| STUDENTS CONTRIBUTION
Environmentally friendly way
of controlling stored product insect pests Mohale S
INTRODUCTION
Storage of agricultural produce is part of the post-harvest system through which food material passes on its way from field to consumer. It is generally accepted that 5-15% of the total weight of all cereals, oilseeds, and pulses is lost after harvest [1]. There is a continuous need to protect the stored products against deterioration, especially loss of quality and weight during storage. Quantitative and qualitative losses of stored grains may result from the feeding and waste production by insects, mites, rodents and birds or from the growth of microorganisms all of which are influenced by environmental conditions. Insects, mites and fungi may cause hydrolysis and oxidation and decrease the level of certain nutrients in stored products or even form toxic substances such as mycotoxins.
The use of pesticides is one means of preventing some losses during storage. However, the choice of pesticides for storage pest control is fairly limited because of the strict requirements imposed for the safe use of synthetic insecticides on or near food. The continuous use of chemical pesticides for control of stored-grain pests has resulted in serious problems such as insecticide resistance [2]. Chemicals used for control of stored product pests or as protectants need also to be compared with the suitability and effectiveness of alternative methods of control. Non-chemical methods are attractive since they neither leave chemical residues in the commodity nor do they cause resistance in insects. The public awareness and concern for environmental quality, has led to more focused attention on research and development of alternative stored product protectants. Use of amorphous diatomaceous earth for pest control in stored products and in the field has increased during recent years. Diatomaceous earth and artificial silica dusts have been reported to be effective against control of insect pests in stored products [3-5].
Diatomaceous earths
Diatomaceous earths (DE) are fossilized remains of diatoms, composed mainly of amorphous hydrated silica, but also other minerals including aluminium, iron oxide, magnesium, sodium and lime. These dusts are either of marine or fresh-water origin. The former are said to be more effective against insect pests [3]. Diatomaceous earths of marine origin are effective against storage insects at 0.1% (w/w), but commercially available diatomaceous earth products are often enhanced by other compounds like ammonium fluorosilicate. Products of fresh-water origin require at least twice the above dosage to produce similar effects on the target insects [3].
Unlike conventional contact insecticides, Des, like other inert dusts function through their physical properties and are therefore generally slower in action [4]. Insect mortality is induced primarily as a result of desiccation. Diatomaceous earths abrade or adsorb the water resistant epicuticular lipid layers of insects. As a result, insects desiccate and die from excessive water loss through the damaged cuticle [3-5].
Some studies on insect mortality as a result of diatomaceous earths activity
Studies on the effectiveness of diatomaceous earths have been reported. Insecto, a diatomaceous earth formulation containing 86.7% armophous silica (silicon dioxide), 3% crystalline silica and 10% food grade additives was shown to effect total mortality of 1st instars of Tribolium castaneum Herbst when applied to maize (Zea mays L.) at 0.5 and 1 g/kg rates. At the same rates of Insecto, mortality of Oryzaephilus surinamensis L. and Plodia interpunctella Hübner 1st instars was 96-97 and 86-97% respectively [6]. Subramanyam et al. [6] reported complete suppression in emergence of T. castaneum and P. interpunctella adults at 1 g/kg Insecto application rate while emergence of O. surinamensis was 98%. Arthur and Throne [5] reported the efficacy of Protect-It (Mississauga, Ontario, Canada) formulation of diatomaceous earth (DE) to control internal infestations of rice weevil and maize weevil. The authors reported that mortality of Sitophilus oryzae L. and Sitophilus zeamais Motschulsky emerging from kernels in wheat treated with DE was always greater than controls and ranged from 56 to 90% at 22 0C and was > 90% at 27 and 32 0C.
Diatomaceous earth product Fossil Shield® was shown to be lethal to adult Tenebrio molitor L. and Tribolium confusum J. du Val [7]. The authors reported that "Fossil Shield®" was lethal to 1st instar larvae of P. interpunctella, but not lethal to older larval stages. They also reported that two-week old larvae of T. confusum were more sensitive to diatomaceous earth than P. interpuctella at the same age. Contact with diatomaceous earth caused adult Sitophilus granarius L. to lose weight and reduced their water content, suggesting disruption of ‘the water barrier' [7]. Arthur [4] reported increased mortality of T. castaneum and T. confusum with increased temperature and exposure interval to diatomaceous earth (Protect-It). On farm trials in Buhera and Binga districts in Zimbabwe demonstrated that maize can effectively be protected from insect damage by admixture with either "Protect-It" or "Dryacide" at 1 g/kg of grain [8].
Studies on diatomaceous earth and biological control of insects
Studies have shown that diatomaceous earth enhances the efficacy of entomopathogens. Akbar et al. [9] reported that the efficacy of Beauveria bassiana Balsamo Vuillemin (Hyphomycete), a well known entomopathogen against larvae of T. castaneum is enhanced by DE applied at 190 mg/kg when the fungus is applied at 33 to 2700 mg of conidia per kilogram of grain. Beauveria bassiana must adhere to, germinate on and penetrate through the host integument; therefore its efficacy is improved in the presence of other surface-active agents. Diatomaceous earth is one such agent that has been reported to cause abrasion and adsorption of the cuticular lipids [3]. On the other hand, Perez-Mendoza et al. [10] reported that Anisopteromalus calandrae Howard, a parasite of rice weevil was sensitive to direct contact with "Protect-It", an inert dust formulation containing 90% diatomaceous earth and 10% silica aerogel. Thus, any natural control of pest insects exerted by local populations of parasitoids, or enhanced biological control by augmentative releases of parasitoids, would be adversely affected by the use of Protect-It or other diatomaceous earth products on stored grain.
Determinants of diatomaceous earth efficacy
Exposure to diatomaceous earth results in loss of insect body weight mainly through water loss [3,6,7]. Relative humidity (r.h.) affects the rate of water loss and therefore determines the effectiveness of inert dusts [3,4,5,9,11]. Arthur [11] reported complete mortality of S. oryzae adults at 40% or 57% r.h., but at 75% r.h. survival was related to population and temperature. A higher percentage of adults survived when 30 were exposed to Protect-It compared to 10 and 20 and within each population density, survival decreased with increasing temperature. The author also reported that survival decreased with increasing exposure interval, but with exposure interval, survival was greater at 75% compared to 57% r.h. Arthur [4] reported that mortality of T. castaneum and T. confusum after initial exposure to Protect-It was lowest at 22 0C but increased as temperature and exposure interval increased, and within each temperature decreased as humidity increased. Death of stored product insects treated with diatomaceous earth decreased with increased r.h., due to reduced transpiration through the cuticle. High relative humidity delays, or above 60% can prevent, the drying action of diatomaceous earth [7]. Mewis and Ulrichs [7] reported that T. confusum, T. molitor, S. granarius and P. interpunctella died within 14 days exposure to Fossil Shield® in the absence of food but mortality was reduced in those fed on grain bran. This showed that access to food increased the chance of survival of these beetles. Le Patourel [12] suggested that whilst feeding, insects may be able to compensate for the loss of water by the production of metabolic water.
Limitations to the widespread use of diatomaceous earths
Unlike conventional grain protectants, diatomaceous earth dusts reduce the grain bulk density (test weight) and flowability [13]. They also produce airborne dust during application and handling and may damage milling machinery during processing. The action of diatomaceous earth is confined to external feeders whose bodies come in contact with the insecticide. Internal feeders developing inside the grain kernels would not die until they emerged from the kernels and encountered the insecticide. However, Arthur and Throne [5] have shown that this is still sufficient to curb population development of rice weevil and maize weevil, which are internal feeders.
ACKNOWLEDGEMENTS
I thank Prof. Allotey and Dr. Siame for their guidance in the writing up and revision of this script.
REFERENCES
1. Padin S, Dal Bello G and M Fabrizio Grain Loss Caused by Tribolium castaneum, Sitophilus oryzae and Acanthoscelides obtectus in Stored Durum Wheat and Beans Treated with Beauveria bassiana. J. Stored Prod. Res. 2002; 3: 69-74.
2. Suchita MG, Reddy GPU and MMK Murthy Relative Efficacy of Pyrethroids against Rice Weevil (Sitophilus oryzae L.) Infesting Stored Wheat. Indian J. Plant Prot. 1989; 17: 243-246.
3. Golob P Current Status and Future Perspective for Inert Dusts for Control of Stored Product Insects. J. Stored Prod. Res. 1997; 33: 69-79.
4. Arthur FH Toxicity of Diatomaceous Earth to Red Flour Beetles and Confused Flour Beetles (Coleoptera: Tenebrionidae): Effects of Temperature and Relative Humidity. J. Econ. Entomol. 1999; 93: 526-532.
5. Arthur FH and JE Throne Efficacy of Diatomaceous Earth to Control Internal Infestations of Rice Weevil and Maize Weevil (Coleoptera: Curculionidae). J. Econ. Entomol. 2003; 96: 510-518.
6. Subramanyam BH, Madamanchi N and S Norwood Effectiveness of Insecto Applied to Shelled Maize against Stored-product Insect Larvae. J. Econ. Entomol. 1998; 91: 280-286.
7. Mewis I and CH Ulrichs Action of Amorphous Diatomaceous Earth against Different Stages of the Stored Product Pests Tribolium confusum, Tenebrio molitor, Sitophilus granarius and Plodia interpunctella. J. Stored Prod. Res. 2001; 37: 153-164.
8. Stathers T and B Mvumi Farmers Help to Test New Grain Protectants. In: Crop Post-harvest Programme Issue No. 16 Jan-March 2001.
9. Akbar W, Lord JC, Nechols JR and RW Howard Diatomaceous Earth Increases the Efficacy of Beauveria bassiana against Tribolium castaneum Larvae and Increases Conidia Attachment. J. Econ. Entomol. 2004; 97: 273-280.
10. Perez-Mendoza J, Baker JE, Arthur FH and PW Flinn Effects of Protect-it on Efficacy of Anisopteromalus calandrae (Hymenoptera: Pteromalidae) Parasitizing Rice Weevils (Coleoptera: Curculionidae) in Wheat. Environ. Entomol. 1999; 28: 529-534.
11. Arthur FH Survival of Sitophilus oryzae (L.) on Wheat Treated with Diatomaceous Earth: Impact of Biological and Environmental Parameters on Product Efficacy. J. Stored Prod. Res. 2001; 38: 305-313.
12. Le Patourel GNJ The Effect of Grain Moisture Content on the Toxicity of a Sorptive Silica Dust to Four Species of Grain Beetle. J. Stored Prod. Res. 1986; 22: 63-69.
13. Korunic Z, Fields PG, Kovacs MIP, Knoll JS, Lukow OM, Demianyk CJ and KJ Shibley The Effect of Diatomaceous Earth on Grain Quality. Post harvest Biol. Technol. 1996; 9: 373-387. |
