Studies onGomphocarpus physocarpus: Further evidence of preferential feeding by the aphid,Aphis nerii on the internal phloem

Penetration of the stems and leaves ofGomphocarpus physocarpus (E.Mey) by the aphid,Aphis nerii (B. de F.) was studied with light and phase microscopes. Penetration of the epidermis and ground tissues was largely intercellular, that of the phloem tissues partly intercellular and in part intracellular. In the large majority of penetrations the external phloem was bypassed, the stylet tracks terminating in the sieve tubes of the internal phloem. Of 75 pairs of stylet tips encountered in presumably functional sieve tubes 73 were lodged in sieve tubes of the internal phloem. This confirms observations of a preliminary study which indicated thatA. nerii feeds preferentially on sieve tubes of the internal phloem. A satisfactory explanation of this preferential feeding has yet to be provided.