Impacts of shelter tree species on the pest status of oystershell scale (Diaspidiotus ostreaeformis Hem: Diaspididae) in Central Otago, and options for integrated pest management.

Oystershell scale, Diaspidiotus ostreaeformis, is a pest of fruit crops in Otago and Canterbury that is well controlled within orchards but becomes a problem at harvest due to invasion from external host trees. Its major pest status on export crops comes from its being indistinguishable on harvested fruit from the critical quarantine pest San Jose scale Diaspidiotus perniciosus. Oystershell scale has a wide host range which includes trees frequently used as orchard shelter. The distribution of oystershell scale in apples adjacent to an infested shelter belt was investigated in Otago in 1995-96. Apple infestation was >90% next to the shelter and declined linearly to <1% 64 m away. The slope of this linear regression was similar on four cultivars of apple but infestation increased substantially from early- to late-harvested cultivars. The phenology of oystershell scale on apples in 1998-2000 was found to precede that on a range of other hosts, which extended the period of risk of fruit infestation by crawlers from December to April each year. Experiments were conducted in 1999-2000 on control of oystershell scale on a range of infested shelter trees using a vertical boom sprayer. Sprays timed for control of adult males and females in spring provided only a short-term reduction of winged males in sticky traps and failed to reduce crawler production and dispersal as measured by sticky bands and traps on tree branches. On the other hand, sprays timed for control of crawlers in mid-summer were effective in greatly reducing catches of crawlers. The options for management of oystershell scale on alternative hosts, particularly shelter, are discussed in relation to phenology, crop invasion by crawlers, host resistance and spraying. [ABSTRACT FROM AUTHOR]