No abstract

1 Attack by shoot borers (Hypsipyla grandella Zeller) is the main factor limiting the cultivation of Cedrela odorata and Swietenia macrophylla, two economically important members of the mahogany family. No viable methods of pest control are currently available. To assess for genetic variation in susceptibility to pest attack, a combined progeny/provenance test of C. odorata and a provenance test of S. macrophylla were established separately at CATIE, Turrialba, Costa Rica, and assessed intensively over an 84-week period. 2 Variation in height growth between provenances was highly signi®cant in both species, provenance means varying by a factor of 2.7 and 1.2 in C. odorata and S. macrophylla, respectively. Cedrela odorata also displayed provenance variation in foliar phenology, as 94.7% of the trees from a`wet zone' provenance remained foliated, whereas > 35% of trees from three`dry zone' provenances abscised their leaves during the dry season. 3 Both species displayed signi®cant genetic variation in susceptibility to shoot borer attack. At the peak of attack during the second year of growth, the effect of provenance was highly signi®cant in both species, the mean number of attacks per tree varying between 0.8±2.4 and 0.6±1.3 in different provenances of C. odorata and S. macrophylla, respectively. A provenance of C. odorata from San Carlos, Costa Rica, displayed consistently lower susceptibility to pest attack, being subjected to fewer attacks during the ®rst year and demonstrating a higher mean height to ®rst point of damage. 4 The nitrogen, total tannin and proanthocyanidin concentration of foliage varied signi®cantly between C. odorata provenances: nitrogen concentration was signi®cantly lower and tannin and proanthocyanidin contents were signi®cantly higher in trees from the San Carlos provenance, Costa Rica. This study therefore suggests that proanthocyanidins may reduce susceptibility of C. odorata to H. grandella, at least during the early period of growth. 5 These results provide support for the development of plant resistance as a strategy for managing shoot borers. In particular, there may be scope for selecting for high foliar proanthocyanidin content and the ability to tolerate attack by vigorous apical growth.

In order to evaluate the relationships between attacks by the mahogany shoot borer Hypsipyla grandella (Zeller) and host growth and phenology, field trials of Spanish cedar Cedrela odorata and American mahogany Swietenia macrophylla established in Costa Rica were assessed intensively over an 84 week period. Height growth of C. odorata was approximately twice that of S. macrophylla, with mean growth rates of 2.0 and 1.3 cm week−1 respectively. Cedrela odorata displayed pronounced temporal variation in leaf phenology, with a minimum of 51% of trees in leaf during a relatively dry period. In contrast, the minimum proportion of foliated S. macrophylla trees was 87%. No shoot borer attacks were recorded on S. macrophylla until week 50 after the onset of assessments, whereas initial attacks of C. odorata were recorded during week 6. By week 84, the proportion of trees attacked was 74% and 77%, respectively. In both species, attacks were concentrated in seasonal peaks, coinciding with the production of new shoots. Pronounced spatial variation in attack was observed in both species. At week 12, the number of attacks per block in C. odorata was negatively correlated with available calcium concentration (r=0.85, P<0.001), whereas mean height growth of C. odorata was positively correlated with available phosphate concentration (r=0.84; P<0.0001). Significant variation in height to first damaging attack was observed in both species. By the end of the observational period, only 6% of C. odorata and 0.4% S. macrophylla remained undamaged to a height of 3 m. It is suggested that control methods for the mahogany shoot borer should take account of temporal dynamics in attack, and relationship to host phenology. Manipulation of the soil nutrient status may also form part of an integrated approach to pest management.

Mahoganies (Swietenia spp. ; Meliaceae) are amongst the most economically important tropical hardwoods, and yet little information exists concerning their patterns of genetic variation. The characterization of this variation is essential for defining more accurately the conservation status of mahogany populations, and for their economic utilization. The loss of genetic variation through deforestation may be critical for these species, which are highly susceptible to pest attacks when grown in plantations. This paper assesses the current state of knowledge concerning the extent of genetic variation in mahoganies, and highlights its potential importance. It is suggested that any conservation strategy developed for mahoganies should include a genetic selection and improvement programme as well as the protection of natural stands in situ. Techniques by which particular genotypes may be captured for ex situ conservation are briefly described.

The development of a strategy for the sustainable management and conservation of mahogany is an urgent priority. Such a strategy should be based upon clear information about the extent of genetic differentiation within and between populations, and on an understanding of the processes maintaining this variation. At present, such information is very limited for mahogany. Preliminary data are presented from two genetic tests of Szmitaia muoophyllu King (Meliaceae), indicating significant differences between provenances and half-sib progenies in both growth and form characteristics. In addition, the use of molecular markers for the characterization of genetic resources of mahogany is discussed. On the basis of the results available, the impacts of deforestation and logging activities on genetic resources are evaluated. Although both deforestation and selective logging may deplete genetic resources, no quantitative information on the extent of such depletion in mahogany is currently available. Additional research is therefore required before clear guidelines can be provided for the sustainable management of mahogany.