Aphis fabae gynoparae occurred in the same large proportion in simultaneous collections of all aphids alighting and probing on, and taking off from, a host plant (spindle) and a non-host (peach), and behaved similarly when approaching and leaving them in the same conditions. Most alighters took off again from leaves of both kinds within a few minutes, staying longer and probing more times on the host. In atmospheric conditions favouring local 'hovering' instead of dispersal, flying and alighting aphids became concentrated around host plants, not through any specific attraction to them, but apparently because more aphids had accumulated upon them and were now taking off.Brevicoryne brassicae occurred in the same large proportion in simultaneous collections of aphids alighting on a host plant (cabbage) and a non-host (sugar beet). No satisfactory evidence was found of preferential alightment on cabbage and there were indications of preferential alightment on the non-host. A very small minority of the A. fabae and B. brassicae that alighted on their hosts stayed there long enough to larviposit. This minority was rather larger among alighters late in the day, but in the absolute sense, more colonization occurred during earlier periods when more aphids arrived.The intensely dispersive type of host-finding behaviour in Myzus persicae, A. fabae and B. brassicae may be common among Aphididae. It seems ideal for the dissemination of non-persistent plant viruses, more particularly among the lessfavoured host plants of each aphid. The tendency to commensal association between virus and vector provides an ecological framework which may govern the incidence of virus-vector specificity and symbiosis.

Aphis fabae gynoparae occurred in the same large proportion in simultaneous collections of all aphids alighting and probing on, and taking off from, a host plant (spindle) and a non‐host (peach), and behaved similarly when approaching and leaving them in the same conditions. Most alighters took off again from leaves of both kinds within a few minutes, staying longer and probing more times on the host. In atmospheric conditions favouring local ‘hovering’ instead of dispersal, flying and alighting aphids became concentrated around host plants, not through any specific attraction to them, but apparently because more aphids had accumulated upon them and were now taking off. Brevicoryne brassicae occurred in the same large proportion in simultaneous collections of aphids alighting on a host plant (cabbage) and a non‐host (sugar beet). No satisfactory evidence was found of preferential alightment on cabbage and there were indications of preferential alightment on the non‐host. A very small minority of the A. fabae and B. brassicae that alighted on their hosts stayed there long enough to larviposit. This minority was rather larger among alighters late in the day, but in the absolute sense, more colonization occurred during earlier periods when more aphids arrived. The intensely dispersive type of host‐finding behaviour in Myzus persicae, A. fabae and B. brassicae may be common among Aphididae. It seems ideal for the dissemination of non‐persistent plant viruses, more particularly among the less‐favoured host plants of each aphid. The tendency to commensal association between virus and vector provides an ecological framework which may govern the incidence of virus‐vector specificity and symbiosis.

With 3 Text-figures)Attempts to test experimentally the theory of olfactory attraction to host plants gave negative results, and more conclusive evidence was sought by recording the behaviour of host-finding migrants in the field. T h e first of three occasions when the natural alighting rate was high enough for comparative purposes was an autumn migration dominated by Myzus persicae returning to its specific overwintering host, the peach tree.The proportion of M . persicae gynoparae among all aphids caught alighting on a spindle tree, where persicae does not overwinter, was as high as on peach. The much heavier accumulation of migrants on the peach was evidently due, not to their differential alightment, but to their differential rate of departure, with a longer average stay on the peach. The proportion of persicae gynoparae was smaller in catches on transparent sticky traps than among alighters on the trees, and they avoided alighting on brussels sprout plants more often than on peach. Only a fraction of those that approached the plants alighted, even on peach; and only a small fraction of those that did alight, stayed, even on peach.There was appreciable colonization of spindle and sprouts by the M . persicae gynoparae and some 'wasted' oviposition by their progeny on spindle. After the migration was over the peach leaves entered the last stages of senescence before the spindle leaves, and persicae gynoparae (no longer able to fly) that were given access to both at that time, settled preferentially on spindle. T h e distribution of males created the impression that they were less host-specific than gynoparae.

Glasshouse studies were made on honeydew production, adult and nymphal body weight gain, fecundity and size of egg groups of brown planthopper Nilaparvata lugens (Stål) (Homoptera: Delphacidae) (BPH) on CVS TN1, ASD7, Babawee, Mudgo, Rathu‐Heenati and Ptb33 rice plants with watering regimes of 10 ml, 20 ml, 30 ml and excess water daily. Honeydew production, weight gain, fecundity and size of egg groups were greatest on TN1, followed by ASD7. BPH did less well on Babawee, Mudgo and Rathu‐Heenati and they did worst on Ptb33. Watering regimes greatly affected performance, with significantly less honeydew, less weight gain, lower fecundity and smaller egg groups on plants with 10 ml water daily than on plants with excess water. The 20 ml and 30 ml watering regimes also significantly reduced honeydew production on TN1 and ASD7 and fecundity and weight gain of adults and nymphs on Mudgo. On Babawee adults gained most weight on the 20 ml and 30 ml treatments, and nymphs gained most weight on the 30 ml treatment. BPH laid most eggs on TN1, ASD7, Babawee and Rathu‐Heenati when plants were given 30 ml water daily.

Host preference, honeydew production, fecundity and size of egg groups of brown planthopper Nilaparvata lugens (BPH) were studied on 30‐, 60‐ and 90‐day‐old rice plants of cultivars TN1, ASD7, Mudgo, Babawee, Rathu‐Heenati and Ptb33. BPH preferred to infest TN1 and ASD7 to Babawee and Mudgo, while Rathu‐Heenati and Ptb33 were non‐preferred. In general, plant age within a cultivar did not affect BPH preference, but on Babawee it settled more on 90‐day‐old plants than on 30‐ or 60‐day‐old ones. Honeydew output was high on TN1 and ASD7 and low on Ptb33. On TN1, ASD7 and Mudgo it was lower on 90‐day‐old than on younger plants, but on Babawee the output was highest on the oldest plants. Over 24 h most eggs were laid on TN1, but there were no differences in egg laying between other cultivars, or between plants of different ages within individual cultivars. Lifetime fecundity was highest on TN1 and low on Ptb33 and Rathu‐Heenati. On TN1 and Mudgo fecundity was lowest on 90‐day‐old plants, but on Babawee it was highest on 90‐day‐old plants. Plant age did not affect the number of eggs laid on other cultivars. The size of egg groups declined in the order TN1, ASD7, Mudgo, Babawee, Rathu‐Heenati and Ptb33, but it was not affected by plant age. Zusammenfassung Zum Einfluß des Pflanzenalters auf die Wirtspräferenz, die Honigtauproduktion und die Fekundität von Nilaparvata lugens (Stål) (Hom., Delphacidae) in Reiskulturen An 30, 60 und 90 Tage alten Reispflanzen (TN1, ASD7, Mudgo, Babawee, Rathu‐Heenati und Ptb33) wurden Wirtspräferenz, Honigtauproduktion, Fekundität und Größe der Eigelege von Nilaparvata lugens untersucht. N. lugens bevorzugte TN1 und ASD7 gegenüber Babawee und Mudgo, während Rathu‐Heenati und Ptb33 nicht zu den bevorzugten Sorten zählten. Im allgemeinen beeinflußte das Pflanzenalter nicht die Akzeptanz einer Reissorte, aber bei Babawee wurden die ältesten Pflanzen bevorzugt. Auf TN1 und ASD7 wurde eine hohe Honigtauproduktion festgestellt, auf Ptb33 eine niedrige. Bei den Sorten TN1, ASD7 und Mudgo war die Honigtauproduktion bei den älteren Pflanzen geringer als bei den jüngeren, bei Babawee war es umgekehrt. Auf TN1 wurden innerhalb von 24 h die meisten Eier abgelegt; zwischen den anderen Sorten konnte diesbezüglich kein Unterschied festgestellt werden. Auch das Alter der Pflanzen hatte keinen Einfluß auf die Eiablage. Die Gesamtfekundität war auf TN1 am höchsten und auf Ptb33 und Rathu‐Heenati am niedrigsten. Das Pflanzenalter beeinflußte nur bei einzelnen Sorten die Gesamtfekundität. Die Größe der Eigelege war abhängig von der Sorte, nicht aber vom Alter der Reispflanzen.