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New Zealand Plant Protection 64 (2011): 285

Outbreak of armyworms in eastern Bay of Plenty

P.J. Gerard, P.J. Addison, P. Hedley, N.L. Bell and C.J. Vink


Tropical armyworm (Spodoptera litura) caused widespread serious damage to Bay of Plenty lucerne crops and clover, plantain and chicory in pastures in April and early May 2011, reaching populations at high as 100/m2. This outbreak was preceded by an outbreak of cosmopolitan armyworm (Mythimna separata) attacking maize and grasses. Tropical armyworm is a minor pest of subtropical crops, such as kumara, in Northland. The last widespread outbreak was in autumn 1975 and resulted in serious pasture and crop damage in Northland and Auckland and minor damage in Bay of Plenty, Taranaki and Manawatu. The current outbreak in Bay of Plenty may be climate related with a mild and extremely wet winter in 2010, above average spring and summer temperatures and near record summer rainfall. These conditions may have allowed tropical armyworm to complete four generations to build up to outbreak levels. In addition, dry spring soil conditions during maize establishment meant poor weed control, a known factor to predispose problems with cosmopolitan armyworm, which has a similar life cycle to tropical armyworm. Similar weather patterns are expected to increase in frequency with climate change.

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