Insecticidal control and the phenology of cherryslug (Caliroa cerasi (L.)) (Hymenoptera: Tenthredinidae) on organic pears in Central Otago

Abstract

Cherryslug, Caliroa cerasi, is an important pest of organic pear production in New Zealand. It is reported to have two generations per year and cause severe leaf damage, particularly through the feeding of second generation larvae in mid to late-summer. This insect has been little studied in Central Otago. Adult cherryslugs were monitored with sticky traps in a block of ‘Taylor's Gold’ pears in the summers of 1999–2001 and the catches were used to assist in timing the single application each season of a range of organically-acceptable insecticides in randomized block experiments which included untreated control trees. The insecticides were: 1999–2000, ryanodine, lime sulphur, kaolin in January, kaolin in February; 2000–01, pyrethrum +mineral oil, lime sulphur, neem extract, kaolin in January. Efficacy was assessed using pre- and post-treatment larval counts and leaf damage. The impact of pre-treatment larval density on post-treatment efficacy was investigated to determine a preliminary spray threshold, specifically for pyrethrum+mineral oil. The adult monitoring indicated that cherryslug is univoltine in Central Otago, with the first adults appearing at the end of December. All the insecticides tested significantly reduced damage from cherryslug larvae and, if registered, could be used by organic growers. In both seasons, kaolin application in January, which targeted the adult cherryslugs, was more effective than all the other products, which were applied in February against the larvae. On the other hand, kaolin applied in February 2000 resulted in greater cherryslug damage than occurred with the other insecticides applied at that time. Larval density had a significant effect on the efficacy of pyrethrum+mineral oil, particularly when >10 larvae could be counted per minute on the trees before treatment. The implications of this result and the comparative efficacy of the different treatments are discussed in relation to organic pear production in Otago.