APHID STYLET ACTIVITIES DURING POTYVIRUS ACQUISITION FROM PLANTS AND AN IN-VITRO SYSTEM THAT CORRELATE WITH SUBSEQUENT TRANSMISSION

The behavioural events associated with acquisition of tobacco etch potyvirus by starved Myzus persicae during single, electrically-recorded penetrations of plants or a Parafilm membrane were compared. Twenty nine percent of aphids acquired virus from plants and subsequently transmitted to test plants. Stylet puncture of the plasmalemma, indicated by a potential drop (pd) to the intracellular signal voltage level, occurred during 84% of penetrations, and virus transmission was always associated with this behavioural event during acquisition. Periods of intracellular stylet tip location, known as pd phase II, ranged from 3.6-12.2s, and always comprised three consecutive sub-phases, designated II1, II2 and II3. Ninety six percent of pds included distinct pulses during phase II3. A waveform which closely resembled these pulses was produced by 59% of aphids that probed a virus suspension through a Parafilm membrane; nineteen percent of the aphids subsequently transmitted membrane-acquired virus and transmission was significantly associated with the occurrence of the phase II3-like pulses during acquisition. The duration of occurrence of recorded phase II3 pulses, either on plants or the in vitro system, did not influence the virus transmission efficiency of aphids. The association of virus uptake from aqueous suspension with a particular behavioural activity is discussed as evidence for the 'ingestion-egestion' hypothesis for non-persistent transmission. Starved aphids acquiring virus from infected leaf tissue or the in vitro system had significantly higher transmission efficiencies than non-starved aphids. Starved and non-starved insects were electrically-recorded penetrating the artificial membrane, and again there was a clear difference in transmission efficiency (starved aphids, 26%; non-starved aphids, 2%). The higher transmission efficiency of starved insects could not be explained by behavioural differences, and the results lend support to the hypothesis that non-behavioural factors determine the enhancement of potyvirus transmission by pre-acquisition starvation.