Aphid galls accumulate high concentrations of amino acids: a support for the nutrition hypothesis for gall formation

Abstract

AbstractThe nutrition hypothesis for the adaptive significance of insect gall formation postulates that galls accumulate higher concentrations of nutritive compounds than uninfested plant tissue, resulting in a high performance of the gall former. This hypothesis has been supported by some taxa of gall insects, but not by taxa such as cynipid wasps. Aphid galls are expected to require higher levels of nitrogen than other insects’ galls with a single inhabitant, because aphid galls are required to sustain a number of aphids reproducing parthenogenetically over two generations. The present study tested this hypothesis by evaluating aphid performance and amino acid concentration in phloem sap, using the aphidRhopalosiphum insertum(Walker) (Homoptera: Aphididae), which establishes colonies on leaves ofSorbus commixtaHedlund or in galls of the aphidSorbaphis chaetosiphonShaposhnikov (Homoptera: Aphididae). We prepared the gall and non‐gall treatments on trees ofS. commixta, in whichR. insertumfundatrices were reared and allowed to reproduce. InS. chaetosiphongalls,R. insertumcolonies propagated more rapidly, and the second generation grew larger and more fecund than on ungalled leaves. The amount of amino acids exuding from cut galled leaves was fivefold that in ungalled leaves; however, there was no significant difference in the amino acid composition between galled and ungalled leaves. In the intact leaves, total amino acid concentration in the phloem sap declined rapidly from late April to late May; however, the galls retained this high amino acid concentration in developing leaves for 1 month. These results indicate that the improved performance inR. insertumis ascribed to the increased concentration of amino acids in galled leaves. We suggest thatS. chaetosiphongalls function to promote the breakdown of leaf protein, leading to an increased performance of gall‐inhabiting aphids.