Inert dusts: using the past to protect the future

Farmers fears about mixing synthetic chemical pesticides with their food, even if approved for use as food additives, have resulted in the successful evaluation of diatomaceous earths (inert dusts) for use as grain protectants in Zimbabwe. However, until the dusts are registered for use in Zimbabwe, treated grain cannot be consumed so diatomaceous earths cannot yet be recommended for widespread use although relevant information on the use of diatomaceous earths as grain protectants is currently being prepared for submission to the pesticide registration committee. Sources of diatomaceous earth have also been found locally in Zimbabwe, which could possibly be exploited to provide a more cost-effective source of inert dusts. A deposit sample from northern Zimbabwe has been shown in preliminary tests to have activity against storage insects and further investigations are planned. Although commercially available diatomaceous earths are comparable in price to chemical grain protectants, these dusts will provide farmers with a safer alternative to synthetic pesticides.

Credit: T.E.Strathers, NRI

Diatomaceous earths consist of the fossils of tiny phytoplanktons (diatoms), and observations of bird and mammals taking dust baths to rid themselves of mites and parasites may have led the Chinese to start using diatomaceous earths in pest control more than 4000 years ago. Composed of mainly amorphous hydrated silicates, contact with diatomaceous earth particles causes the waxy fat and lipids to be adsorbed from the cuticle of insects resulting in water loss, dehydration and death. Diatomaceous earths have extremely low toxicity to mammals, and are commonly used by cattle, poultry and dog owners as a feed mix to combat internal parasites. Diatomaceous earths are also currently registered for use as grain protectants in a number of developed countries where they are used in the top dressing of grain in silos (acting both as a pesticide and to improve the effectiveness of phosphine fumigation). They are also used in the surface treatment of storage structures. However, very little information exists as to the efficacy of diatomaceous earths under tropical small-scale farming conditions.

In 1996, farmers in four districts in Zimbabwe ranked the need for improved methods of storage pest control as a priority issue during a post-harvest needs assessment. In response to these findings a collaborative project between NRI and the Institute for Agricultural Engineering (IAE) in Zimbabwe was developed to study the potential of diatomaceous earths as alternative grain protectants to organophosphates. The project set up on-farm trials in three regions during 1998 using locally produced sorghum, maize and cowpeas. Treatments included the inert dusts, Protect-it™ and Dryacide® at two concentrations (0.1% and 0.2% w/w), a pesticide dust Actellic Super™ and an untreated control. Damage levels remained very low in all the grain protectant treatments throughout the 40-week storage period, in contrast to the untreated controls where damage levels increased rapidly from 24 weeks storage onwards. No significant difference was found between damage levels in the Actellic Super™ dust treatment and the diatomaceous earth treatments with the exception of lower concentrations of Dryacide in sorghum, which was attributed to the problems of controlling the lesser grain borer, Rhyzopertha dominica. On-farm trials in 1999 using even lower concentrations of the inert dusts (0.1% and 0.05% w/w) have also proved particularly effective in maize.

During the initial trials, local farmers became interested in evaluating the grain protectant treatments using presence of adult insects, larvae, trash, poor taste, smell, damage, weight as criteria. Protect-it™ (0.1% w/w) scored very highly during farmers' evaluations with the result that, in 1999, farmers in two of the project districts set up their own participatory trials, involving treatment of their own grain (pre-purchased by the project) in their own stores. In comparison, with their traditionally treated or untreated grain, Protect-it™-treated grain again scored very highly in terms of reduced insect damage and presence, whiter and less contaminated mealie meal and increased sale price.

Although the current project is coming to an end in Zimbabwe, assistance in registering the commercially available dusts will continue. In addition, proposals for using diatomaceous earths are currently being put together with the aim of further investigating the efficacy of locally available African sources of diatomaceous earth, and the field efficacy against the Larger Grain Borer, which is not yet present in Zimbabwe but continues to be a major problem in many other African countries.

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