Mineral Dusts

For some time researchers have known that certain powdery minerals can kill insects. The sharp-edged dust particles "spear" through the thin joints between the horny plates of the animal's exoskeleton. This was first recognized with diatomaceous earth, a widely available, completely safe powder that kills cockroaches almost on contact. Now scientists in Nigeria have found that a common local mineral called "trona" also works in the same way—at least on certain storage pests.

In experiments, powdered trona proved lethal to the maize weevil (Sitophilus zeamais), causing almost 100 percent mortality after 15 days of exposure. It also reduced the maize weevil's fecundity in grains treated with the dust.14

Trona, Na2CO3·NaHCO2H2O, is a crystalline carbonate/bicarbonate that occurs naturally in several parts of Africa. It is apparently not toxic to humans and livestock. Indeed, in most African countries, rural people use it as a food additive.15 For example, they commonly drop it into okra soups to increase the mucilaginous quality or into boiling cowpeas to reduce the cooking time. In northern Nigeria, farmers add trona to their cattle's drinking water.

Mixing trona dust with maize grains (at 1.5 percent by weight or more) killed or inhibited the biological activities of the most ubiquitous pest of stored maize, the maize weevil; but another noxious pest, the red flour beetle (Tribolium castaneum), was unaffected.16

Mineral dusts may never be fully reliable in grain-store insect control, but their permanence, low toxicity, and ready availability make them attractive possibilities for a simple, cheap, and ubiquitous answer to at least part of the massive and widespread storage losses.

14  

L.C. Emebiri and M.I. Nwufo. 1990. Effect of Trona (Urao) on the survival and reproduction of Sitophilus zeamais and Tribolium castaneum on stored maize. Agriculture, Ecosystems and Environment 32:69-75. (L.C. Emebiri and M.I. Nwufo, Department of Crop Production, S.A.A.T., Federal University of Technology, Owerri, P.M.B. 1526, Nigeria.)

15  

It is known locally as kaunin Nigeria and kanwe in Ghana.

16  

A similar finding has been reported in respect to an inert diatomaceous earth, which was lethal to eight pests of stored products but was harmless to the red flour beetle. S.D. Carlson and H.J. Ball. 1962. Mode of action and insecticidal value of a diatomaceous earth as a grain protectant. Journal of Economic Entomology 55:964-970.



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