs) that are released in response to pest damage. Furthermore, synthetic HIPVs have been proposed as tools to enhance the biological control of crop pests. The sugarcane borer, Diatraea saccharalis Fabricius (Lepidoptera: Pyralidae), is a key herbivore pest of sugarcane, Saccharum spp. (Andropogoneae), in the Americas. To manage D. saccharalis in Brazil, more than 3 million ha of sugarcane have been treated with the larval parasitoid, Cotesia flavipes (Hymenoptera: Braconidae). In this study, the role of sugarcane HIPVs as cues in the host-finding process of C. flavipes was investigated using a combination of dynamic headspace collection, chemical analysis and laboratory behaviour experiments. Comparison of volatile organic compounds (VOCs) collected from D. saccharalis-damaged and healthy sugarcane revealed very similar VOC profiles apart from significantly higher levels of the sesquiterpene (E)-caryophyllene released from damaged plants. Naive female C. flavipes spent significantly more time in the olfactometer arm containing VOCs from D. saccharalis-damaged plants but showed no preference to VOCs from healthy plants. When (E)-caryophyllene was added to VOCs from healthy plants, parasitoids spent more time in the arm containing the combined treatment. Furthermore, in a dose-response experiment with synthetic (E)-caryophyllene, naive parasitoids preferred the compound across a dose range of 3-300 ng, and experienced parasitoids (pre-exposed to (E)-caryophyllene) responded to doses of (E)-caryophyllene as low as 0.03 ng. These results suggest that C. flavipes can use (E)-caryophyllene as a cue to locate D. saccharalis-infested sugarcane plants. Moreover, experienced females appear to respond to lower doses than naive females. These results potentially pave the way for increasing the efficiency of C. flavipes in biological control of D. saccharalis, the most important pest of sugarcane and maize crops in the Western Hemisphere, and also a major pest for rice and sorghum crops.