Evaluation of indigenous parasitoids of the Iroko (Milicia excelsa) gall bug, Phytolyma lata Scott (Homoptera: Psylldae)

Abstract

Phytolyma lata Scott (Hymenoptera: Psyllidae) is a gall-forming insect pest, which threatens the sustainable production of Milicia species (Iroko) in the humid tropics of Africa. As part of an integrated approach to manage this pest a survey was conducted at the nursery of the Forestry Research Institute of Ghana to identify parasitoids of P. lata, and determine their impact on P. lata population. Four parasitoids were found on P. lata nymphs and were identified as Psyllaephagus phytolymae Ferriere (Hymenoptera: Encyrtidae), Aprostocetus roseveari Ferriere, A. salebrosus Robinson, and A. trichionotus Robinson. This is the first record of A. trichionotus in West Africa. Parasitoids occurred at the nursery year round, with the peak population in February 1997 (82.2 parasitoids/1000 galls) and the lowest population in October 1996 (9.6 parasitoids/1000 galls). There were significantly more parasitoids on pole size trees (67.3 parasitoids/1000 galls) than saplings (27.6 parasitoids/1000 galls) or seedlings (18.6 parasitoids/1000 galls). The primary Phytolyma parasitoid, P. phytolymae was absent from seedling galls during the evaluation period, and coupled with the overall low monthly percent parasitism (4-16%) could explain why the worst cases of Phytolyma damage to Milicia species occurred in nurseries and young plantations. In general, parasitism increased as gall size decreased. Also, parasitism was significantly higher on resistant- (25.5%) than intermediate- (11.9%) or susceptible genotypes (7.6%). This inter-relationship, whereby increased resistance in M. excelsa results in increased parasitism has potential to enhance the biological control of P. lata in plantations planted with resistant M. excelsa genotypes.

Keywords: Milicia excelsa, Phytolyma lata, galls, Phytolyma parasitoids, host resistance

Ghana Journal of Forestry Vol. 15&16 2004: 1-12