BYDV threat grows as aphids multiply

MILD WEATHER coinciding with early emergence of winter cereals means aphid populations could be quick to establish this autumn, according to ADAS.

Aphids are already being found in areas traditionally prone to BYDV (Barley Yellow Dwarf Virus), such as Herefordshire and growers should consider an earlier aphicide application, said ADAS entomologist, David Green.

“While trials suggest the optimum timing for single sprays is between mid October and early November, depending on emergence dates, any crops that were through the ground in late September or early October may benefit from an earlier spray.”

BYDV spread and optimum spray timing depend on a T-sum system of accumulated day degrees above 3oC from the date of emergence, he said. If the T-sum reaches 340, significant spread of BYDV can be imminent, he warned.

“If it stays mild into November, it may well be necessary to consider follow-up treatments in higher-risk areas to provide protection of new leaf material and prevent secondary spread.”

Where an aphicide is normally applied with a post-emergence herbicide, some growers may need to rethink their BYDV strategy this year, added Makhteshim Agan’s Elizabeth Spence.

She reckons that untreated wheat and barley will probably need spraying within the next two weeks if mild weather continues. “Because a pyrethroid is not systemic, follow-up protection around three to four weeks later is likely against continued [aphid] build-up.”

Growers should use an insecticide that does not harm natural aphid predators, such as parasitic wasps, which were present in high numbers over the summer, she noted. Makhteshim Agan recommends a split application of Mavrik (tau-fluvalinate).

See FWi’s Crop Watch for weekly reports from agronomists around the country.