Stylet penetration activities by aphids

The composition of stylet penetration behaviour in aphids and its possible role in food-plant selection is the subject of these studies. Evidence is presented that the labium is devoid of external chemoreceptors (Chapter 1). In addition to other morphological data (Wensler & Filshie, 1969; Wensler, 1974) this suggests that internal chemoreceptors are involved in assessing plant quality. Internal gustation can only occur during stylet penetration. Another indication of a central role of stylet penetration in food plant selection is provided by behavioural observations (Klingauf, 1970; Tjallingii, 1976; Chapter 6) which show that differences in the reactions to host and non-host plants are inconspicuous before stylet penetration, but very distinct from the time of first penetration.<p/>For further investigation of stylet penetration a DC modified method of electrically recording this behaviour was developed. In the recorded signal, the electrical penetration graph (EPG), six different wave form patterns, A to F, have been distinguished. Some of these patterns have been correlated experimentally with penetration activities (Chapter 2). Pattern A, B, and C were correlated principally with saliva secretion and pattern D with ingestion, but some ingestion occurs during A, B, and C. Pattern E and F occur almost exclusively in combination with pattern D. During pattern D+E saliva pump muscle activity was correlated with the pulses of the E pattern. Pattern D+F could not be correlated with ingestion. These relations are somewhat coarse and mainly based on statistical correlations, not on causitive relations. A more refined analysis of the EPG relations requires knowledge of the physical backgrounds of the electrically recorded signals. In Chapter 3 it is shown that the signal is caused by resistance fluctuations in the stylet region (resistance component) and by electro motive force (emf) sources in the aphid as well as in the plant (emf component). The contribution of each component to the signal depends on the input specifications of the amplifier used, especially its input resistance. The DC method is compared with the AC method used by McLean and Kinsey 1964, 1965).<p/>In Chapter 4 and 5 the 'potential drop' (pd), a phenomenon due to an emf source, is described and experimentally related to the penetration of a plant cell membrane by the stylet tip. The membrane potential causes the sudden drop of the potential in the signal, which may last for short or longer periods. A short pd reflects a protoplast puncture for 5 to 20 s. During long pds, lasting about 10 min to several hours or longer. the signal pattern D+E is recorded (abbr.: D+E(pd)). When stylets were amputated during long pds. phloem sap immediately exuded from the severed stump in the leaf. Subsequent EM showed the maxillary stylet tips being penetrated beyond the plasmalemma into a sieve element cell.<p/>It was shown that penetration behaviour is affected by wiring and connecting an aphid to the electrical circuit (Chapter 6). Since toxic and electrical effects can probably be ruled out, the effects are presumably due mainly to locomotion restraints. The effects of wiring on some penetration behaviour parameters of aphids on host plants appear to be opposite to that of aphids on non-host plants. It is concluded that electrical recording of stylet penetration should not be applied without control experiments with free aphids. especially when susceptibility or resistance of plants to aphids is studied.