Competition between honeydew producers in an ant–hemipteran interaction may enhance biological control of an invasive pest

AbstractAsian citrus psyllid,Diaphorina citriKuwayama (Hemiptera: Liviidae), is an invasive citrus pest in southern California, which secretes honeydew and has the potential to spread a lethal bacterial disease, huanglongbing, of citrus. In urban citrus, Argentine ant,Linepithema humile(Mayr) (Hymenoptera: Formicidae), also an invasive pest, tends honeydew-producing hemipterans. We used field data to determine whether the mutualistic relationship betweenL. humileand six established species of honeydew producers may hinder or favor the establishment ofD. citriand its biological control withTamarixia radiata(Waterston) (Hymenoptera: Eulophidae) in citrus via competition or mutualism for ants, respectively. In the field,L. humileandD. citriare engaged in a mutualistic relationship. Ants harvest solid honeydew secreted by psyllid nymphs and tended more than 55% of observedD. citricolonies.Linepithema humiledisplayed a preference hierarchy when tending honeydew producers infesting citrus. It responded equally or less intensively toD. citrithan to other honeydew-producing species. Consequently, the mutualism betweenL. humileandD. citriwas affected by the presence of other honeydew-producing species, and the percentage ofD. citricolonies tended byL. humile. The number of ants perD. citricolony also decreased as the number of other honeydew producers increased.Diaphorina citridensity was also affected by the presence of other honeydew producers. Both colony size and the number ofD. citrinymphs counted per tree decreased as the number of other honeydew producers increased. Our results indicate that competition between honeydew producers for the mutualist antL. humilemay hinder the establishment ofD. citriby possibly facilitating increased biological control.