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[] Without Gliricidia
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1/92 2/92 3/92 4/92 9/9211/92 1/93 3/93 5/93 7/93
Sample Date
Figure 3.--Influence of companion planting Gliricidia sepium with Milicia excelsa on the total number of Phytolyma lata
galls on M. excelsa. P-value based on paired t-test.
Shading Trial
Shading had a significant positive impact on height (Fig. 4). After 1 year, trees grown at the highest level of shade
(82%) were nearly 50% taller than trees at 57% shade and 100% taller than trees grown in full sun. In general, there were
more leaves produced under shade than in full sunlight, but this pattern was not consistent across sample dates (Fig. 5).
Despite the fact that more leaves were available to Phytolyma lata under shade, there were fewer galls per plant under shade
(Fig. 6). While this pattern is statistically significant at only two of the sample dates, the pattern is very consistent across all
sample dates.
Fertilizer Trial
Two of the fertilizer treatments, 15-15-15 and 10-4-22, significantly affected Milicia height when compared to the
control (Fig. 7). In general the higher the nitrogen the greater the impact. The highest nitrogen content fertilizer also resulted
in significantly fewer branch diebacks. Diebacks are positively related to the number of galls (Cobbinah and Wagner 1994).
DISCUSSION
We are unaware of other studies that have clearly linked companion planting or agroforestry techniques and modifications
in the incidence and impact of damaging insects in a tropical ecosystem. While the patterns we observed are not
absolutely consistent for all the sample periods, there is sufficient evidence to warrant further study.
There is, however, a considerable body of evidence on the relationship between fertilization and insect attack (see
Stark 1965, and Schowalter et al. 1986 for reviews). Strauss (1987) observed that fertilization tended to have a positive
impact on sap feeding insects and no effect on chewing insects. Several studies have examined the relationship between
fertilization and population parameters of the gall-forming adelgid Adelges coolyei (Mitchell and Paul 1974, Johnson et aL
1977, Mitchell and Miller 1976). In each case they found a general trendof fertilization increasing population parameters of
Adelges coolyei. While these results are opposite to our own findings, the environmental conditions and plant species are
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