IMPACT OF 2 SPECIALIST INSECT HERBIVORES ON REPRODUCTION OF HORSE NETTLE, SOLANUM-CAROLINENSE

The frequency of coevolution as a process of strong mutual interaction between a single plant and herbivore species has been questioned in l ight of more commonly observed, complex relationships between a plant and a suite of herbivore species. Despite recognition of the possibili ty of diffuse coevolution, relatively few studies have examined ecolog ical responses of plants to herbivores in complex associations. We stu died the impact of two specialist herbivores, the horse nettle beetle, Leptinotarsa juncta, and the eggplant flea beetle, Epitrix fuscula, o n reproduction of their host, Solanum carolinense. Our study involved field and controlled-environment experimental tests of the impact on s exual and potential asexual reproduction of attack by individuals of t he two herbivore species, individually and in combination. Field tests demonstrated that under normal levels of phytophagous insect attack, horse nettle plants experienced a reduction in fruit production of mor e than 75% compared with plants from which insects were excluded. In c ontrolled-environment experiments using enclosure-exclosure cages, the horse nettle's two principal herbivores, the flea beetle and the hors e nettle beetle, caused decreases in sexual reproduction similar to th ose observed in the field, and a reduction in potential asexual reprod uction, represented by root biomass. Attack by each herbivore reduced the numbers of fruits produced, and root growth, when feeding in isola tion. When both species were feeding together, fruit production, but n ot root growth, was lower than when either beetle species fed alone. E cological interactions between horse nettle and its two primary herbiv ores necessary for diffuse coevolution to occur were evident from an o verall analysis of the statistical interactions between the two herbiv ores for combined assessment of fruit and vegetative traits. For eithe r of these traits alone, the interactions necessary to promote diffuse coevolution apparently were lacking.