FOOD PLANT CHOICES OF 2 GOLDENROD BEETLES - RELATION TO PLANT-QUALITY

The leaf beetles Trirhabda borealis and T. virgata are specialist herbivores of meadow goldenrods, Solidago spp., in central New York [USA]. Upon emergence, both larvae and adults must actively search for food plants, and must choose among the 5 goldenrod species (S. canadensis, S. gigantea, S. graminifolia, S. juncea and S. rugosa) that co-occur in old fields. The relationship between Trirhabda foraging decisions and plant quality was examined by comparing food preferences in the field with the performance of beetles caged on each host. Trirhabda adults were highly selective in their use of food plants. Adults of T. borealis preferred a single host, S. canadensis; T. virgata adults were most common on S. canadensis and S. gigantea. These preferences were not strictly related to variation in plant quality. In the laboratory, T. borealis performed equally well on 4 goldenrods (but completely failed to reproduce when fed S. graminifolia), and T. virgata performed equally well on all 5 hosts. Larval feeding preferences in each beetle species were broader, and were more in accord with subtle variation in plant quality. Newly-hatched T. borealis larvae readily colonized 4 hosts and rejected only S. graminifolia, which conferred the lowest survivorship and the slowest growth. Larvae of T. virgata accepted each host, even though growth rates were somewhat slower on S. graminifolia and S. juncea. Ontogenetic differences in host preference and host tolerance may have evolved because of the different host-finding abilities of each beetle stage. During host search, the sluggish, newly-emerged larvae may be more food-limited and accept even marginally inferior food plants. The relatively mobile adults are more discriminating and use a subset of suitable hosts. The intermingled dispersion of Solidago species in old fields results in frequent larval colonization of hosts seldom used by adults. In diverse plant communities, ultimate patterns of food plant choice can be a complex function of at least 3 factors; intrinisic plant quality, local plant dispersion, and the host-search abilities of the insect forager.