Stylet penetration by larvae of the greenhouse whitefly on cucumber

Probing behaviour of Trialeurodes vaporariorum (Westwood) larvae was monitored using the DC electrical penetration graph (EPG) technique on the host plant cucumber. EPGs were recorded for 16 h, simultaneously with honeydew excretion using a 'honeydew clock'. Three waveforms were distinguished: a pathway waveform (C), and two phloem waveforms, one with a high (H), and one with a low frequency (L) signal. The C waveform mainly occurred in the crawler stage of the Ist instar larvae. EPGs recorded from larvae during and after moulting indicated that the process involves stylet withdrawal; hence the stylets of each new instar need to penetrate again from the leaf surface to the phloem. All sessile stages, from L1 to pre-pupa, spent almost their entire time in waveforms H and L. These waveforms alternated more frequently in the early instars than during the later ones, in which the H waveform became predominant. The H waveform was highly correlated with honeydew excretion and thus phloem sap ingestion. The L waveform was not related to honeydew excretion but EPGs indicated that the stylet tips remain in a sieve element during both waveforms. Periods of honeydew production demonstrated a delay of 30-40 min in relation to the onset and end of H and L waveforms. This delay is presumably related to the time needed for food passing through, or emptying of, the insect's gut. From the Ist instar to the pre-pupa, the frequency of excreted honeydew droplets decreased but their size increased, causing a net increase of the excretion rate.