Birds using honeydew reduce leaf herbivory on scale insects' host plant in fragmented cloud forests of Mexico

Honeydew, a sugar solution produced as waste by phloem-feeding insects, is commonly a prized food for ants, which may serve to protect the insects' host plant by reducing the number of other herbivorous insects. A wide variety of birds both eat arthropods and use the honeydew produced by the scale insect Stigmacoccus garmilleri (family Margarodidae), which is associated with oak trees (Quercus spp.) in humid, tropical, montane forests of Mexico. By removing potential leaf-chewing insects, birds may aid the plant by reducing the amount of leaf area lost, as do the ants in ant-honeydew systems. We used bird exclosures to examine the indirect effects of birds on leaf damage on oak trees harboring scale insects. Twenty-one experimental trees were chosen with a range of scale insect densities along a gradient of habitat fragmentation. The amount of leaf damage was recorded on two exclosure branches and two control branches in each of the study trees. Leaves on the branches from which birds were excluded sustained substantially more damage than those on control branches, a result that suggests that birds may decrease herbivory on trees harboring both scale insects and other herbivorous insects. The magnitude in reduction of herbivory by birds was also affected by the local habitat structure, intensity of scale insect distribution and honeydew food source available to birds. These results suggest that scale insects may benefit plants indirectly and may support the presence of an indirect mutualism between oaks and scale insects via birds.