A sampling system was developed for monitoring population levels of woolly apple aphid, Eriosoma lanigerum (Hausmann), by counting colonies on half of each of 25 apple trees per 2 ha block. Sampling error was affected by whether the colonies were found in wounds or in leaf axils. Parasitism of colonies in leaf axils had a slight effect on sampling error. Sampling error for colonies in leaf axils was high at just over 40 %. However, decisions regarding intervention were not markedly compromised by simply classifying the 25 trees as infested or uninfested, as opposed to counting colonies in leaf axils. The presence-absence system greatly reduced the time spent monitoring E. lanigerum population levels, making it an attractive system for assessing woolly apple aphid infestations in commercial orchards.
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