SummaryThe disease was transmitted by Orosius argentatus (Evans) collected in infected lucerne crops, and also by groups of virus-free individuals reared under experimental conditions, after having been fed on plants infected with witches' broom virus.The following plants became infected with the virus under experimental conditions: Beta vulgaris L.; Datura stramonium L., Erodium cicutarium (L.) L.Herit, Hypochaeris radicata L., Lycopersicon esculentum Mill., Medicago sativum L., Vinca rosea L. Lucerne was difficult to infect and the first symptoms took about seven months to appear (but see note on infection of lucerne).In all plants the symptoms caused were severe stunting, proliferation of axillary shoots, and the production of green flowers. In tomato and other hosts these symptoms closely resembled those of tomato big bud virus. The same vector transmits them and it is suggested that the two diseases may be caused by the same or closely related strains of virus.

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Tobacco yellow dwarf virus disease was experimentally transmitted to 15 species of plants by the leafhopper, Orosius argentatus (Evans). Eight of these plants are summer annuals and seven are autumn-spring growing plants which provide a continuous succession and wide range of hosts. Their occurrence in the districts where tobacco is grown makes control by the elimination of alternative host plants appear impracticable.

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The increased activity of insects before weather changes, particularly, certain types of weather, has been the subject of theorising that atmospheric e l e c t r i c i t y , among other things, might influence insect activity and behaviour before and during these changes (Fabre 1918;Uvarov, 1931;Wellington, 1957). Henson (1951), Rainey (1951) and Wellington (1957} have studied m a s s weather effects on insects and have shown the apparent effects of approaching and receding weather fronts on insect activity. Wellington suggests that p a r t i c u l a r types of weather have their own p e c u l i a r identifying features, which influence a p a r t i c u l a r behavioural r esponse in certain insects, and concluded that insect activity should be classified in relation to general weather patterns r a t h e r than on static r e c o r d s alone. Because populations, weather, and climate are all dynamic and not static as fixed climatic readings a r e , then one dynamic happening (such as flight behaviour) should be related to another (such as a i r m a s s meteorology) r a t h e r than to a specific set of static data at a fixed point in time.Pottinger (1966) showed that adult weevils, HYPERODES BONARIENSIS, in Canterbury, New Zealand, have dispersive flights throughout a period c o m m e ncing in late August and ceasing in late April. Flight activity is related to a change in weather from a n o r t h -e a s t e r l y to a south-westerly air s t r e a m . Flight was observed to commence only when wind speed was below 13 km p e r hour, relative humidity below 6%, and temperature higher than 16°C. Helson (1967) found in light trap studies that the porina moth, WISEANA SPP. begins its flight activity in early October and this continues until late March. As with stem weevil, m a s s flight activity is related to a change in weather from a northerly to a southerly a i r s t r e a m . This generally takes place 24 + 9 hr before the a r r i v a l of a low p r e ssure system and usually rain, and from observations made at Makara R e s e a r c h Station (Fig. 1)over s e v e r a l y e a r s , after ground t e m p e r a t u r e s at 4" depth have risen to 10°C in spring. In New Zealand these low p r e s s u r e cold fronts pass over the country approximately every 3 days in October. The aim of the p r e s e n t investigation was to study the relationship between flight activity and weather changes. Satellite photos of approaching cold fronts with associated rain were also studied to evaluate these as a possibly better method of predicting the approach of the fronts after they have reached approximately 160°E longitude, providing some means can be found to distinguish between those fronts which stimulate emergence and those which do not.