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EAP Publication - 4

DIATOMACEOUS EARTH: A Non Toxic Pesticide

MACDONALD J. 47( 2): 14, 42 (May, 1986)
by Professor Stuart B. Hill
Department of Entomology and
Ecological Agriculture Projects

For centuries stored grain has been protected from insect attack in much of the less developed world by adding some form of powder or dust to it. Common materials include plant ash. lime. dolomite. certain types of soil, and diatomaceous earth (DE) or Kieselguhr.

With me introduction of synthetic pesticides in the 1940s, and modern fumigants some time later, it was felt that a scientific solution to pest problems had been found. Although tnese materials provided enormous local benefits. a number of problems are beginning to be recognized. These include the development of resistance by insects. pollution of the environment. contamination of foodstuffs with residues, and exposure of users to toxic chemicals. This has led a small group of researchers and developers .to look again at the different powders to see which are most effective and how they can be improved.

Probably the most effective naturally occurring protective powder is diatomaceous earth. This is a geological deposit made up of the fossilized skeletons and tests of siliceous marine and fresh water organisms, particularly diatoms and other algae. These skeletons are made of hydrated amorphous silica or opal. When crushed, they break up into tiny pieces of glass'' (so tiny that the material feels like talcum powder). This is easily picked up by the hairy bodies of most Insects. whereupon it scratches through their protective wax layers; and they also absorb some of this material. the result being that the insects lose water rapidly . dry up and die Further protection is provided by the powder's property of repelling many insects. A similar principle probably accounts for the fact that birds frequently take dust baths, presumably to rid themselves of parasites.

Although patents for diatomaceous earth formulations were issued in the United States in the late 1800s it was not until the 1950s that the first commercial formulations of it became widely available, and between 1963 and 1970 a series of studies on DE were conducted by the U.S. Department of Agriculture.

In several tests, DE gave better protection of grain than malathion, particularly over the long term, without exposing anyone to the dangers of toxic chemicals. At that time relatively large amounts of DE were added to grain to provide protection, e.g., 3-1/2 kg/tonne. The main problem with using this amount was that it tended to make the grain very dusty and it reduced its flow rate and test weight.

Today this problem has been greatly reduced through the use of improved DE formulations that contain baits and attractants. Such formulations have been developed and tested in Quebec through the collaborative work of Mr. Arthur Carle (P.l.P. Products Inc., 2721 Plamondon, Longueuil, Que., J4L lSl) and myself. Using NCr, one of these formulations, as little as 0.5 kg/tonne may provide full protection. Despite this, very little grain in Canada is treated with these DE formulations. One of the main reasons for this is that present regulations prevent the adding of any powder to grain destined for export. Until such rules are changed the full potential of DE will not be realized. This is especially frustrating in the case of grain going to developing countries as aid. While this grain may be pest-free when it leaves Canada, it is often rapidly invaded by insects when it reaches its Third World destination. It is not uncommon for 20 per cent of this grain to be subsequently lost to pests. If DE had been added prior to export, however, it would have been protected indefinitely. Fortunately, DE can be added to domestic grain as long as it doesn't pass through licensed elevators. It can also be used in grain and food handling and storage areas such as flour mills, empty grain bins, box cars, ships' holds, warehouses, food processing plants, etc.

In houses it can be used effectively to prevent the entry of certain insects such as earwigs, ants, and cockroaches, and to control these and others that are present in cupboards containing food, carpets, basements, attics, window ledges, pet areas (for fleas), etc. In all of these examples it is important to place a small amount of the powder in corners, cracks, crevices. and other areas where insects might hide.

Whereas with a contact pesticide the insect dies quite quickly, with DE control may take several days. The more important difference is that the effect of the protection provided by the chemical is short-lived. whereas DE will control the pests as long as the powder remains. In this respect DE is an ideal pesticide; it is residual but nontoxic. The only health precautions that need to be taken are that if large areas are being treated with a power duster, the applicator should wear a mask to prevent inhalation. Because DE is made of silica, people sometimes mistakenly think that DE causes silicosis. As indicated above, however, pesticide quality DE is usually over 97 per cent amorphous silica, which does not cause silicosis, which is associated only with crystalline silica. Indeed, inhalation of road dust and grain dust IS likely to be more harmful than DE.

In the field DE has potential in certain restricted uses such as treating the bark of fruit trees in spring using an electrostatic duster, or the roots of plants when transplanting: but because it is non-selective and also kills beneficial insects, its use here should be carefully controlled.

Another use is in animal production units for the control of external parasites and flies. This is achieved by dusting the animals and the litter or bedding area. It has also been included in the diet (two per cent in the grain ration) to control certain internal parasites, and this practice is said to result in lower fly populations in the resulting manure.

In the future, improvements in the formulation of DE to reduce dustiness and more effectively lure insects to it to ensure their rapid exposure will no doubt extend its use. In the meantime, it is perhaps the safest effective pesticide for use in the home and has a valuable place in the protection of stored food and control of insects in animal production units.

DE may be purchased in Quebec from Chemfree Environment. Inc. (514-630-4400), and from garden centres.

Copyright 1986 Ecological Agriculture Projects


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