4.1 Relative status of major pest species
The estimated grain losses in storage due to pest infestation account for some 0.35 - 4.55 percent out of the total estimated grain losses in the post-harvest system of 2.63 - 31.3 percent. An insect population attains pest status through invasion, ecological changes and economic changes.
Invasion. The development of international trade has contributed greatly to the widespread habitat of insect pests. It is therefore important that the strict quarantine be part of the pest control system in a country.
Ecological change. The use of wide spectrum pesticides has reduced the population of natural enemies such as predators and parasites of major rice storage pests. The conditions therefore become more favourable to the rapid multiplication of major pests.
Socio-economic changes. The economic threshold is determined by market value of the grain, cost of control measures and consumer habits and taste. The rice damaged by a certain insignificant pest may lessen in value because of reduced tolerance of people for such condition of the commodity. The pest therefore becomes a significant or serious pest, although its population density did not change.
4.1 Relative status of major pest species
A pest is major if it is found in great number or in abundance. The rice weevil and the flour beetle are examples of such pest. A pest is minor if it is found in small number. A pest is primary if it attacks undamaged or sound grain, completes its development in it and initiates a chain of events in which other insects, fungi or bacteria feed on the damaged grain. The rice weevil is a primary pest. A pest is secondary if it attacks grain damaged or processed mechanically by grinding, milling, and handling or by the action of primary pests. The flour beetle, saw-toothed grain beetle and the rice moth are secondary pests.
4.1.1 Details of each major pest
(See Table 5.2.1, Annex 5.2).
Pesticide residues. (See Table 5.3.1, Annex 5.3).
Integrated pest management (IPM).
IPM is the use of all available tactics in a program to manage pest problems and minimise economic damage and environmental side effects (NAPHIRE, 1997). It involves the integration of biological, chemical and physical methods ones to control pests as well as other suitable and compatible to the system to keep the pest population at a level that will not cause any economic damage. The IPM program is built around the concepts of efficient warehouse design, high standard hygiene, minimum use of insecticides, use of methods that obtain as complete kill as practical and thorough inspection program.
The warehouse or storage structure at any level or size should protect the grain from water and moisture, keep the pests out or given no access for entry, facilitate loading and unloading, and be conveniently sited for handling and transport. A high standard of cleanliness and strict hygiene will prevent the build-up of damaging pest population by denying the pests of food and reduce the residual populations of insects and rodents in grain stores, surroundings and machinery. The program will also avoid seeding of clean grain with insects and infestation of adjacent stores as well as reveal possible structural weaknesses and paths ingress of water and soil moisture into the store.
The restriction of the use of pesticides to combinations with other control methods will enhance the oval effectiveness of the pest control program. Such restriction will help in reducing the potentially harmful residues, selection for resistance, environmental hazards and cost of treatment.
The complete kill is important in delaying population build-up and in reducing the rate of selection for resistance. Proper fumigation will control established infestations and in combination with barrier sprays of residual insecticides, will protect the grains against re-infestation.
Inspection and sampling activities are the means to obtain accurate qualitative information and data on the status of infestation of the grain. A pest and damage monitoring system is important in the overall efficiency of the pest control measures.
Elements of IPM
There are four basic elements of IPM, namely, natural control, economic levels, sampling and insect biology and ecology.
Naturally occurring growth suppressive factors may be utilised. The gas composition may be manipulated to hinder the growth and development of the pest
The economic threshold level (ETL) rather than the calendar-based application of pesticides will minimise the use of harmful chemicals and maximise the use of no-chemical methods of control.
The status of pest infestation, damage, level of losses and the population of beneficial insects and population trends of the harmful ones are determined by sampling which is a tool to apply ETL.
The knowledge of the biology and ecology is essential in utilising effectively the above three elements.
Components of IPM
The following components are utilised in the practice of IPM:
Biological control. The pest is controlled by a parasitoid, a predator or a pathogen to manipulate the reproductive processes, behaviour, feeding and other biotic aspects of the pest. A parasitoid resides and feeds on the host pest itself which eventually dies. An example is Anisopteromalus calandrae a small wasp which feeds on the beetle larvae of Sitophilus spp. and Rhyzopherta spp. A predator kills and feeds on one or more hosts and seldom resides or rides on them. A pathogen is a disease-causing organism, which is normally targeted specific and harmless to non-target organisms. It is easy and cheap to culture. Pheromones are chemical messages released by organisms to influence the behaviour (usually sexual) of the other organisms of the same species. Pheromones may be employed as attractant to prevent mating of the target species by misleading them. Host resistance to insect attack makes use of the natural built-in protection. For example, it is more advantageous to store rice as paddy than as milled rice because of the protective husk. Some varieties may have degree of tightness of the husk, which make them either susceptible or resistant to certain pest species. The use of sterile insects may be effective but it is resisted by those concerned. Neem (Azadirachta indica) leaves are used as insect repellent in the grain store in Indian households.
The constraints on the potential use of biological control of stored grain pests have been cited as follows:
(a) Predators, parasites and sterile insects found among the grains are themselves contaminants;
(b) Chemicals used to treat grains in store are toxic also to the beneficial insects;
(c) Predators and parasite attacks on pests are usually limited to the superficial layers of the large bulk of the stored grain;
(d) Pathogens do not thrive well in conditions of grain stored according to the recommended practices;
(e) Consumer rejection of the pathogen-contaminated rice.
Physical control. The physical method of controlling pests in storage includes the following:
(a) Hygiene and physical removal of infestation nuclei, including commodity residues, secondary or unproductive primary hosts for field pests. Cleaning should involve brushing and washing and disposal of all residues containing or supporting live insects. The rice mill machinery and premises are always a potential hosts of insects and should be cleaned regularly with special efforts made before any long gaps in operation to prevent the old and new resident pests from multiplying before the next milling season comes.
(b) Physical exclusion of the pests from the stored grain in the form of hermitic and controlled atmosphere storage requires gas tightness to be effective. Gas tightness through sealing is effective not only in keeping insects out but also in fumigation, single-treatment controlled atmosphere and heat disinfestation.
(c) Drying hinders the attack of most insects and fungi on the stored grain.
(d) Cold ambient temperatures during harvest and storage reduce the activities of insects. Refrigeration is the alternative in places like the tropics to take advantage of the effect of low temperature on insect pests.
(e) Aeration with cold ambient air as a means of effectively controlling pests in cold and temperate climates will not be as effective in semi-tropical and tropical conditions where temperatures are above 15 to 18oC required to prevent rapid increase of insect population. However, aeration of the grain bulk can be effective in preventing local hot or wet spots, which favour insect growth and development.
(f) Hermetic (airtight) storage confines the grain inside a sealed enclosure wherein the respiration of the grain and the associated insects and fungi will deplete the oxygen and replace it with lethal amounts of carbon dioxide generated by them. The process is simple, self-regulating, and requires no added pesticides, inert gas or energy inputs. Maintenance of the hermetic seal of the storage structure and moisture migration are problems involved in this method. Studies of the use of plastic enclosure for outdoor storage intended to be used by farmers' co-operatives and small-scale traders and millers were conducted by NAPHIRE (1997) in collaboration with the Commonwealth Scientific and International Research Organisation (CSIRO) of Australia and the Agricultural Research Organisation (ARO) of Israel. Results indicated that paddy at 14 percent moisture content could be stored for three months without being damaged. The locally available nylon fibre-reinforced polyvinyl chloride (PVC) plastic material of thickness 0.60 mm could be used for five years. The capacity of the heat-welded enclosure was 6 tons of paddy. The volcani cube (Israel) made of PVC food grade liner of 0.83 mm thickness and closed by polyurethane zipper, could be made for capacities of 5, 10 and 20 tons of paddy. The volcani cube could be re-used and last for 10 years provided that it is cleaned and properly stored after each use.
(g) Controlled atmosphere which although similar in form to hermetic storage, is different from it in that the carbon dioxide gas is supplied from the outside. Storage period could last from 9 to 16 months. No risks of toxic chemical residues are present.
(h) Inert dusts in the form of ground rock or wood ash have been used to control insects in subsistence level grain storage systems. They may be used at rates above 30 percent of the weight of the grain. Dusts made of silica aerogels, various clays, diatomaceous earth, activated carbon, pyrophylite and a number of other silicates kill insects by absorbing or abrading the waxy layer from their cuticle causing desiccation and death. Dusts do not eliminate the residue problem although they are not toxic. They are unpleasant to handle. Promising inert dusts for on-farm storage in developing countries are the low-cost and easily applied silica aerogels and the diatomaceous earth as they are not toxic.
(i) Physical shock and disturbance can kill insects due to physical stress and damage due to handling and processing of the grain.
(j) Artificial light regimes can affect the photoperiodic responses and mating of insects. However, the potential for response by insects to visible, ultraviolet and infrared radiation for control has yet to be realised.
Chemical control. Chemical control methods have the advantage of effectiveness, simplicity, versatility, low cost and immediate availability. However, synthetic insecticides must be regarded as adjunct to good warehouse management, to reinforce hygiene and sanitation, to enhance effectiveness of available storage facilities, and to complement physical methods. It is not intended to replace good warehouse keeping or regular inspection for infestation or deterioration. The main types of insecticide treatment are as follows:
(a) Structural treatment (residual spray application). The surfaces of warehouses, storage bins, transport vehicles and other structures and machinery are sprayed with chemical which will not only kill the insects directly hit by the spray but will also leave a deposit on the treated surface which will be toxic to walking insects. Spraying may be done during the cleaning of the storage facilities before intake of new stocks or along with fumigation or spraying of stock in storage. The residual deposit decays with time and its effectiveness depends on the chemical and the climatic conditions prevailing.
(b) Space treatment. Fogging or space spraying is intended to control flying insects not controlled by the residual spray and those coming into the storage warehouse. Spraying is done usually at dusk hen insects are most active. Chemicals with knockdown action such as pyrethrin, lindane, and dichlorvos aerosols and strips or smoke or fog are used in space spraying. Dichlorvos plastic strips hung inside the warehouse at a density of 1 strip /30 cubic meters of space will be effective for flying moths.
(c) Grain protectants. These insecticides will prevent infestation when applied on grains. It is intended for light infestation only at the time of treatment and is not a substitute for fumigation in case of heavy infestation. In general, it should be avoided as it may accelerate the selection of resistant strains. The choice of insecticides to use is quite limited, as safety should be a prime consideration. Studies by Sayaboc, et al., 1996, of resistance of major insect pests to pyrethroids and organophosphates, revealed high resistance of the lesser grain borer (Rhyzoperhta dominica) to phosphine, in contrast to the low resistance of flour beetle (Tribolium castaneum) and susceptibility of rice weevil (Sitophilus oryzae).
(d) Surface spraying. The insecticide is applied on the surfaces of bulk grain or bags of grain. Examples are pyrethrum synergized with piperonyl butoxide, pirimiphos-mythl, chlopyrifos-methyl, tetrachlorvinphos, fenithrothion and metacrifas at about 1 to 2 percent concentration.
(e) Fumigation. A fumigant is a volatile pesticide, which exerts toxic action in the gaseous or vapour phase. The most common fumigants used world-wide are methyl bromide and phosphine. Fumigation is effective because of thoroughness of application but fumigants used to control pests are generally toxic to humans and plants; may be corrosive, flammable, leave harmful residues and produce offensive odours. In application, it is best to monitor the atmosphere inside the warehouse with appropriate test equipment and the threshold limit value should conform with that recommended by the American Conference of Government Industrial Hygienists (1983-1984). The most common deficiency in fumigation is the neglect of hygiene and stock management resulting in the necessity of frequent fumigation and consequently, the hazards of excessive bromide residue accumulation in the grain. Phosphine complements the use of methyl bromide, especially in vertical storage, Sayaboc et al., (1996) found that the use of 2 g phosphine per ton of grain at seven-day exposure time will effectively control insects at all their life stages. Other fumigants, which are used occasionally, are hydrogen cyanide, carbon disulfidem ethylene dibromide, chloropicrin, methyl chloride and carbon tetrachloride. A tolerance value of 0.1 ppm expressed as PH3 , is recommended in international trade of cereals.
Two or more components of pest control can be combined in an integrated program. However, in a storage ecosystem, hygiene and good warehouse management are basic requirements and the IPM system is but supplementary. The combination of two or more of the following practices will constitute an effective pest control program.
(a) Improved harvesting and threshing techniques;
(b) Judicious use of insecticides;
(c) Use of ambient aeration and refrigerated aeration;
(d) Atmospheric gas modification;
(e) Thermal disinfestation;
(f) Irradiation techniques;
(g) Insect resistant packaging;
(h) Insect growth regulators;
(i) Biological control;
(j) Use of resistant varieties.