Interspecific Competition among Phloem‐Feeding Insects Mediated by Induced Host‐Plant Sinks

The role of interspecific interactions among herbivorous insects is considered to be limited, especially in specialist communities. In the current study we report on exploitative interspecific interaction between two closely related phloem—feeding species of gall—forming aphids (Homoptera; Pemphigidae; Fordinae), mediated by the supply of photoassimilates from the host plant. Geoica sp. forms a spherical gall on the leaflet midrib of Pistacia palaestina (Anacardiaceae), while Forda formicaria forms crescent—shaped galls on the leaflet margin of the same host plant. Using ¹ ⁴C labeling, we were able to trace the food supply (assimilated carbohydrates) from the leaves to galls of each species. We found that Geoica galls are strong sinks. These galls divert the normal phloem transport of the plant and reduce the amount of assimilates imported by F. formicaria, especially when they are located on the same leaflet. By the end of the season Geoica caused death of 84% of F. formicaria galls that were located on the same leaflet, and reduced reproductive success in the surviving galls by 20%. This is because the presence of Geoica causes early senescence (but not abscission) of the leaflet it is on (whether or not F. formicaria is present). The interaction is asymmetrical: F. formicaria did not affect reproductive output of Geoica nor did it cause visible damage to the leaflets. To our knowledge, this it the first demonstration of exploitation competition for plant assimilates between two insect—induced sinks. This exploitative competition, mediated by manipulation of plant phloem transport, stands in contrast to the absence of interference competition for galling sites between the two aphid species. Although their spatial distributions partly overlapped, the niche breadth of each species (measured from gall positions on leaves along the shoot axis) was not affected by the presence of the other. Moreover, when both species were located on the same leaf, they formed galls independently on the same or different leaflets, and there was no indication of interference competition over galling sites.