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Biological Agents for Cocoa Crop Protection

Biological control (biocontrol) can be defined as the deliberate use of living organisms (control agents) to suppress unwanted organisms (pests). It is generally recognised as one of the most environmentally sound and sustainable methods of controlling pests (pathogens, invertebrates, or weeds), but has a mixed history of success. Although it does not always work, there have been some spectacular success stories, with biopesticides replacing chemicals in certain situations and some classical biological programmes showing several hundred-fold benefit / cost ratios.

There are a number of theoretical approaches, with various techniques appropriate for different situations. Of the main biological control strategies, three are of potential importance in cocoa:

  • Classical biological control: a co-evolved natural enemy from the area of origin of the target pest is released as a biocontrol agent to reduce and suppress the target population;
  • Conservation of natural enemies: activities which enhance the activity of indigenous natural enemies, in contrast to the use of broad spectrum pesticides and their potentially negative impact;
  • Biopesticides are one example of inundative biological control using entomopathogens and other microbial antagonists which may be either indigenous or "exotic" (from another agro-ecological zone).
Healthy cocoa
 

Some cocoa pests and diseases respond to a classical approach, while others call for an inundative/biopesticide solution. The interactions between biological and chemical control agents may also have to be investigated, as part of an ICM strategy. The term "biopesticide" is most useful when applied strictly to living organisms which are (a) specific as individual products and thus confer some environmental advantage (unlike many but not all chemicals), and (b) have a limited ability to persist in the environment, and are therefore usually released using pesticide application techniques (although, more persistent, classical agents may be utilised as biopesticides). In practice, biopesticides often are most acceptable to policy makers and the public when indigenous isolates of control agents are used.

Trichoderma spp. are present in substantial numbers in most soils and in environments such as decaying wood. They are being increasingly exploited as disease control agents against plant diseases in a wide range of field and tree crops, and promising candidates for tackling cocoa diseases include: T. asperellum, T. koningiopsis, T. ovalisporum and T. stromaticum. The USDA provides a very helpful key to these species. Mechanisms of action may include direct anti-fungal activity through mycoparasitism of hyphae and antibiotic production, and indirect effects such as inducing host plant resistance and competing for nutrients and space.

Originally identified as a strain of Trichoderma viride on cocoa witches' broom, in Brazil's Amazon basin, T. stromaticum (Samuels et al., 2000) has proved to be an effective mycoparasite of witches' broom disease: Moniliophthora perniciosa. Initial small-scale field and lab trials conducted at CEPLAC showed that it could reduce formation of basidiocarps (fruiting bodies) by 99% when brooms were in contact with the soil, and by 56% in brooms remaining on trees. It also reduced pod infection by 31%. A commercial formulation 'Tricovab' was developed by CEPLAC in Brazil and is now available for control of witches' broom, with improved formulations under development.

Left: unimpaired sporulation of Trichoderma stromaticum on a witches' broom in a plot that has clearly been treated with a copper fungicide
Right: a cocoa weevil Pantorhytes plutus infected with Beauveria bassiana

Over recent years there has been the development of biologically based methods against insect pests including the development of mycoinsecticides, use of botanicals, natural enemies and pheromones (sometimes called "biorational" agents). The potential of using oil-based formulations to enhance the infectivity of potential mycoinsecticidal agents such as Beauveria bassiana was first identified with the cocoa weevil in Papua New Guinea by Dr. Chris Prior. Based on this work, a technology was subsequently developed for control of locusts and grasshoppers, with the development of the Metarhizium-based 'Green Muscle' mycoinsecticide. Unfortunately, this remains a "latent technology" for a number of potential insect pests, including cocoa insect targets, partly because of development costs. For example, in order to turn 'Green Muscle' from an idea into a commercial product, approximately €15 million were spent by the international LUBILOSA Programme. Although much of this technology could be adapted for other pests, a considerable investment would still be required for efficacy trials, optimising production, adapting formulations, safety testing, registration etc. Nevertheless, there are examples where biopesticide approaches might be effective against major cocoa pests such as cocoa capsids (various species of Miridae) and cocoa pod borer: Conopomorpha cramerella. These show promise in the laboratory, but require further verification in the field.

 

Current Research & Development Activities

Development of delivery systems for Trichoderma stromaticum. A 'MycoHarvester' Mk 3 has been installed and is operational at the biological control unit in CEPLAC. It has completely replaced previous 'Tricovab' product handling equipment and can therefore be considered a considerable success (having also been subject to attention in the local media), but optimisation can be considered a continuing process.

Reducing the 'Tricovab' product to a pure single-spore powder creates a number of opportunities:

  • Accurate dosage control is now possible: 'Tricovab' is a reliable product, akin to its chemical rivals, which will not block filters and nozzles;
  • Packaging and formulation are considerably easier: with a sachet system for mixing individual sprayer tank loads under development;
  • Shelf-life of the product should be enhanced by improved control of product moisture content (by reducing the bulk of the product, control of ambient temperature is easier).

Key processes to achieve optimise delivery systems for biological agents:

Associated production issues, including:
* selection of suitable isolates
* spore separation from substrate
   (e.g. using the MycoHarvester)
* optimisation of storage and packaging

Formulation options under investigation/development:
* water miscible suspension
* spores in adjuvant oil
* oil based formulation for ULV application

Application (special issues with particulate suspensions)

 

The biocontrol activity of 'Tricovab' has been assessed, but more work is needed on its persistence in the environment. It may be compatible with chemicals such as copper fungicides; improved formulation will aid tank mixing with chemical fungicides (e.g. for pod protection).

Medeirosa FHV, Pomella AWV, de Souza JT, Niella GR, Valle R, Bateman RP, Fravel D, Vinyard B, Hebbar PK (2010)  A novel, integrated method for management of witches' broom disease in Cacao in Bahia, Brazil.  Crop Protection, 29(7): 704–711.

Links

Global research updates:

USDA key to Trichoderma spp. (Dr. Gary Samuels et al.)

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