Section 10 - Non-chemical control methods

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Physical and mechanical methods of stored product insect control
Controlled atmosphere storage technology (CAST)
Comparative analysis between traditional cereal preservation methods and controlled atmospheres utilizing high | N2 | /low | O2 | in sealed bulk storage: Current methods

- CAST/SEALED STORAGE
- PHYSICAL METHODS
- GAP IN STORAGE MANAGEMENT

Physical and mechanical methods of stored product insect control

by A.C. QUINONES

The problems associated with chemical control of insects and increased demand for hygienic food supplies have stimulated a search to alternate control of storage-insect pests. Among the approaches being explored are use of temperature, moisture and atmosphere control, airation, grain fuming facilities, cleaning operation and radiation.

1. Temperature Control - Temperature plays a significant role in such physiological functions as oviposition, fecundity, generation time, longevity which essentially determine the growth and abundance of insect populations in grain. Temperature ranges of 2238C are considered favorable to most insects, but optimal temperature preferences are quite variable which accounts for geographical distribution patterns often associated with particular species.

Heat was used to kill insects at least 75 years ago. Temperatures of 120 to 130F maintained for 1012 furs. are effective. Actually, these temperatures kill most insects quite quickly but where insulation, grain and materials are involved, the general temperature must be kept up for several hours to ensure complete penetration of heat. Heating vaults are used for control of insects in used bags and in bagged or packaged products. With vault temperatures of 180 to 200 F, 100 1b bags of feed or flour can be completely freed from insect life in 24 hours' exposure.

Low temperatures are probably the most important single factor in making long-term storage possible and economical. Freezing quickly kills many forms of organisms and low temperature is also important in maintaining seed viability.

Temperatures above 115F may damage seed viability unless the grain is cooled soon after drying.

Several methods of heating grain in bulk are now available. A recent method is the use of fluidised-bed heating (Thorpe, 1985) where air is used as the heat transfer medium because it transfers heat rapidly, can combine with good solid mixing ensuring that individual grains in the bulk will each receive appropriate treatment. Batches of each receive appropriate treatment. Batches of grains in fluidised beds could be disinfected by heating them to a temperature of 65C in 4 minutes or even in 30 sec. or less. It was also found out that heating did not adversely affect the moisture content, germination or baking quality of wheat. At this temperature of the grain all developmental stages of insect pests are killed.

Frying a small quantity of rice (22.7 kg) for about 7 minutes was found sufficient for control of common insects, but this method is ineffective against S. oryzae despite reduction in the rate of build-up of the weevils (Lim 1975). On the other hand microwave heating complete mortality of adult s oryzae was achieved in rice (13% moisture content) after 10 min. (Lim & Tea 1978)

2. Moisture Content Seeds are hydroscopic, thus grain either absorbs moisture from the environment (under high relative humidity conditions) or loses moisture (under low relative humidity conditions). This relationship is represented graphically by typical zymoldal curves, dependent on whether the grain is absorbing or desorbing moisture, until a situation where relative humidity of the surrounding air and moisture within the grain kernels reach a dynamic equilibrium. Thus, the maintenance of "safe storage moisture content" requires a corresponding low level of relative humidity, which can be achieved by:

  1. Positioning of the storage in a region or location where relative humdity is naturally low.
  2. Reducing relative humidity to an acceptable level by conditioning the storage environment with refrigeration and dehumidification.
  3. Storing dry seeds in impervious sealed containers.

In regions where climate is more humid, grains are nearly always harvested with moisture favorable for insect development and unfavorable for safe storage. In this case, post harvest drying using heated air is a necessity.

The phenomenon of moisture migration is very pronounced in large grain bulks stored in warm subtropical climates where bulks of grain initially stored at elevated temperatures remain warm, while ambient temperature drops at night. A consequent increase in moisture content of the grain bulk periphery or condensation of moisture (sweating) on the bulk surface is generally imminent in vertical storage systems initiating microfloral activity and promoting excessive losses. Heavy insect infestation also causes moisture migration as a result of heat and moisture given off by the insects. Under this situation, the insects must be destroyed by fumigation. Methods of eliminating accumulations of moisture and reducing moisture migration are as follows: 1) drying the grain to a low, uniform moisture content, 2) keeping the grain uniformly cool by aeration or ventilation and 3) fuming the grain to cool it and disperse accumulation of moisture.

Most stored grain insects are unable to survive and reproduce in grain with moisture content below 9 percent. If by various means, it is possible to reduce and maintain the moisture below that favorable for reproduction and development, then we have, in effect, controlled the insects.

3. Grain Aeration - Another non-chernical method of insect control is grain aeration or ventilation. This is the process of cooling a grain bulk by passing air of suitable temperature and humidity through it. This process involves the formation of temperature and moisture fronts which move through the grain bed in the direction of the air flow. These fronts can interact which leads to an attenuation of grain temperature and moisture content profiles (Sutherland et al., 1971).

Grain temperatures of 15-18C prevent reproduction of Rhyzopertha dominca and Tribolium species and severely limit other Sitophilus spp. Dryness of grain also lowers reproduction of Sitophilus. Cooling to suppress moisture movement and removal of heat resulting from grain respiration, also decreased chemical protectant breakdwn rates, maintenance of grain freshness, expulsion of odors, and distribution or removal of fumigants.

Grain cooling can be achieved by aeration using ambient or artificially cooled air. The term aeration is usually taken to mean the forced movement of air of low airflow rates for the purpose of cooling rather drying, although some drying may occur. In tropical climates storage temperature are usually in the range of 25-30C, and although some cooling by aeration may be possible to prevent grain deterioration, it is often necessary to pay more attention to reducing grain moisture content as a means of suppressing biological, particularly microbiological, activity and use other methods to control insect infestation when this occurs.

An aeration system comprises a fan ductwork to distribute the air through the grain bulk, a controller to operate the fan only when aeration will be beneficial and for conventional systems, temperature monitoring equipment to observe and optimize the cooling performance. Fans and ducts are the two major pieces of equipment needed for the proper delivery of aerating air through grain mass in terms of the required rate and uniformity of the airflow.

Grain aeration is widely used in Australia, and the United States and is now practiced in Asean countries like Malaysia, Philippines and Indonesia. In Australia, about 30% of the permanent storage capacity have aeration facilities. Aeration has been applied to vertical silos (concrete and steel) and horizontal sheds of various shapes and sizes. Table 1 summarizes date on refrigerated aeration trials at Dalby Oueensland in 1967. This type of grain aeration was used in areas of Australia where the climate is too warm for the use of ambient air to be effective. Results of three trials with wheat carried out over two seasons showed that insect free wheat could be safely stored for 10 months and that insect population could be held at relatively low levels.

The airflow rates used in grain aeration systems must be adequate to cool all the grains before undesirable changes takes place. The airflow rate required depends on the purpose of aeration, the kind of grain, the size and type of stage structure and climatic conditions.

The recommended air flow rates in USA are measured in cubic feet per minute per bushel (c.f. m/bu), where 1 cfm/bu is equal to 0.0344 m/sec/m (Chung, et. al., 1985)

Sorenson and Crane (1960) recommended an airflow rate 12.9 x 10-3 m/sec/m for rouch rice with initial moisture content of 25% wet basis. With this airflow rate, the rice bed usually should be no deeper than 2.44 to 3.01 m (8-10 ft) for economical use of power.

In California where the climate in drier and cooler than in the Texas rice-growing area, Hederson (1958) recommended airflow rate of 5.7 x 10-3/sec/m and 9.2 x 10-3m/sec/m3, for rough rice with moisture content of 20-25%.

In Malaysia, studies carried out by Mohd/Jantan et. al., (1983), to determine optimum level of aeration for avoiding grain deterioration revealed that aerated storage at airflow rate of more than 0.1 m/t/m can reduce yellowing in paddy bulk stored up to 4 months in concrete tower silos to about 2% (weight rates of yellow kernels to that of total bulk). There was also a slight drying effect.

In the Philippines paddy held in sacks and aerated 8 hours a day with an airflow rate of 28 m/min/t showed no decline in grade up to 9 days, while paddy in bulk aerated with an airflow rate of 35 m/min/t lasted for 14 days without losing its initial grade quality. Wet moisture of 23-25% maintained milled rice quality within acceptable limits for about one week by aerating at an airflow rate of 6.0-35.0 m/min/t for 8 hours a day.

4. Turning of Grain - is the process of moving grain by transferring the contents of a silo to a nearby empty one. The result is a break-up of pockets of insect infestation, a slight loss of heat to the atmosphere, an averaging out of temperature and hence a reduction in maximum temperature and the mixing of any moist grain with the rest of the bulk. However, moisture content of the bulk of the grain is not lowered appreciably by turning.

With limited handling facilities, grain turning is a tedious and time-consuming process. A complete turning of 750 t of grain in one silo may take 40 hours and often interferes with other drying and transfer operations of the plant (Loo, 1 985)

Caution must be exercised in turning grain which is already infested. It will diperse the existing infestation and subject more grain to attack. Surface-infesting insects should be destroyed and the damaged grain removed before the grains are turned.

5. Atmosphere Control - Insects are essentially obligate aerobes, hence, the atmospheric gas composition has a dramatic effect on insect populations in grain bulks. Under hermetic conditions, oxygen depletion will eventually wipe out insect populations. The application of lethal concentrations of CO2 is an established, commercial control tactic in many countries. The level of gastightness achieved by sealing is of paramount importance for successful application of the technique.

Cleaning Operations - Most modern storages have elevators with equipment designed to clean or condition grain before it is stored. Most of this equipment employ a shifting and/or an aspiration process. These two processes are capable of removing most of the free-living insects but have no appreciable effect on internal infestation. Proper disposal of the residues from cleaning restricts cross - infestation.

6. Radiation - The use of radiant energy for insect control offers many promises. Research in this subject is very active and it is anticipated that the practical use of radiant energy for insect control will increase. Several types of radiation are useful for stored-product insect control.

Many insects are attracted most strongly to "black light" or ultraviolet light. Advantage is taken of this attraction by using such light in insect traps to indicate population levels or degree of infestation.

Infrared radiation produces high temperatures in relatively short exposures. This radiation can be produced electrically (heat lamp), but gas-fired ceramic panel heaters produce it more economically on a large scale. Temperatures in the order of 160 F in irradiation periods of 10 to 20 seconds are said to be effective in drying grain and controlling insects.

Ionizing radiation (atomic energy) can be used to "cold sterilize" food products or grain. X-rays, accelerated electrons, and gamma rays are deadly to insects. These forms of radiation have been seriously considered for large-scale processing applications.

Table 1 Summary of results of three grain refrigerated aeration trials at Dalby, Queensland.

Trial No. Season 1
1967 - 68
2
1967 - 68
3
1968 - 69
Average airflow rate      
(L/sect/t of silo capacity) 0.45 0.15 0.6
Quantity of wheat (t) 594 594 570
Storage periods (months) 10 10 10
Mean grain temperature (C)      
initial 26.4 33.0 28.3
final 10.4 12.1 15.5
Time to cool to 15C (months) 0.6 2.1 15.5
Mean grain moisture content      
(% wet basis)      
initial 10.8 10.7 11.1
final 12.5 10.9 10.9
Total operating time (hours) 5600 5600 1900
Energy used KWh/t of silo capacity) 34 11 18
Number of live insects      
inloading 0 0 0
outloading 0 35 3
Quantity of grain sieved (kg)      
inloading 76 72 82
outloading 214 94 91

 

REFERENCES:

1. Elder, W.b., t.F. Ghally and G.R. Thorpe. 1984. Grain refrigeration trials in Australia. In Ripp. B.E. ed. Controlled atmosphere and fumigation in grain strages. Developments in agricultural engineering 5. Amsterdam, Elsevier. 623-643.

2. Chung, Do B. Kanuyoso, L. Erickson, and C:H. Lee. 1985. Grain aeration and in-store drying in the USA. In: Champ, B.R. and E. Highly ed. Preserving grain quality by aeration and in stone drying. ACIAR Proceedings No. 15. pp. 224-237.

3. Lim, G.S. 1975. Insects in rice packaging. Development and pattern of infestation. MARDI Report 33: 1-11.

4. Lim, G.S. and S.P. Tee 1978. Insects and rice packaging. Effect of trying for varying periods and with different quantities. Malaysian Agric. J. (in press).

5. Loo, K.F. 1985. Silo Storage in Malaysia. In: Champ, B.R. and E. Highly ed. Preserving grain quality by aeration and moisture drying. ACIAR Proceed. No.15. International Seminar, Kuala Lumpur, Malaysia 9-11 October 1985.

6. Mohd, Nour. D. 1985. Aerated bulk storage of dry padi in LPN concrete silos. ASEAN Food Journal 1:39-40.

7. Mohd, Nour/Jantan, l.C. D. Tech , l. H. Rukunudin, R. Abdullah and S. Said. 1983. Aerated bulk storage of paddy in vertical concrete silos. In: Teter, N.C. Semple, R.l. and Frio, A.S., ed. Maintaining good grain quality. Manila, ASEAN Crop Post-Harvest Programme. 254265.

8. Sutherland, l.W. 1985. Preserving Grain Quality by Aeration and In-Store Drying. In Champ, B.R. and E. Highly ed. Proceedings of an international seminar held at Kuala Lumpur, Malaysia, 9-11, Oct. 1985.

9. Thorpe, G.K.1985. Using heat to disinfect stored grain. Aciar Grain Storage Newsletter, Asean Food Handling Bureau. July Issue, p.8.

10. Whitney, K.W. and J.R. Pederson, 1974. Manual of Grain and Cereal Product Insects and Their Control, pp. 213-218.


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