However, the plantations are under attack from numerous insects, several of which cause considerable damage. In particular, this applies to lepidopteran borers such as Eldana saccharina (Pyralidae), whose caterpillars bore into sugarcane stalks, causing losses in terms of both sugar and biomass. The economic impact is significant: estimates in Réunion showed that an infestation rate of more than 90% of affected canes (20% of internodes with holes) causes losses of up to 30 tonnes a hectare in susceptible varieties.
Concentrating on slowing the pest’s development
Chemical control against this type of pest is difficult to implement in that the caterpillars and larvae that cause the damage are sheltered inside the stalks. Moreover, biological control, which has been tested in recent years, has failed to give any conclusive results. Research is now centring on identifying the agronomic factors that slow the borer’s development.
A collaboration project between CIRAD and the South African Sugar Research Institute (SASRI) enabled a vast research programme from 2004 to 2006 in South Africa, one of the world’s leading sugarcane producers. The work was based on an agroecological approach combining silicon applications in the field and the water stress that occurs in certain production zones.
The results of the laboratory trials are very convincing: silicon applications significantly reduce the damage caused by the various varieties of borer, with or without water stress. In the case of susceptible varieties and with water stress, damage is kept at very low levels, equivalent to those measured in resistant varieties, with or without water stress. It is now estimated that applying silicon to susceptible varieties prevents the loss of 20 if not 30% of the sugar yield, not counting the biomass losses caused by the plant. On the other hand, silicon applications do not modify stem hardness. Moreover, no impact on sugar quality has yet been observed.
Silicon apparently triggers a barrier effect against the larvae
One suggestion has been made concerning the active role played by silicon in improving the plant’s defence system. AS regards plants subjected to water stress, it may be that a lack of water leads to modifications in silicon concentration and structure within the plant tissues. These modifications apparently reinforce the barrier effect against larva penetration, without affecting tissue hardness. One other possibility is that the plant’s natural defence mechanisms, whether chemical or physiological, may be strengthened. However, those defence mechanisms have yet to be fully elucidated. Experiments are planned, notably with the University of Kwa-Zulu Natal, in South Africa, to study the role of silicon within the plant. The aim is to locate the silicon deposits in the stalk that are involved in the barrier effect and to determine the nature of that barrier.
The results obtained in the laboratory suggest that it should eventually be possible to extend the use of this agrobiological control method to all sugarcane producers. The cane production basins of South Africa are particularly concerned: 60% of the region’s soils are deficient in plant-available silicon, deficiency that is sometimes exacerbated by a lack of water. There are hopes of controlling the borer more effectively in the field by applying calcium silicate, including on susceptible varieties. Field trials are planned for 2007 and 2008, following which development technicians in the regions concerned will be extending the results to producers.
This research was awarded the Kynoch prize for the best contribution to the South African Sugar Technologists' Association (SASTA) congress in Durban (South Africa) in July 2006, for its merits in terms of potential applications.
Helen Burford | alfa
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