Distribution modelling of leptocybe invasa (hymenoptera: eulophidae), and an assessment of host susceptibility and relative efficacies of systemic insecticides for gum seedling protection in Zimbabwe

Abstract

Studies were conducted from February 2015 to December 2015 to predict the potential distribution of the blue gum chalcid or Eucalyptus gall wasp, Leptocybe invasa Fisher and La Salle in Zimbabwe, to screen locally available Eucalyptus species for tolerance and susceptibility to the pest and to test the efficacy of imidacloprid (Imidacloprid® 20% SL) and thiamethoxam (Thiamethoxam® 25% WG) in protecting gum tree seedlings. The Maximum Entropy method (MaxEnt) was used to model the extent of distribution of L. invasa using environmental variables, namely elevation, temperature and rainfall. Data for the environmental variables were obtained from the Wordclim database while occurrence records for L. invasa in Zimbabwe were obtained from forest research centres, forestry companies and district forestry offices from cases reported between December 2008 and August 2012 as well as countrywide surveys conducted between February and April 2015. Experiments were also conducted at Chesa Forest Research Station to screen Eucalyptus tereticornis, E. camadulenisis, E. grandis and E. propinqua seedlings for susceptibility to L. invasa and to test the efficacy of the systemic insecticides in protecting E. tereticornis seedlings against L. invasa attack. Seedlings were exposed to natural infestation and after a month of exposure, assessments were carried out at 2 week intervals to assess for susceptibility and severity of damage. A positive relationship was observed between altitude and the occurrence of L. invasa as well as between rainfall and the occurrence of the pest. Temperature was showed to have a negative relationship with the probability of occurrence of L. invasa. There is a high probability of finding L. invasa in areas where annual temperatures range between 10o and 26oC. The Eucalyptus seedlings screened for susceptibility to L. invasa attack were all equally damaged by the pest. The insecticides used in this study were not effective in protecting E. tereticornis seedlings against L. invasa attack during the period of assessment. The preliminary results of this study together with data from other related distribution surveys of L. invasa may aid in focusing control in areas that have a high risk of invasion by L. invasa. It is recommended that further studies to determine the susceptibility of different Eucalyptus species and hybrids to blue gum chalcid attack as well as screen locally available insecticides for use in protecting seedlings from L. invasa attack be conducted.