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COMPANION PLANTING OF THE NITROGEN-FIXING GLIRICIDIA SEPIUM
WITH THE TROPICAL TIMBER SPECIES MILICIA EXCELSA AND ITS IM-
PACT ON THE GALL FORMING INSECT PHYTOLYMA LATA
MICHAEL R. WAGNER 1, JOSEPH R, COBBINAH 2, and DANIEL A. OFORI z
_School of Forestry, Northern Arizona University, P.O. Box 15018, Flagstaff, Arizona 86011, USA
2Forestry Research Institute of Ghana, UST P.O. 63, Kumasi, Ghana
INTRODUCTION
In Ghana, West Africa, there is increasing concern over the loss of valuable commercial timber species that contribute
to the economic development of this country (Wagner and Cobbinah 1993). Part of the diminishing supply of timber relates
to the high rates of deforestation (Zerbe et al. 1980, Mergen 1983, Repetto 1988). But an equally important factor that is
often overlooked is the failure of native species to regenerate following disturbance in the natural forest or when planted in
plantations (Fontaine 1985). Forest insects can significantly contribute to the poor regeneration rates of tropical timber
species. An example of this is the complete failure to artificially regenerate Milicia excelsa, an extremely valuable tropical
timber species in Africa, because of heavy damage by the gall-forming psyllid, Phytolyma lata (White 1968, Wagner et al.
1991).
Milicia excelsa and M. regia are important timber species in Africa, known in the timber trade as lroko, Mvule, and
locally in Ghana as odum. The wood is durable, uniformly grained, and extremely resistant to termites. While the species is
a common component of the natural overstory in much of the remaining tropical forests of West Africa, it is not currently
regenerating and has a predicted resource life of less than 10 years in Ghana (Alder 1989). Reforestation efforts have focused
on establishing plantations of Imko which have failed in part because of massive damage by Phytolyma lata, the odum
gallfly.
The odum gallfly oviposits on new foliage of Milicia spp., and the first instar nymphs damage leaf tissue and induce
galls (White 1968; Orr and Osei-Nkrumah 1978; Cobbinah 1986, 1988). The nymphs feed within the gall tissue. Numerous
attack sites result in large gall masses. After about 2-3 weeks, the galls burst open releasing the adult psyllid. The gall tissue
that has burst open exudes gall and leaf fluids that are heavily colonized by saprophytic fungi. These saprophytic fungi result
in decay of leaf and stem tissue and dieback of young shoots (Wagner et al. 1991). Multiple attacks within the same season
result in heavy damage and frequent death to young seedlings.
Considering that conventional plantations had largely failed, we decided to examine the influence of companion
planting of Gliricidia sepium, a well-known nitrogen-fixing agroforestry species, and Milicia excelsa on odum gallfly attack.
We hypothesized that Gliricidia sepium would provide nitrogen and overstory shade more similar to conditions that occur in
the natural tropical forests. In addition, we examined the influence of shading and nitrogen fertilization independently in an
effort to determine which factor contributed more to the observed effects. In this manuscript we report that companion
planting, shading, and fertilization all reduce attack and damage by the odum gallfly on Iroko.
Mattson, W.J., Niemela, E, and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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