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Aphid vectors of barley yellow dwarf virus (BYDV) were sampled from three 7.5 m high suction traps in Canterbury from April to October 1998. Aphid numbers were very high (up to 193 aphids per trap per week) in April and May and medium to low (0 to 44 aphids per trap per week) from June to September. A BYDV forecast based solely on numbers of aphids caught in traps (aphid flights), or primary infestation, accurately predicted high virus incidence for wheat sown before mid-May. However, an exceptionally mild winter encouraging the secondary spread of aphids within crops was implicated in the high level of BYDV found in wheat crops sown after mid-May that was not predicted by aphid flights. A measure of secondary aphid infestation and virus infection may need to be included in future BYDV forecasts.