Shade tree management affects fruit abortion, insect pests and pathogens of cacao

doi:10.1016/j.agee.2006.09.004

Agriculture, Ecosystems & Environment

Volume 120, Issues 2–4, May 2007, Pages 201-205

https://doi.org/10.1016/j.agee.2006.09.004 Get rights and content

Abstract

The mortality of cacao fruits caused by early fruit abortion or insect and pathogen attacks was investigated in differently managed agroforestry systems in Central Sulawesi, Indonesia. Nine agroforestry systems shaded by three different types of tree stands were selected, which represented a decrease in structural heterogeneity: forest remnants, diverse planted trees and one or two species of planted leguminose trees. After standardized manual cross-pollination, the development of 600 fruits on 54 trees (6 trees per agroforest) was followed during 18 weeks of fruit development. In total, 432 of all fruits were lost before maturity, which seriously undermined yields. The proportion of harvested fruits per tree (overall average: 27 ± 4%) was not affected by canopy type. Although shade cover did not have a significant effect, losses due to fruit abortion were most likely under forest shade, where nitrogen-fixing leguminose shade trees were absent. Fruit losses due to pathogenic infections and insect attacks increased with the homogenization of the agroforests, supporting the hypothesis that agricultural homogenization increases risks of pest outbreaks. In conclusion, shade management may be improved to increase yields from cacao using highly diversified natural shade agroforestry systems.

Introduction

A common phenomenon in plants is that numbers of flowers exceed final numbers of mature fruits (Stephenson, 1981) and fruit mortality due to internal (e.g., Nichols and Walmsley, 1965, Pías and Guitián, 2006) and external (e.g., Louda, 1982, Arnold et al., 2003) factors can be a major bottleneck in plant reproductive success. Cacao (Theobroma cacao L.) is among the most common crops grown in tropical agroforestry systems. Its reproductive system is characterized by high numbers of flowers, of which generally no more than 5% develop into mature fruits (Entwistle, 1972, Young, 1994). The highest ever annual global production of 3.5 million t dry cacao beans was reached in 2004, which equaled a total of 3.7 billion US$ of income to farmers (International Cocoa Organization, 2005). Despite the increasing economic importance of cacao, surprisingly little is known about the mechanisms that determine its yields.

Because productivity of cacao is predicted to decrease under dense shade regimes (Zuidema et al., 2005), recent agricultural intensifications led to large scale landscape homogenization, turning heterogeneous, shaded agroforestry systems into poorly shaded monocultures at local and regional scales (Siebert, 2002). Such intensifications and consequent landscape homogenizations can threaten tropical biodiversity and profitable ecosystem services (e.g., Rice and Greenberg, 2000, Klein et al., 2002) and increase risks of pest outbreaks (Schroth et al., 2000, Wilby and Thomas, 2002, Tscharntke et al., 2005).

Cash crops grown in tropical agroforestry systems depend strongly on ecosystem services provided by naturally occurring species (Schroth et al., 2000, Tylianakis et al., 2005). In cacao, pollination is carried out by small insects such as midges (Entwistle, 1972, Young, 1994) and some ants are suggested to play important roles in the regulation of insect pests (Entwistle, 1972, See and Khoo, 1996). However, a wide range of herbivorous insects and pathogens attack cacao, and many of them are able to develop high densities, causing severe harvest losses and even regional abandonment of cacao farming (Fowler et al., 1956, Entwistle, 1972, Purdy and Schmidt, 1996, See and Khoo, 1996, Krauss and Soberanis, 2001).

In addition to pest attacks, a major cause of fruit mortality on cacao trees is active abortion, or “cherelle wilt” (Nichols and Walmsley, 1965, Valle et al., 1990, Young, 1994, Falque et al., 1995; Hasenstein and Zavada, 2001). Such losses are regulated by the plant, primarily as a response to pollen incompatibility (Hasenstein and Zavada, 2001) and nutrient limitations that result from low photosynthetic rates or poor soils (Nichols and Walmsley, 1965, Valle et al., 1990).

Here cacao fruit losses in shaded agroforestry systems are investigated, evaluating internal causes (fruit abortion) and external causes (insect attacks and pathogens) of fruit mortality. The question is whether shade density and composition of shade trees in agroforestry systems affect mechanisms of fruit loss. Management recommendations are derived for increasing cacao yields from shaded agroforests.

Section snippets

Study sites

The study was conducted in nine cacao dominated agroforestry systems in the Toro village, about 100 km southwest of Palu, the capital city of Central Sulawesi, Indonesia. The systems were characterized by three different types of shade tree stands: trees remaining from previous rainforest cover, diverse planted trees (secondarily grown forest trees, fruit and timber trees and leguminose trees) and stands dominated by one or two species of planted trees (dominated by the leguminose Glyricidia

Results

In total, 600 fruits on 54 trees (average per tree: 11.3 ± 0.58) were monitored. Of the monitored fruits, 432 (72%) were lost before harvest. One-half (n = 300) of all fruits were lost due to abortion by the plants themselves. A further 111 fruits (19%) did not reach maturity due to pathogens and 21 fruits (4%) were lost due to insect attacks, which in all cases was due to attacks by Helopeltis sulawesi Stonedahl (Hemiptera: Miridae). In total, H. sulawesi fed upon 55 fruits, of which 62% still

Discussion

The results of this study show the enormous influence that fruit mortality had on potential yields of cacao: 72% of the pollinated flowers did not develop into mature fruits. The majority of fruit mortality was driven by within-tree factors (i.e., abortion), over one-half of which took place during the first 3 weeks of fruit development. The recorded fruit mortality is within the range of reported estimates in poorly shaded, intensive cacao plantations in Brazil (79%; Hasenstein and Zavada, 2001

Conclusions

Fruit mortality is an important bottleneck in the reproductive success of flowering plants. The results of this study show that mechanisms of cacao's fruit mortality within well-shaded agroforestry systems may differ greatly among shade management types. The distinction between fruit abortion and insect and pathogen attacks as causes of fruit mortality revealed an as yet unexploited management potential for realizing higher yields from cacao in shaded agroforestry systems. Natural shade was

Acknowledgements

This study was funded by the German Research Foundation (DFG) under grant SFB-552 (“Stability of Rainforest Margins”—STORMA) and we thank coordinators Daniel Stietenroth, Adam Malik, Wolfram Lorenz, Surya Tarigan and the many other colleagues, in particular Damayanti Buchori, for their help and collaboration. We thank Drs. Rita Muhamad and Siswanto for identifying the mirid bugs and Pak Man and Arifin for their help during the fieldwork. Further, we thank the cacao farmers for their permission

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Shade tree management affects fruit abortion, insect pests and pathogens of cacao

Abstract

Introduction

Section snippets

Study sites

Results

Discussion

Conclusions

Acknowledgements

Crop Prot.

Acta Oecol.

Agric. Syst.

Fungal endophytes limit pathogen damage in a tropical tree

Proc. Natl. Acad. Sci. U.S.A.

Shade management in coffee and cacao plantations

Agroforest. Syst.

Are partial nutrient balances suitable to evaluate nutrient sutainability of land use systems? Results from a case study in Central Sulawesi, Indonesia

Nutr. Cycl. Agroecosyst.

Pests of Cocoa

Effect of pollination intensity on fruit and seed set in cacao (Theobroma cacao L.)

Sex. Plant Reprod.

Evaluation of certain factors affecting the yield of cacao in Ecuador

Ecology

Auxin modification of the incompatibility response in Theobroma cacao

Physiol. Plantarum

Annual Report for 2003/2004

Effects of land-use intensity in tropical agroforestry systems on coffee flower-visiting and trap-nesting bees and wasps

Conserv. Biol.

Rehabilitation of diseased cacao fields in Peru through shade regulation and timing of biocontrol measures

Agroforest. Syst.