Survey and Evaluation of Natural Enemies of the Iroko Gall Bug Phytolyma Lata Scott (Homoptera: Psyllidae) in Ghana

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Field and laboratory studies were conducted in three forest zones in Ghana to identify indigenous natural enemies of Phytolyma, and determine their potential as biocontrol agents. Weekly and monthly surveys of predators and parasitoids respectively were carried out at the main study site at Mesewam nursery in the Moist Semi-Deciduous Forest zone from January 1996 to November 1997. Two other sites at Daboase in the Moist Evergreen Forest zone and Manso Amenfi in the Wet Evergreen Forest zone were surveyed twice yearly for two years. Ten predators from five families namely; Pseudocreobotra ocellata (Beauvois), Sphodromantis lineola (Bum) (Mantidae) Rhinocoris bicolor (Fabr.), R. rapax (Stal.), R. camelita (Stal.), R. tibialis (Stal.), Phonoctonus fasciatus (P.B) (Reduviidae), Connocephalus sp. (Gryllidae), a Coccinellid sp and an unidentified vespid (Vespidae), and four parasitoids from two families namely; Psyllaephagus phytolymae Ferriere (Encyrtjdae). Aprostocetus salebrosus (Robinson), A. roseveari Ferriere, and A. trichionotus (Robinson) (Eulophidae) were identified. The record of A. trichionotus in this study is the first of its kind in West Africa. Some pathogenic fungi were isolated from dead or diseased nymphs and adults of Phytolyma collected from the nursery; Aspergillus niger Tiegh., A. flavus Link:Fr, Penicillium cyclopium Westling, P. brevi-compactum Dierckx, Fusarium sp. and Cladosporium sp. These pathogens, however, occurred at enzootic levels in the Phytolyma population. The most important predator was the mantid S. lineola. It had the highest mean per capita rate of predation (PCRP) of 22.29 ± 2.25 which was significantly different (p < 0.05) from that of all the other predators. It was also the only predator whose population markedly varied seasonally, with peaks occurring in April for both 1996 and 1997. The populations of S. lineola and exposed adult psyllids in the field were found to be positively density dependent. However, evaluation experiments carried out in field cages showed that the regulatory effect of S. lineola on Phytolyma was insignificant. Parasitoid population dynamics were evaluated on three growth stages (seedlings, saplings and crown of pole-size trees) at Mesewam. There was an observed relationship between growth (age and/or height) of Milicia and parasitoid numbers; the number of parasitoids increased with height or age of plant as seedling < saplings < pole-sized. This relationship was, however, not statistically significant (p> 0.05). Percent parasitism was low, between 4 and 16% with fairly higher rates in the drier months than in the wetter months of the year. There was a significant negative correlation between parasitism and the size of gall formed with smaller galls recording higher rates of parasitism than larger galls (p= 0.0012, df =1, 10, R2 = 78.5%). Psyllaephagus phytolymae, even though it has been cited in literature as the primary parasitoid of P. lata, did not occur on seedling galls. The absence of this important parasitoid from seedlings may explain why the worst cases of Phytolvma damage to Milicia occurred at the nurseries and young plantations. Increasing plant species diversity with respect to Milicia did not lead to increased natural enemy populations and/or efficiency in the field within one year of evaluation. Therefore, the hypothesis that increased vegetational diversity would lead to a corresponding increase in natural enemies’ populations could not be supported. There was, however, increased parasitism rates on plant resistant genotypes compared to susceptible genotypes because of the predominantly smaller galls that developed on the resistant genotypes. Parasitoid activity in the field was quite substantial and accounted for almost 20% of mortalities in Phytolyma populations. It may be possible to achieve an appreciable control of Phytolyma populations if the parasitoid populations could be increased by employing some silvicultural techniques or augmentative release.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirement for the award of the Degree of Master of Science, 1999