Storage trials in Zimbabwe found diatomaceous earths (DE) offered an alternative to organophosphate insecticides and are effective in controlling post-harvest insect pests in maize, sorghum and cowpeas stored for >8 months, enabling households to increase their food security and control over grain sales.

This project will verify these findings under semi-arid conditions in Tanzania where producers have constantly prioritised storage losses, and where the devastating larger grain borer (Prostephanus truncatus) is endemic.

Local sources of DEs are being evaluated for their potential as sustainable low cost sources of DE.

New knowledge will be promoted amongst intermediary and end-users in forms they can utilise and adapt, building on the multiple information networks used by the respective user groups.

The project flyer [PDF, 321kb] provides an overview of the project.

Background to the current project

The study of diatomaceous earths (DE) as grain protectants for small-scale producers began in Zimbabwe in 1998. Producers were expressing a need for a relatively cheap and safe method of storage insect control. DEs offer safer alternatives to synthetic chemicals, but information on their efficacy under tropical small-scale farming conditions was lacking. The initial storage trials in Zimbabwe (from 1998 -2000) used two commercially available DE products Protect-It® and Dryacide®. Both of these DEs gave significant protection against insect damage when admixed with farm stored maize, sorghum and cowpeas for periods of 40 weeks and were as effective as Actellic Super dust. However, efficacy of these DEs is closely linked to the application rates and differs between commodities, locations and insect pests. Details of these trials can be found in two publications.

In 1999 the Crop Post Harvest Programme (CPHP) funded a regional East African workshop in Dar Es Salaam to assess farmers strategies for coping with the Larger Grain Borer. The main message to come out of this workshop was that farmers wanted alternatives to organophosphates for grain protection so that they had options to choose from. Conventional insecticides are often unavailable when needed, adulterated, of poor quality, expensive and many farmers are afraid to use synthetic chemicals on their stored food because they are inherently poisonous.

Following the workshop, Mr Riwa of the Ministry of Agriculture and Food Security contacted NRI to discuss the potential for the use of diatomaceous earths (DEs) as grain protectants and the trials that we were conducting in Zimbabwe in collaboration with the University of Zimbabwe and the Institute of Agricultural Engineering. These trials had found that DEs were effective in controlling post-harvest insect pests in maize, sorghum and cowpeas stored for >8 months under small-scale farmer conditions in Zimbabwe and offered an acceptable alternative to the locally recommended insecticide, Actellic Super Dust (ASD).

The existing evidence suggested that was potential for DEs to be used in Tanzania to provide an alternative option for farmers in the battle against LGB. We submitted an outline proposal to the DFID Crop Post Harvest Programme (CPHP) in April 2000. The CPHP Project Advisory Committee were keen to fund the work, but were short of funds, so decided to release limited funds to enable the full project proposal to be collaboratively developed during a workshop in Shinyanga in August 2001.

At this workshop, participants related how farmers constantly mentioned the threat posed by storage pest damage, to NGO staff and other field workers and that the adulteration of Actellic Super dust had reached such a serious scale in Tanzania (one farmer in Shinyanga region actually managed to breed storage insects in what had been sold to him as pesticide). The scale of damage caused by LGB and the widespread adulteration of pesticides had made stored product pest damage a political issue and parliamentary members had been asking the Ministry of Agriculture what they were doing about this problem. In June 2002, the project was funded and storage trials were set up in three regions of Tanzania, Dodoma, Manyara (formerly in Arusha) and Shinyanga.

Diatomaceous Earths

Diatomaceous earths (DE) are formed from the fossils of microscopic planktons called diatoms, which are found in fresh and salt water.

When diatoms die, they sink to the bottom of oceans or freshwater lakes and accumulate into a sedimentary layer, which over the centuries builds up and becomes compressed and fossilised into a soft chalky rock called diatomaceous earth.
DE is a porous material and is used: in filters to help clarify fruit juices, beers, wine, pharmaceuticals, swimming pool waste; dry cleaning solvents; as food additives; in baby powders; to remove oil from concrete floors and as an insecticide.

DE looks similar to talcum powder, however if you were to put your finger into the DE you would find it makes your fingers feel quite dry. DEs exert their effect on insects through physical means. When insects come into contact with the DE particles, waxes are absorbed from the cuticle (or outside skin) of the insect, as a result the insect loses its waterproofing and dehydrates and dies.

However, insect species differ in their sensitivity to the various DEs and laboratory trials at NRI have found that LGB (Prostephanus truncatus) is less susceptible to DEs than some of the other storage insect pests such as Sitophilus spp. In the field trials in Zimbabwe another Bostrichid beetle Rhyzopertha dominica was also found to be less susceptible to DEs than other storage pests. As a result a range of application rates of DEs and a mixture of DE with the pyrethroid permethrin have been included along with traditional farmer grain protection practices such as grain admixture with ash, in the Tanzanian field trials to determine how DEs can best be used to reduce damage caused by LGB and other bostrichid beetles.

The use of diatomaceous earths in pest control is not new. Observations of birds and mammals taking dust baths to rid themselves of mites and parasites is believed to have led the Chinese to start using diatomaceous earths in pest control more than 4000 years ago.

DEs have extremely low toxicity to mammals (e.g. DE rat oral LD50, >5000 mg/kg), and are considered 'Generally Regarded As Safe' by the USA Environmental Protection Authority. The US Food and Drug Agency has exempted DE from requirements of fixed residue levels when added to stored grain. The only possible negative health effect comes from long-term chronic exposure to quantities of inhaled dust, and safety precautions such as the wearing of masks need to be taken by workers during DE application. Many DE dusts are now commercially available, and are registered for use as grain protectants in Australia, Brazil, Canada, Croatia, China, Germany, Indonesia, Japan, Philippines, Saudi Arabia, United Arab Emirates and USA. DEs can be used for managing stored product insects and mites, improving fumigation and aeration efficiency, and structural treatments. DEs are approved for organic processing.

The Small scale farmer utilisation of diatomaceous earths during storage project has six main outputs that it plans to achieve.