Nutrition as a facilitator of host‐race formation: the shift of a stem‐boring beetle to a gall host

The importance of host‐race formation to herbivorous insect diversity depends on the likelihood that successful populations can be established on a new plant host. A previously unexplored ecological aid to success on a novel host is better nutritional quality. The role of nutrition was examined in the shift of the stem‐boring beetle Mordellistena convicta to fly‐induced galls on goldenrod and the establishment there of a genetically distinct gall host race. 2. First, larvae of the host race inhabiting stems of Solidago gigantea were transplanted into stems and galls of greenhouse‐grown S. gigantea plants. At the end of larval development, the mean mass of larvae transplanted to galls was significantly greater than the mass of larvae transplanted to stems, indicating a likely nutritional benefit during the shift. This advantage was slightly but significantly diminished when the gall‐inducing fly feeding at the centre of the gall died early in the season. Additionally, there was a suggestion of a trade‐off in the increased mortality of smaller beetle larvae transplanted into galls. 3. In a companion experiment, S. gigantea gall‐race beetle larvae were likewise transplanted to S. gigantea stems and galls. Besides the expected greater mass in galls, the larvae also exhibited adaptations to the gall nutritional environment: larger inherent size, altered tunnelling behaviour, and no diminution of mass pursuant to gall‐inducer mortality. 4. In a third line of inquiry, chemical analyses of field‐collected S. gigantea plants revealed higher levels of mineral elements important to insect nutrition in galls as compared with stems.