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A method for amputating homopteran stylets using a pulsed dye laser

The important contribution made by techniques of stylet amputation to the study of the host plant relationships ofhomopterous insects has been evident ever since the work of Kennedy & Mittler (1953) who used a small piece of razor blade to cut the mouthparts of probing and feeding willow aphids, Tuberolachnus salignus Gmelin. This operation made it possible to collect samples of pure phloem sap for subsequent sugar amino acid analysis (Mittler, 1958). With T. salignus the operation was facilitated by the use of the woody stem; from which the insect feeds, as an 'anvil'. This is however not possible when studying insects which colonise non-lignified tissues. Van Soest (1955) constructed an ingenious pair of micro-scissors using a pair of forceps and pieces of razor blade. McLean & Kinsey (1967) found that the stylets of the pea aphid Acyrthosiphon pisum feeding on Vicia faba could be severed by careful use of ophthalmic micro-scissors; the resulting stylet stumps were used as a basis for the histological correlation of stylet position with electrical recordings of feeding behaviour. In their recent survey Fisher & Frame (1984) reported that the mouthparts of armoured scales (Family Diaspididae) are so firmly fixed in the plant that they may be broken off simply by flicking the insect body from the plant. Unfortunately few Homoptera are as simple to work with and in most cases it has been necessary to develop more sophisticated techniques. Probably the most successful and widely used to date was developed by Downing & Unwin (1977) and described in detail by Unwin (1978). The method relies on the use of high voltage, at radio frequency (27 MHz) fed through a fine and tapering tungsten electrode to cauterise the mouthparts. If the power is high then the rate of temperature rise will be great enough to sever the stylets without blocking However, during our work using the brown planthopper, Nilaparvata lugens (Sfftl) we found this method unsuitable for stylet amputation. It requires the tungsten electrode to touch the rostrum of the insect, at which point the radio frequency supply is activated and the stylets are cauterised. When using N. lugens it was impossible to achieve this without disturbing the insect and causing it to withdraw its stylets from the plant. Therefore a remote method was needed, involving no physical contact between the cutting device and the rostrum. The use of a focussed laser beam was introduced by Barlow & McCully (1972), who succeeded in amputating the stylets ofA. pisum using a ruby laser, emitting a beam at a wavelength of 694.3nm. Similar results were obtained by Kawabe et al. (1980) who amputated the stylets of N. lugens using a Yttrium-Aluminium-Garnet (YAG) laser. This paper describes a technique for stylet amputation using a pulsed dye laser which incorporates an accurate targetting system and has been successfully used in our studies of the host plant relations of N. lugens.