‘Attract and reward’: Combining a herbivore-induced plant volatile with floral resource supplementation – Multi-trophic level effects

Abstract

Two field experiments were conducted to assess whether a concept termed ‘attract and reward’ (A&R) could enhance conservation biological control (CBC). In A&R, a synthetically-produced herbivore-induced plant volatile (HIPV) (‘attract’) is combined with a floral resource (‘reward’). It is anticipated that the two will work synergistically, attracting natural enemies into the crop (‘attract’) and maintaining them within it (‘reward’).The study was conducted in Canterbury, New Zealand and the system consisted of brassica crop, commonly occurring brassica herbivores, their natural enemies and higher order natural enemies. The HIPV deployed was methyl salicylate (MeSA) and the floral resource was buckwheat Fagopyrum esculentum.The first experiment assessed the abundance of arthropods from three trophic levels and the second evaluated herbivore abundance, parasitism and hyper-parasitism rates. No synergistic effect of ‘attract’ and ‘reward’ was observed in either experiment. Populations of three parasitoids, one hoverfly and one lacewing from the third trophic level and a fourth trophic level lacewing parasitoid increased significantly in treatments with buckwheat. One hoverfly species was significantly more abundant in treatments with MeSA, but less abundant in treatments with buckwheat. The effect of MeSA on Diadegma semiclausum abundance depended on sex, with fewer males and more females being caught. Treatments with MeSA had significantly higher aphid parasitism rate.Combining MeSA and buckwheat could be beneficial because the two techniques increase the abundance of different natural enemies. Thus, these results indicate that A&R has potential as a CBC technique, as long as any unwanted side effects can be managed.