STYLET PENETRATION BY LARVAE OF THE GREENHOUSE-WHITEFLY ON CUCUMBER

Probing behaviour of Trialeurodes vaporariorum (Westwood) larvae was m onitored using the DC electrical penetration graph (EPG) technique on the host plant cucumber. EPGs were recorded for 16 h, simultaneously w ith honeydew excretion using a 'honeydew clock'. Three waveforms were distinguished: a pathway waveform (C), and two phloem waveforms, one w ith a high (H), and one with a low frequency (L) signal. The C wavefor m mainly occurred in the crawler stage of the Ist instar larvae. EPGs recorded from larvae during and after moulting indicated that the proc ess involves stylet withdrawal; hence the stylets of each new instar n eed to penetrate again from the leaf surface to the phloem. All sessil e stages, from L1 to pre-pupa, spent almost their entire time in wavef orms H and L. These waveforms alternated more frequently in the early instars than during the later ones, in which the H waveform became pre dominant. The H waveform was highly correlated with honeydew excretion and thus phloem sap ingestion. The L waveform was not related to hone ydew excretion but EPGs indicated that the stylet tips remain in a sie ve element during both waveforms. Periods of honeydew production demon strated 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 i nstar to the pre-pupa, the frequency of excreted honeydew droplets dec reased but their size increased, causing a net increase of the excreti on rate.