Each insect species has its own optimum temperature and moisture range conditions for development as well as a preferred feeding habit.
The optimum temperatures for most insect species range between 25 and 32°C. At temperatures below 14°C and above 42°C, the rate of development is reduced and most storage pests will die at temperatures below 5°C and above 45°C.
The optimum relative humidity for most species is around 70%, with the minimum 25 - 40% and the maximum 80 - 100%. Very few species are able to survive in extremely dry conditions.
Under good conditions, the duration of the development cycle from egg to adult is 18 - 25 days for beetles and 28 - 35 days for moths. In unfavorable conditions, this period may be extended to several months.
Insect activity and fertility are also affected by the change in light. Moths are most active at dawn and at dusk. Inspections to check and control flying insects are best made at these times. Artificial light can help to reduce the movement and fertility of moths. Most storage pests are able to penetrate a stack of bags more quickly and thoroughly than bulk grain because of the gaps between the bags. The size, the surface texture and nutrients in the grain also influence the ability of the pest to attack the grain.
While many different species of insects are found in rice only a few are major pests. Insects in stored rice can be classified as either primary or secondary insects.
Rice Weevil (Sitophilus oryzae (Linnaeus)) Adults and larvae feed on a wide variety of grains and a female can deposit up to 150 eggs. A single egg is laid in each grain after boring a hole inside. The egg stays in the grain until it becomes an adult, and this completely damages the grain. The life cycle takes approximately 35 days under good conditions such as 28 °C and 70% relative humidity.
Angoumois Grain Moth (Sitatroga cerealella (Olivier)) Eggs are laid on or the near grain. The white larvae bore into the kernels of the grain and feed on the inside. When the larvae mature they eat their way to the outer portion of the grain, leaving only a thin layer of the outer seed coat intact. Pupation takes place just under the seed coat. When the adult emerges from the grain, it pushes aside the thin layer of seed coat leaving a small trap door covering its exit point from the kernel. They infest only the surface layer of bulk-stored grain, as adults are unable to penetrate deeply.
Lesser Grain Borer (Rhyzopertha dominica (Fabricus)) The eggs are laid in the grain mass and the larvae may enter the kernels and develop within or they may feed externally in the flour-like dust that accumulates from the feeding of the adults and other larvae. The optimum temperature for reproduction is 34°C and the optimal relative humidity is 60 -70% Females lay 300 – 500 eggs and the life cycle lasts 20 - 84 days.
|Secondary Insects These are insects that feed from the outside of the grain even though they may chew through the outer coat and devour the inside. Two of the more prevalent secondary insects are the Saw-toothed Grain Beetle and the Rust-red Flour Beetle.|
|Saw-toothed Grain Beetle (Oryzaephilus surinamensis (Linnaeus)).
Eggs are usually laid, either singly or in small masses in a crevice in the grain but in products such as flour they are laid freely.
|Rust-red Flour Beetle (Tribolium castaneum)
The red flour beetles primarily attacks milled grain products. Both adults and larvae feed only on the grain dust and broken kernels and do not attack the undamaged whole kernels. Infestation leads to persistent and disagreeable odors of the rice. The optimum temperature for reproduction is 35°C and a relative humidity of 75%. The red flour beetle can lay up to 500 eggs and has a life cycle of 20 days under optimum conditions
The management of stored grain pests should be done in a sequential and integrated manner.
An effective pest control system involves:
A. Harvesting, drying and storage
Grain must be dried to at least 14% moisture (wet basis) and seed grain should be dried to 12% moisture before storage. Grain needs to be harvested and dried so that it will not cause cracking of the grain, as cracked grains are easier for insects to infest.
Harvesting and threshing at the correct stage of maturity (20-25% moisture content)
Drying the grain at a rate and temperature that will not damage the grain. The first stage of drying from 25% moisture to 18% can be done at high temperatures e.g. above 50-60oC. After this the grain needs tempering or cooling for at least 4 hours. Drying from 18% to 14% moisture should be much slower and the temperature should not exceed 42oC.
When sun drying, the grain should be spread in thin layers, 2-5cm, and turned every 1-2 hours. When sun-drying seed, the grain should be turned more often and not exposed to temperatures above 42oC. If high temperatures occur the seed should be dried in the shade.
New grain should not be stored near older grain unless all insects have been eliminated from the older grain. It is preferable to store grain as paddy or rough rice as this is less susceptible to insect attack than milled rice. Parboiled rice is also less susceptible to damage than raw rice.
Grain stores must have a damp proof floor and have waterproof walls and roof. It is preferable to be able to seal the storage so fumigation is possible should the need arise. Sealing the storage also helps exclude rodents and birds. Where grain is to be stored in bags, the bags should be stacked on pallets at least 50cm away from the walls. Hermetic storage systems have proved to be an effective means of storing grain. By having a sealed atmosphere the insects utilize the O2, expire CO2 and eventually die through suffocation and dehydration. This will occur within 5-10 days depending on the level of insect infestation. Other benefits of a hermetic system are that the moisture content of the grain and storage environment remains constant, and the sealed system reduces the chance of damage by rodents and bird.
B. Disinfesting the storage system
Disinfestations require a systematic and thorough cleaning of all sources of infestation before storage. Old grain residues in the storehouses, grain bins, harvesting and threshing equipment should be treated, removed or destroyed.
Storage containers, structures and equipment can be treated with:
If thorough cleaning of containers is not possible, the containers may need to be sealed and fumigated with phosphine. All second hand bags should be examined and where necessary treated with either a fumigant, insecticide or dipped in boiling water. Solutions of Malathion (50EC) and Fenitrothion (50EC) at 5ml/20l of water and Deltamethrin (2.5% WP) at 1.5g/l water @20ml/m2 can be used for dipping the bags.
C. Controlling infestations within the grain
Consumers are increasingly demanding grain that is free from live insects and free from chemical residues caused by controlling pests. While many chemical sprays are registered for rice, some markets will not accept grain treated with these registered chemicals. Farmers should always check with buyers to ensure that the pest control methods intended for use will be acceptable.
The first step in controlling any infestation is to determine the level of infestation and then select an appropriate method for control. All storage should be checked, preferably every fortnight, and at least monthly. Random samples need to taken from all grain and tested for infestation. If there are more than 4 insects per kg some form of treatment is required. A simple rule of thumb for the number of bags to be sampled is to use the square root of the lot size. For example if there are 100 bags in the lot, samples should be taken from 10 bags.
It is not recommended to use synthetic insecticides with rice that is for consumption. Only chemicals registered for direct application to rice grain should be used and these should be applied according to the label.
Malathion is a widely used chemical and is toxic to insects if it comes into direct contact with the pest. Malathion is considered one of the safest organophosphate insecticides as it is not highly toxic to humans or pets, and breaks down fast under tropical conditions. Malathion will not penetrate piles of grain. Although it is not usually recommended, it is still legal to treat grain with Malathion at 8 parts per million concentrations. As a grain treatment Malathion is applied at the time grain is stored. Treated grain should not be sold for at least 7 days nor eaten within 60 days of treatment. Safety precautions must be observed when applying Malathion or any other chemical.
Fumigants are effective against storage pests because as gases they can reach the pests in the most remote hiding place. The range of safe fumigant chemicals that can be used is now restricted to phosphine and carbon dioxide.
Phosphine fumigation is undertaken using tablets and pellets. These tablets and pellets release phosphine gas when they come into contact with humid air. Phosphine is toxic to all insects. When insects are exposed to fumigation in a sealed environment all stages of development from the eggs, larvae, pupae to adults are killed.
Phosphine does not impair the grain nor leave residues that could be hazardous to the consumer when correctly applied and the grain aerated. Care must be taken when using phosphine as a gas as it is very toxic to humans. Fumigation must take place in an enclosure that can be tightly sealed. Once the exposure time is ended, the grain must be aerated and the bin checked for residual phosphine gas before entry.
|Temperature(in o Celsius)||Tablets (days)||Pellets (days)|
|Under 5||No fumigation||No fumigation|
Carbon dioxide fumigation
Insects need oxygen for respiration. With carbon dioxide fumigation, much of the oxygen in the storage bin is replaced by carbon dioxide that suffocates, dehydrates and also produces toxic chemicals in the blood of the insects. To be effective, elevated carbon dioxide levels must be maintained until all insects die. The required exposure time depends on the percentage of carbon dioxide and the temperature of the grain. The cost of CO2 fumigation is high.
|Grain temperature(in o Celsius)||Minimum CO2 Levels (%)||Days for control|
|25-30||20||Weeks to months|
The ideal temperature for stored product insect growth is 25-30oC. The lower the grain temperature the slower the insect populations increase. Aerating the grain immediately after harvest so the grain is cooled will significantly reduce insect infestation. At 15oC the insects stop laying eggs and development stops. At lower temperatures insects will die.
High temperatures will also kill all stages of insects (eggs, pupae, larvae and adult) if exposed for a sufficient period of time. The most realistic use of high temperature is at drying or in some instances when the grain is being removed from storage for sale. Generally insects need to be exposed to temperatures of 50-55oC for at least 15 minutes.
Certain insects are vulnerable to the physical impact caused by moving grain. Pneumatic conveyers subject the grain kernels to large forces and operate at high pressure. As insects go through the pneumatic conveyer they are killed. Like high temperature this method may be useful when the grain is being moved for sale.
Control of some insects (e.g. Rusty Grain Beetle) can be achieved by using a non-toxic dust made from prehistoric diatoms. When the insect comes in contact with this dust, the waxy covering on the exoskeleton is absorbed, leaving them prone to dehydration and death. The product is applied as the grain is loaded into the bin and is most effective when applied to dry grain at harvest. Control can take up to 5-6 weeks.