John Kennedy was one of the most influential insect physiologists of the 20th century (Brady, 1997), so it is not surprising to find one of his papers in the top 20 of those published by the Annals of Applied Biology over the last 100 years. The paper Kennedy et al. (1961) was the third in a series of papers on host finding by aphids published in the Annals by Kennedy and his coworkers. The previous two dealt with the gynoparae (the aphid morph produced on the secondary host that migrates back to the primary host and gives birth to the sexual females, oviparae) of Myzus persicae (Kennedy et al., 1959a) and the gynoparae of Aphis fabae and Brevicoryne brassicae (Kennedy et al., 1959b), which have been cited 80 and 89 times, respectively. The third and most cited paper (192 citations by the end of 2013) describes experimental and observational data gathered over more than a decade.
Kennedy very strongly believed that aphids found their hosts using visual cues and had, as early as 1950, postulated an optomotor effect (Kennedy, 1950), which is essentially a reflex involving the turning of head or body in response to moving stripes of differing luminosity. By the late 1950s, Kennedy was absolutely convinced that vision was the only way in which aphids found their hosts and the first sentence in the introduction to their Kennedy et al. (1961) paper makes this abundantly clear ‘Apart from olfactory attraction to plants, of which there is no evidence whatever (Kennedy, Booth & Kershaw, 1959a,b) two types of visual response have been suggested…’ Brave words indeed and this must certainly have had an effect on research workers in the field.
What has made this paper such a well cited one? I think that there are two main reasons why this paper has become an Annals classic. First, it reported very detailed work on the colour response of aphids in both field and laboratory experiments, analysing the reflectance of different coloured surfaces. The research confirmed that yellow was an attractive colour to aphids, as demonstrated some years previously by Moericke (1952) and that there were seasonal changes in the response to yellow that helped explain the way in which host-alternating aphids were able to find their primary and secondary hosts at the right time of year. Secondly, the paper demonstrated that if the alighting substrates were of contrasting colours and wavelengths, black paper versus white for example, then the aphids could be manipulated into delaying their settling, or indeed, not settling at all.
Both of these findings, plus the very definite statement that aphids did NOT use odour to find their host plants, meant that other scientists could directly test and apply the results in both the laboratory and field. Despite this, the paper was only cited 12 times between publication and 1970. Since then it has gained a wider and wider audience, with 92 papers citing it since the turn of the century. The immediate applications of the paper were in field studies on other aphid species in relation to cropping patterns (A'Brook, 1968, 1973). This also fuelled the interest seen in using intercropping as a means of pest control, suggested ironically enough in the pioneer work of Tahvanainen & Root (1972) who postulated that specialist herbivores were confused by the combination of odours emanating from mixed cropping. Notwithstanding, many of the modern papers still refer to visual cues as being a very important component of host location by aphids (Pitan & Olatunde, 2006; Wang et al., 2011; Le Guigo et al., 2012).
The belief that aphids only used vision to locate their host was not disproved until 9 years after Kennedy's bold statement, by the often overlooked but pioneering work of Pettersson (1970) who, using an olfactometer, demonstrated that aphids were able to detect and respond to plant odours from a distance without visual cues. The first field proof of this effect was not published until 1981 (Chapman et al., 1981) but it has since then been shown to occur widely amongst aphids (Döring, 2014).
Kennedy's work as a whole stimulated huge interest in research on host location by phytophagous insects, not just aphids (Marucci et al., 2005), and as mentioned earlier continues to be cited to this day as the exemplar of the field.