Economic impact and re-establishment of biological control for Gonipterus sp. 2 in Eucalyptus plantations in South Africa

Abstract

Gonipterus sp. n. 2 (Coleoptera: Curculionidae), native to Australia, was reported in South Africa in 1916 and has become a serious pest in Eucalyptus plantations. The adults and larvae feed on young and tender leaves of Eucalyptus which can result in a reduction in tree growth and/or extension of rotation length and thus negatively affect economic benefits derived from plantations. Despite the economic importance of Eucalyptus and the threat posed by Gonipterus sp. n. 2, there is a lack of knowledge on the economic impact of this insect in South Africa. The main control strategy for Gonipterus sp. n. 2 is biological control, using a solitary egg parasitoid wasp, Anaphes nitens Girault (Hymenoptera: Mymarid). However, there are still outbreaks of Gonipterus sp. n. 2 despite the presence of this biological control agent, thus necessitating the use of alternative control measures for effective integrated pest management. In this study, the tree growth losses and corresponding economic impact of Gonipterus sp. n. 2 were quantified from insect exclusion trials, and the effect of cypermethrin (pyrethroid insecticide) on the re-establishment of A. nitens was also investigated. The insect exclusion trials consisted of chemically treated plots for the exclusion of Gonipterus sp. n. 2 and untreated plots for this beetle to freely feed. Results from insect exclusion trials showed that Gonipterus sp. n. 2 can result in a significant 17.5% (p = 0.022) reduction in wood volume from cumulative 18% defoliation of two assessment events which translates to a projected economic impact of between R6 399.22/ha (344.11 USD) and R29 224.53/ha (1571.51 USD) at harvest (8 years). The effect of cypermethrin on the re-establishment of A. nitens was investigated by comparing parasitism rates from chemically treated and untreated treatments, before and after spraying of ~2-year-old (at time of last egg collection) Eucalytpus dunnii. Gonipterus sp. n. 2 egg capsules were collected from the field for assessment of parasitism rates. It was found that the insecticide did not influence the parasitism rate (re-establishment) of the parasitoid, indicating potential compatibility between biological and chemical control of Gonipterus sp. n. 2, although larger trials are needed to confirm the results. This is the first study in South Africa to quantify the economic impact of Gonipterus sp. n. 2 and the compatibility of cypermethrin (one of the most used broad-spectrum insecticides) with A. nitens in the field.