Diffusion of the maize root signal (E)-β-caryophyllene in soils of different textures and the effects on the migration of the entomopathogenic nematode Heterorhabditis megidis

Maize roots respond to feeding by larvae of the beetle Diabrotica virgifera virgifera by releasing (E)-β-caryophyllene (Eβc). This insect-induced root volatile attracts entomopathogenic nematodes (EPN) and thereby helps to protect the roots against herbivore damage. Previous studies suggest that diffusion of Eβc occurs through the gaseous rather than the aqueous phase in sand and its diffusion is best at low levels of humidity. However, it remains largely unknown how Eβc diffuses in typical natural and agricultural soils. To fully understand the function and efficiency of root-produced Eβc as a belowground signal it is important to know how it spreads in real soils and how soil properties affect its diffusion. Using gas chromatography-mass spectrometry analyses, the diffusion of two doses of synthetic Eβc (200 ng and 20,000 ng) injected in sand was compared with the diffusion of Eβc injected in clay, clay-loam and sandy-loam soils, at 3 moisture levels (5, 10 and 20% water), and at two distances (5 and 10 cm) from the Eβc injection point. The diffusion of the compound was measured with a Solid Phase Micro Extraction (SPME) fiber every 30 minutes over a period of 9 hours. We found that, in contrast to the pattern observed for pure sand, Eβc diffused best when humidity was high in the three agricultural soils. In subsequent experiments we used glass-trays to create two types of mesocosms to assess the effect of synthetic or root-produced Eβc on the dispersal of the EPN Heterorhabditis megidis and its infection of sentinel hosts in the trays. The presence of synthetic Eβc did not affect the ability of H. megidis to infect the sentinel host. However, under the test conditions, Eβc released from maize roots influenced the migration behaviour of H. megidis depending on soil type. The results suggest that D. virgifera-damaged maize plants may recruit H. megidis more efficiently in clay loam soils than in other types of soil. These new insights into the diffusion dynamics and attraction efficiency of the root-produced signal Eβc may help efforts to develop novel strategies for the sustainable management of the maize pest D. virgifera.