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very different from the Milicia/Phytolyma system we examined. There is very little consistency in the response of herbivores
to fertilization of their hosts (Stark 1965, Schowalter et al. 1986). Insufficient evidence exists to postulate any general
pattern in this regard.
Hall and Swaine (1983) reported that Iroko is not abundant in the evergreen tropical forest because it requires high
light for germination and establishment. However, as previously noted, plantations of this species planted in full sun have
been heavily attacked by the odum gaUfly and have largely failed. In contrast to the report of Hall and Swaine (1983), we
have been told by a tropical tree physiologist that the optimal photosynthesis for Iroko seedlings occurs at 8-10% of full
sunlight, suggesting this is a shade tolerant species (E.M. Veenendaal, pers. comm.). The results we report in this paper
indicate that the abundance and subsequent impact of this insect is greater in full sun than under shade.
We recognize that there would be considerable utility in examining the relationship between Iroko and the odum
gallfly under natural field conditions. However, naturally regenerating seedlings of this species are difficult to find. In a
recent field excursion to the Tain II Forest Reserve, which is noted for its high density of Iroko in the overstory, we were able
to locate only a single seedling in 12 person/hours of searching. The low incidence of Iroko seedlings has been confirmed
anecdotally by several botanists and forestry field surveys. Growing stock as of 1987 was immature stock (1-7ft girth)
834,000m 3and mature stock (> 7ft girth) 5,269,000m 3 (Alder 1989).
The results we report suggest that standard agroforestry practices such as mixed planting with nitrogen-fixing trees
may have utility as part of a pest management program to control the odum gallfly. Other approaches including selection for
genetic resistances have resulted in considerable success (Cobbinah and Wagner 1994). Cooperative effort between silviculturists,
physiologists, and genetists is required to fully understand how to reduce the impact of this insect and provide for a
continuing supply of one of Africa's most valuable timber species.
SUMMARY
Planting the nitrogen-fixing Gliricidia sepium in conjunction with the tropical timber species Milicia excelsa reduces
the abundance and damage caused by the gall-forming pysUid, Phytolyma lata. This effect appears to be related to both the
increased shading and enhanced nitrogen environment. Shading independently has a strong impact on gall abundance with
the lowest gall abundance occurring under the highest level of shade (82%). Gall abundance is reduced under shade even
though more leaves are available to the insect. Fertilization at the highest levels of nitrogen also reduces the abundance of
galls.
ACKNOWLEDGEMENT
These studies were supported by the International Tropical Timber Organization through research grant No. PD75/90
to J.R. Cobbinah and M.R. Wagner.
LITERATURE CITED
ALDER, D. 1989. Naturalforest increment, growth and yield, p. 47-52. In Ghana Forest Inventory Proceedings. Overseas
Dev. Agency/Ghana For. Dep., Accra.
COBBINAH, J.R. t986. Factors affecting the distribution and abundance of Phytolyma lata (Homoptera:PsyUidae). Insect
Sci. Appl. 7: 1111-115.
COBBINAH, J.R. 1988. The biology, seasonal activity and control of Phytolyma lata. IUFRO Regional Workshop on Pests
and Diseases of Forest Plantations, June 5-10, 1988, Bangkok, Thailand.
COBBINAH, J.R. and WAGNER, W.R. 1994. Phenotypic variation in Milicia excelsa toattack by Phytolyma lata.
Biotropica. (in review).
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