Annual Report: CIRAD 2006

With 20.4 million hectares in 2005, sugarcane plays a major role in the global economy. Moreover, it has recently seen unprecedented development to produce bioethanol, a biofuel for which demand is almost certain to grow in the coming years. However, 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 show 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.

Limiting the damage on susceptible varieties

Chemical control against this type of pest is difficult to implement since the larvae and caterpillars that cause the damage are sheltered inside the stalks. Moreover, biological control, which has been tested on E. saccharina for many years, has failed to give any conclusive results. Research is now centring on identifying the agronomic factors that slow the borer’s development. In partnership with SASRI (South African Sugar Research Institute), CIRAD recently conducted a three-year research programme in South Africa, one of the world’s leading sugarcane producers.

The results obtained in pot trials are convincing: silicon treatments significantly reduce the damage caused by the borer, regardless of the variety, 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, irrespective of water stress conditions. It is now estimated that applying silicon to susceptible varieties prevents the loss of 20 or even 30% of the sugar yield, not counting the biomass losses caused by the insect. On the other hand, silicon applications do not modify stem hardness or sugar quality.

A new agrobiological control method

In the event of water stress, the active role played by silicon in the plant’s defence system could be put down to changes in silicon concentration and structure in the plant’s tissues. These changes apparently reinforce the barrier effect against larva penetration, without affecting tissue hardness. One other possibility is that silicon may strengthen the plant’s natural defence mechanisms, whether chemical or physiological. 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.

These results suggest that it should eventually be possible to extend the use of this agrobiological control method to all sugarcane producers. The cane production areas of South Africa are particularly concerned: 60% of the region’s soils are deficient in plant-available silicon, a deficiency that is sometimes compounded by a lack of water, which increases infestation levels. 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, before extending the results to producers.

Contact

François-Régis Goebel, e-mail , UPR: Sugarcane Farming Systems

Partners

• South African Sugar Research Institute (SASRI, South Africa)
• University of Witwatersrand (South Africa)

For further information

• Kvedaras O.L., Keeping M.G., Goebel R., Byrne M. Eldana saccharina Walker (Lepidoptera: Pyralidae) larval performance and stalk damage in sugarcane: influence of plant silicon and feeding site. International Journal of Pest Management (sous presse) .
• Kvedaras O.L., Keeping M.G., Goebel F.R., Byrne M. Water stress augments silicon-mediated resistance of susceptible sugarcane cultivars synergy in resistance of sugarcane cultivars to the stalk borer, Eldana saccharina Walker (Lepidoptera: Pyralidae). Bulletin of Entomological Research (sous presse) .
• Kvedaras O.L, Keeping M.G., Goebel R., Byrne M., 2006. Silicon and water stress synergy in resistance of sugarcane cultivars to the stalk borer, Eldana saccharina (Lepidoptera: Pyralidae). Proceedings of the South African Sugar Technologists' Association, 80 : 291-296.
• Kvedaras O.L., Keeping M.G., Goebel R., Byrne M., 2005. Effects of silicon on the African stalk borer, Eldana saccharina (Lepidoptera: Pyralidae) in sugarcane. Proceedings of the South African Sugar Technologists' Association, 79 : 359-362.

This research was awarded the Kynoch prize for the best contribution at the South African Sugar Technologists’ Association Congress in Durban (South Africa) in July 2006.