Shade tree management affects fruit abortion, insect pests and pathogens of cacao
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
References (27)
- et al.
Damage and crop loss relationships of Helopeltis theivora, Hemiptera, Miridae and cocoa in Malaysia
Crop Prot.
(1995) - et al.
Breeding system and pollen limitation in the masting tree Sorbus aucuparia L. (Rosaceae) in the NW Iberian Peninsula
Acta Oecol.
(2006) - et al.
A physiological production model for cocoa (Theobroma cacao): model presentation, validation and application
Agric. Syst.
(2005) - et al.
Fungal endophytes limit pathogen damage in a tropical tree
Proc. Natl. Acad. Sci. U.S.A.
(2003) - et al.
Shade management in coffee and cacao plantations
Agroforest. Syst.
(1998) - et al.
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.
(2005) Pests of Cocoa
(1972)- et al.
Effect of pollination intensity on fruit and seed set in cacao (Theobroma cacao L.)
Sex. Plant Reprod.
(1995) - et al.
Evaluation of certain factors affecting the yield of cacao in Ecuador
Ecology
(1956) - et al.
Auxin modification of the incompatibility response in Theobroma cacao
Physiol. Plantarum
(2001)
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.
Cited by (81)
Effect of land-use history on tree taxonomic and functional diversity in cocoa agroforestry plantations
2024, Agriculture, Ecosystems and EnvironmentNegative effects of water deficit on cocoa tree yield are partially mitigated by irrigation and potassium application
2024, Agricultural Water ManagementCombined effects of shade and drought on physiology, growth, and yield of mature cocoa trees
2023, Science of the Total EnvironmentCross-pollination with native genotypes improves fruit set and yield quality of Peruvian cacao
2023, Agriculture, Ecosystems and EnvironmentHand pollination under shade trees triples cocoa yield in Brazil's agroforests
2023, Agriculture, Ecosystems and Environment