Coffee chemistry and the search for the tasty, pest-free bean
Scientists at Kew and the U.S. Department of Agriculture (USDA) are exploring areas for future research to combat coffee pests, using naturally occurring coffee chemicals and pesticidal plants.
Coffee is a commodity of global significance and is a crop key to the livelihoods of more than 100 million smallholder farmers engaged in its production. Despite the existence of 124 species, commercial and agricultural activity is almost exclusively confined to two species, Arabica coffee (Coffea arabica) and robusta (C. canephora). These two crops are under increasing threat from climate change and constant attack from a range of pests and pathogens.
The most widespread insect pest is the coffee berry borer (CBB; Hypothenemus hampei), a bark beetle with a worldwide distribution. CBB is difficult to control as it burrows into the developing coffee berry and only emerges to disperse. So, for this pest there needs to be some way of preventing it infesting the coffee plants. CBB can be lured into traps, using mixture of volatile coffee-derived compounds, but this is not a practical method of controlling CBB when there are millions of insects per hectare in a typical infestation.
Another group of emerging pests, currently more restricted in their geographical range, are stink bugs from the genus Antestiopsis. These stay exposed on the surface of the plant, where they feed using their sucking mouthparts. The feeding of Antestiopsis contaminates the coffee beans with chemicals, although it is not known whether they are from the insect or from bacteria and fungi that are associated with the bug. What is known is that the chemical contamination results in beans exhibiting a “potato taste”, making the beverage undrinkable.
Different cultivars of coffee vary in their chemical profiles, producing different “chemotypes”. The balance of chemical compounds in these varieties can be altered by different growing conditions, such as increasing altitude. Coffee can also be bred to express different chemical profiles. Production of caffeine in coffee probably developed as part of the arms race against insect pests. Unfortunately, the major pests of coffee are able to tolerate high concentrations of caffeine by utilising symbiotic microorganisms in their digestive systems to break it down into harmless chemicals.
A lot of coffee-compounds are insecticidal and could potentially affect the insect pests feeding on coffee. So, by exploring the diversity of chemotypes within cultivated and wild coffee there is the possibility that the crop could be made more insect resistant. This chemical resistance approach would be best applied to the CBB, which is hidden in the beans. Care must be exercised, however, as each cultivar of coffee produces a beverage with distinct taste characteristics. The flavour and aroma of coffee is imparted by a complex mixture of compounds and some of these (e.g. caffeoyl quinic acids) could also be utilised for their insecticidal effects. There is no point in producing a CBB resistant variety that produces an espresso tasting of wood or rubber!
In many parts of Africa, pests are controlled using preparations of indigenous plant species. The ’pesticidal plants’ offer a cost-effective and sustainable method of pest management for subsistence farmers and the rural poor .
Unfortunately, they are under-utilised for control of CBB, and there are only a few plants that have been reported as having some toxicity against CBB. As part of an Integrated Pest Management (IPM) strategy for coffee, they could be tested for their effectiveness against pest-insects, pathogens and beneficials. This approach could be used for CBB, but would be best applied to stink bugs, which are exposed on the plant surfaces.
So how do we improve and sustain yields of coffee in the face of existing and emerging pests, particularly as the climate changes? The answer lies in a multi-disciplinary approach, involving integrated pest management, breeding for resilience and a better understanding of pest biology. Action is required in order to sustain the rural livelihoods and to ensure that we can continue to enjoy that wonderful aroma, flavour and kick-start that only coffee can provide.
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