Xylem ingestion by winged aphids

When aphids and their host plant are incorporated in a DC electrical circuit, phloem and xylem ingestion register as separate waveforms of the electrical penetration graph (EPG) signal. Aphids are primarily phloem feeders; xylem ingestion is seldom reported but can be induced experimentally by fasting the insects in desiccating conditions. In experiments with the black bean aphid, Aphis fabae Scop., young winged (alate) and unwinged (apterous) virginoparous adults were collected from their natal host plants (broad bean, Vicia faba L.) and allowed 3-h continuous EPG-recorded access to V. faba seedlings. Several aphids (47% of both morphs) showed ingestion from phloem sieve elements. Alate aphids also showed frequent xylem ingestion (60% of individuals), but no apterous aphids exhibited this activity. The EPG technique involves attachment of a fine gold wire electrode to each insect, a process that may affect normal behaviour at the plant surface. However, when the technique was modified to monitor the stylet activities of freely-settled aphids, high levels of xylem ingestion by alates were also recorded. The results suggest that the developmental physiology of winged aphids somehow predisposes them to xylem ingestion, possibly as a result of dehydration during the teneral period. Alate aphids may reduce their weight by fasting before take-off, giving aerodynamic benefits, but making rehydration, via xylem uptake, a priority following plant contact.