Joseph R. Cobbinah

Survival, growth and gall formation by Phytolyma lata on Milicia excelsa established in mixed-species tropical plantations in Ghana

1 Milicia excelsa (Moraceae) is an important timber tree in much of Africa and when grown in monocultural plantations has been subject to nearly complete destruction by gall‐forming psyllids in the genus Phytolyma.

Patterns of seedling survival in the tropical African tree Milicia excelsa

The spatial patterns of canopy tree and seedling life stages in the African Tropical Tree Milicia excelsa were examined to test the Janzen-Connell hypothesis (escape hypothesis). The Janzen-Connell hypothesis states that the mortality of seeds and seedlings is highest near the adult tree and declines with distance away from the adult tree of the same species. This hypothesis argues that this effect is partially responsible for maintaining the high tree species diversity of tropical forests. Solar radiation and fruit dispersal were investigated as possible explanations of observed patterns in seedling distribution. Milicia excelsa trees ≥ 30 cm DBH were found to be randomly dispersed and occurred at a density of ≤ 1 ha. Milicia exceisa seedlings were located in small clumps between parent trees and up to 150 m from parents at a mean density of 65 seedlings ha -1 . Fruits were in abundance (> 9000 on the ground) beneath female trees. Light levels at locations of natural seedlings ranged from 2-29% of full irradiance. Planted seedlings did not suffer greater mortality closer to adult conspecifics, and survived at light levels between 2-37% of full irradiance. Phytolyma spp., gall-forming psyllids which cause high mortality in Milicia seedlings in large openings and plantations, were not observed on any natural or planted seedlings, although galls were present in the crowns of dominant trees.

Phenotypic variation in Milicia excelsa to attack by Phytolyma lata (Psyllidae)

Abstract A study was conducted to screen 21 half-sib Milicia excelsa progenies for pest resistance to the gall-forming psyllid Phytolyma lata. Results of incidence of large galls indicate that there were significant difference among individual progenies. The mean percentage of large galls ranged from 15.4 to 99.6% and 39.5–99.9% for 1992 and 1993 respectively. Four progenies (AA8, AA35, EA9 and EM1) appeared consistently resistant while five others (BK3, EM2, AA24, EA7 and VH3) were consistently susceptible to P. lata. A highly signficant relationship was shown between the incidence of dieback and relative abundance of large galls (R2 = 0.82, d.f. = 2,18, F = 39.9, P = 0.001) and was described by a curvilinear model. Relationships among incidence of dieback and height, number of leaves, girth and branches were also evaluated.

Chaenothecopsis khayensis, a new resinicolous calicioid fungus on African mahogany

The new species Chaenothecopsis khayensis (Ascomycota, Mycocaliciaceae) is described from Ghana, western Africa, on the resin of Khaya anthotheca and K. ivorensis. The species is distinctive in forming asci without crosiers and in possessing ascospores that are faintly longitudinally striate. Analysis of large subunit rDNA gene sequences positioned this species within a clade corresponding to the Mycocaliciales and identified its closest relative as Sphinctrina leucopoda. Chaenothecopsis khayensis occurs commonly on resin exuding from trees damaged by the larvae of the mahogany shoot borer (Hypsipyla sp.), and we discuss the possible ecological relationship between the fungus and these moths.

Using Resistance in Tropical Forest Plantations

Tropical forest plantations are a major source of wood fiber throughout the world. These plantations are subject to depredation by pest insects much like other forests of the world. The use of exotic species is a common form of managing forest pests, but this approach is not without limitations in general and is certainly not a sufficient control strategy to limit all pests. Substantial work has been done to understand mechanism of resistance in tropical tree species such as mahogany, teak, pine, eucalyptus, Leucaena, Acacia, and Cupressus. A pest-resistant transgenic Eucalyptus has been developed but is not operationally deployed. The substantial diversity of tropical trees represents both an opportunity to search for resistance to pests and a challenge to manage the huge array of current and potential forest insect pests.

Wood borers of living trees

Stems of living trees provide food and shelter for many species of boring insects. Plants in various stages of development from very young seedlings to mature trees may be attacked. The damage caused by these insects results in the death of living stems and shoots and reduction in wood quality due to boring and weakening of standing trees. In addition to these direct effects, borers may open the way for secondary attack by wood rotting or staining fungi, thus hastening decay.

Sap-feeding insects

The insects that we discussed in the previous section feed upon the foliage of trees by ingesting the solid parts. There is another important group of phytophagous insects which live upon plant sap. These sap feeders have haustellate or sucking mouthparts in which the mandibles and maxillae have become slender bristle-like organs enclosed in a sheath formed by the labium. The mouthparts thus form a beak which is used to pierce the tissues and suck the fluid from the leaf.

Pests of logs, lumber and forest products

In the previous section, we discussed the ecological group of organisms that bore into the woody tissue of living trees. In this section we will discuss insects that generally attack wood after trees have been cut or processed into wood products. This arbitrary ecological categorization does not necessarily follow taxonomic lines. For example, in the previous section, we discussed insects representing the families Scolytidae, Platypodidae, Cerambycidae, and Bostrichidae that damage living trees. We will again discuss members of these families that have feeding habits quite distinct from their taxonomic relatives. Most of the examples cited in the previous section that are from these families are exceptions to the general ecological pattern for these taxonomic groups.

Pests of flowers, fruits, and seeds

In the natural forest, pests of reproductive structures (flowers, fruits, and seeds) tend to be of little economic importance. Most tree species produce far more seed than is necessary to establish regeneration. Other factors such as competition tend to be far more important in tropical forests. However, when high-value species are artificially regenerated through the production and planting of seedlings, a reliable seed source becomes critical. The need to improve the seed production capability is well established in West Africa (Okoro and Dada 1987, Ouedraogo and Verwey 1987). Many species like wawa are known to have irregular seed years (Taylor 1960, Danso 1970, Kudler and Jones 1970). Insects that infest seeds can be a significant factor in this variability in seed production (Kudler and Jones 1970). The significant role insects play in reducing the availability of seeds is recognized in other parts of Africa as well (Shakacite 1987, Hassani and Messaoudi 1986, Ross 1979, Rasplus 1988).