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Understanding the Undesirable Effects of Grain Beetle Pests and How to Control Its Infestation

Grain Beetle

Grain beetle undergo the four distinct stages and these are egg, larva, pupa and adult. Egg are either laid in dust or in crevices of kernels. The grain beetle lay their eggs inside the kernels. Larva stage is when the pest grows. Grain beetle pests result to huge destruction of stored products such as grains and cereals. At this stage, the pest consumes higher amount of food its weight and the larval skin do not stretch. However, it molts periodically making it to increase in size. It is possible to determine whether the grains were infested by the beetle as they leave cast-off skin. At pupa stage, the beetle does not feed and it is enclosed by the larva, cocoon or cell. The beetle undergoes extreme internal and external changes that result into its development to an adult. The adult is the last stage of the beetle and it is about 0.1 to 1.2 centimeters long. It has three pairs of legs and divided into three body parts. These are;

  • Thorax which has wings and legs
  • Head that have sense organs and mouthparts
  • Abdomen which contains the reproductive organs

General appearance of the grain beetles

Grain beetles are found in all parts of the world. These pests’ attacks food stuffs such as biscuit mixes, breakfast cereals, cereals, rice, dried fruits dried meats, flour, macaroni, sugar among many others. These pests are found primarily in areas where dry foodstuffs are stored. This not only makes the food undesirable for human consumption but also results to huge losses to farmers. For this reason, it is important to understand grain beetles as pests and how to control them.

Usually, grain beetles are unable to eat or break through hard-shelled cereals such as corn but they eat those with broken kernels and also processed cereal products.  There are no diseases that have been associated with the insects. However, market value of the produce is greatly affected due to the destruction caused by these pests.

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The adult grain beetles are brown in color and about 2.5mm long. They have a flat, narrow body that allows them to enter into crevices of the stored materials. The middle section and the thorax have saw-like projections. There are two types of grain beetles and these are saw toothed grain beetle which is flightless and merchant grain beetle which is capable of flight.

Different species grain beetle pest 

a)  Merchant grain beetles

This is a grain feeder pest that belongs to order Coleoptera and family Silvanidae. The Merchant grain beetle (Oryzaephilus Mercator) are strong fliers and less cold tolerant. Female merchant grain beetle lays about two-thirds of eggs as compared to the saw toothed grain beetles. These prefers food sources that have a higher oil content for instance nuts.

However, it still feeds on grain and cereal products.

The reproduction of the merchant grain beetles takes place in four different stages. This starts by the female laying eggs in small batches on the food material. It lays an average of 200 eggs and this is in a period of about 29 to 42 days. The eggs hatch after 8 days, the larvae mature in 37 days and pupa takes about 67 days. Depending on the temperature, the cycle may take about 35 days or up 51 days. Under warm climatic conditions, there are 6 or 7 generations per year. However, during winter months, there are fewer generations with adults remaining active and feeding.

b)  Saw toothed grain beetles

The Latin name of saw toothed grain beetle is Oryzaephilus surinamensis. This is major pest of stored foods at homes and also in commercial facilities. The pest is about 2.5 to 3 mm long and has a flat body. On each side of its body, it has six projections which are located just behind its head.

The saw toothed grain beetle does not fly and is also not attracted to light. Flight is a characteristics that is used to differentiate the saw toothed grain beetle from merchant grain beetle. Unlike the saw toothed grain beetle, merchant grain beetle flies. The Latin name Oryzaephilus surinamensis means ‘rice loving’. The small body of this beetle allows it to enter into tiny cracks of the grains and also the infested broken kernels. However, saw toothed grain beetle pest does not infest whole grains. The pest feeds on birdseeds, pet food and cereal. It also feeds on chocolate, spices, nuts and tobacco.

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*  Reproduction of saw toothed grain beetles

When it comes to reproduction of saw toothed grain beetles, the female beetle deposits its eggs inside cracks found on grains or on food that have been grounded for instance, floor. The female lays up to 300 eggs either singly or in batches. After approximately 8 days, eggs hatch to become larvae. The larvae is fully matured after about 37 days and this is after molting for about two to four times. The pupa stage of development lasts for about 4 days. At this stage, they heavily feeds and when it comes for them to change into adults, they form cocoon out of the food particles.

The climatic conditions affects the process of reproduction with the eggs that are hatched in warm conditions taking approximately two months to complete the entire life cycle. Usually, under normal conditions, there are six generations per year.  The signs that will make one know that there are saw toothed infestation when they see small crawling insects on the ground or at stored dry produce.

c)  Square necked grain beetle

The square necked grain beetle is known as Cathartus quadricollis. It is closely related to the saw toothed grain beetle in terms of its size, color and form. It is a flattened, polished, oblong and reddish-brown beetle of about 1/10 inch long. However, it differs from the saw toothed beetle in that it has an almost square thorax and lacks saw toothlike projections.

This species of grain beetle in found in the South America. It is usually attracted by moldy and damp grains and is rarely found in clean grains.

d)  Siamese grain beetle 

Siamese grain beetle (Lophocateres pusillus) is a reddish brown beetle that is flattened and about one-eighth long. It is also characterized by a flatted margin of the thorax and the wing covers. The Siamese grain beetle was first seen in Liberia, Siam and Ceylon.

e)  Foreign grain beetle

Ahasverus advena is a small and reddish brown grain beetle that is have a similar appearance to that of square necked grain beetle. However, it is shorter and stouter. It is also attracted to damp and moldy grains hence not rarely seen in clean grains.

f)  Mexican grain beetle

Pharaxonotha kirschi is a deep brown beetle that is highly polished. It resembles the confused flour beetle in general appearance but can be distinguished by its more polished surface. It also have a large antennae.

g) Rusty grain beetle

Cryptolestes ferrugineus grain beetle is usually similar to the flat grain beetle in terms of appearance and habit. However, it differs as the antennae of the male beetle is not half as long as its body. It is also more tolerant to cold weather as compared to many other species. It is commonly found in stored grains in Northern States.

h)  Flat Grain beetle 

Cryptolestes pusillus is the smallest grain beetles found in stored grains. It is a minute, oblong, flattened and reddish brown. It has an elongated antennae which is about two-thirds that of its body. It is one of the commonest grain beetle and it is cosmopolitan. The adult of this species is unable to survive in grains that are not broken.

Usually, it follows other pests that are able to destroy the grains. This pest is a scavenger and therefore infests those grains that are already destroyed and in poor condition.

When it comes to reproduction, the females deposit small white eggs in the crevices. It may also lay them on farinaceous materials. The larvae is more likely to be found in infested grains or where kernels are injured. The larvae also survives by feeding on the dead insects. Upon developing fully, the larvae forms cocoons by gelatinous substances.  The larvae transforms into pupa stage in the cocoons and later emerge as adults.

Undesirable effects of grain beetle pests

*  Post -harvest losses

These are losses that takes place through the process of crop processing, marketing, food preparation and storage. These may be in terms of caloric and nutrient composition. It is important to ensure that food is well stored throughout the entire food chain. 

Food insecurity can be solved by ensuring that there is less wastage of foods.Grain beetle pests contribute to post harvest losses by attacking food at storage warehouse or at homes.

The grain beetle makes food produce such as grains undesirable as creates cracks and crevices and leaves waste excretion on the entire batch. Usually, these pests feed on the germ which is soft and at times leaves a hollows on the grains. These makes the food not desirable to be consumed by both man and animals.

*  Reduced market value

This is another damage that is caused by grain beetle infestation. Usually, market value of the produce depends on its dry weight which greatly reduces once the grains are affected. In most cases, when a person realizes that the grains are being infested by the grain beetle pests, they embark on the process of separating those have already been affected to those that are yet to be affected. This will greatly impact on the weight of the end produce hence affecting its market value. This will affect the financial yield of the produce and may make the farmer face financial difficulties.

*  Makes the produce vulnerable for molds 

Grain beetle pests do not transmit any diseases but makes the produce susceptible to molds. Usually, grain beetle such as saw toothed beetles are able to penetrate into grain products that have been processed by tearing the cover. At home, this is one of the causes of spoilage of both packaged cereal and grain products. By creating small holes on the product, moisture penetrates in and it results to growth of molds. More also, by creating hollows on the stored grains, it makes its porous and absorbs moisture more. For this reason, it is important to ensure that the grain beetles are managed to avoid cases of mold spoilage.

*  Increased cost of handling grains

Usually, a warehouse that has a history of grain beetle infestation contributes to increased cost of post-harvest handling. This is because, there are higher chances that eggs are found in cracks on the floor and walls. Lack of a well-laid control measures and relying on destroying the beetles in short-term results to increased cost at long-run.  Therefore, it is important to implement a control measure that will eliminate the grain beetle pests for a long time. More also, the method chosen should be cost-effective to avoid incurring unnecessary costs.

Having a traceability system in place is of great important and cost-effective especially when the warehouse receives grains from more than one farmer. The grain sacks should have tags that makes it easier to be identified and traced to the owner if an issue with molding is reported. This is a sign that the grains were not dried well and there are certain species of grain beetles that are attracted by moldy and damp grains. It also ensures that the entire grain batch is not subjected to infestation by the grain beetles.

How to control infestation of the grain beetle pests

1  Microbial control

This is one of the effective method to control grain beetle pests in which microbial insecticides is used. The microbial insecticide is in form of toxins and spores is used. One of the strain that is used is the BT toxins which is produced by Bacillus thuringensis. The strain is used to control these pests in stored grains. Entomopathogens is also applied when it comes to controlling of the pests microbiologically. In order for the microbial control to be effective, certain botanicals are usually mixed with B. thuringensis. For effective control of saw toothed grain beetles, Beauvesia bassiana, an entomopathogenic is used.

2  Biological control

This control method involves use of different biological agents to suppress the population of the grain beetles. Usually, certain living organisms are used to reduce the population of these pests. They include pathogens, predators, parasitoids and other living organisms. The most common living organisms that is used is hymenoptera which is a parasitoids. The insect pathogens are also used and a few predators such as hemimpteran bug known as Xylocoris flavipes. However, most of these living organisms are able to suppress the population of grain beetles that are not capable of penetrating the grains and not those that are inside the grains. The common parasitoids that are used in stored grains include Braconidae and Gravenhorst. These are used to suppress E. cautella.

3  Cultural control

This involves practicing good store hygiene to minimize grain beetle pests. It is important to ensure that the grains store houses are cleaned to remove dirt, dead larvae and egg shells. Usually, the broken infested grains need to be removed and burnt in order to ensure that the eggs are destroyed. More also, there is need to ensure that prior to storing of new grains, the room is well cleaned and fumigated.

Grain beetle pests reproduce through laying of eggs either inside the broken grain kernels or on the ground. Therefore, as a control measure, it is important to ensure that all crevices and cracks on the floor, walls and ceiling are filled up with cement and repellant paint applied. Coal-tar has been is good to be used as paint.

Another cultural control method is superheating grain stores to a temperature of 150 °F by used burning charcoal. This heat treatment should be done while the doors to the stores are closed for about 48 hours after which it is then allowed to cool down and cleaned prior to storage of new grains. Sulphur has also been used to fumigate the stores. Dusting of the floors, walls and ceiling should be done by the used of 10% DDT or 5% dust. Mixing of the inert dusts with the stored grains also helps prevent infestation.

4  Behavioral control by use of Insect Pheromones

By applying either female specific or male specific pheromonal substances, it is possible to control grain beetle pests. They are used to detect any infestation in stored grains. These are used in minute amounts in traps which are then placed at considerable distance in the warehouse. Controlling these pests using this method depends on the efficacy of the traps in catching the attracted populations. However, a more effective suppression of the grain beetle pest population involves use of pheromones to disrupt mating.

5  Control of the grain beetle pests by use of insect growth regulators

Insect growth regulators cab be used to control the menace of grain beetle pests. These are used in closed environment and it is more successful when controlling these beetles. The insect growth regulators works by causing impairment of reproduction and disrupting of oviposition behavior in the pests. A good example is methoprene which shows inhibition of adult merchant grain beetle. To acheve more control, these insect growth regulators are added to impregnated baits as opposed to being added directly to the stored grains.

6  Physical control by heat, temperature and pressure

*  Temperature and heat treatment

This is one of the best physical control method of the grain beetle pest. It successfully kills several life stages of these pests at a go. Grain beetle pests cannot tolerate extreme temperatures for instance heating and chilling. Raising of the temperature of the grains to around 55-65°C for about 12 hours effectively destroys the grain beetle at any stage. More also, lowering temperature to below 12°c makes the pests to become inactive and they eventually die. Low temperatures affects the reproduction of the grain beetle pests contributing greatly to suppression of the population. Proper temperature management is therefore important to ensure that the pests are controlled.

*  Pressure 

Low pressure is an excellent pest management tool. This is a non-chemical method that is used to suppress the grain beetle population. Usually, a low pressure environment creates a low oxygen controlled atmosphere which kills the pests. Higher temperature combined with low pressure is effective when it comes to destroying of the grain beetles.

7  Ionizing Radiation

This is one of the environmental friendly method of grain beetle control. It involves treating of the stored grains with ƴ and β radiation. Usually, ƴ-radiation is generated by use Cobalt 60 while β-radiation is electrically generated. The low ionizing radiation works by resulting into production of highly reactive free radicals. Strong ionizing radiation damages the pests by sterilization. Irradiation should be carried out in an open chamber which kills all the grain beetle pests despite the stage they are in. Light and color are used to lure and trap the pests.

8  Inert dusts, silica aerogel and sands

These are unreactive and kills the grain beetle pests through physical contact. The usually, when the pests comes into contact with the inert dusts, they die due to dehydration and desiccation. In order to get the maximum benefit of these inert dusts, it is important to ensure that the relative humidity is kept low. Sand has been on use for long as a traditional insecticide. It provides a protective layer on the stored grains. Silica aerogel has sodium silicate, a non-hygroscopic powder that is used to control the grain beetle.

9  Chemical control

This involves use of fumigation method to control grain beetle pests. This is one of the most effective chemical method which involves exposing of the pests to poisonous gaseous environment. This is achieved by applying a grain fumigant. The generated fumes enters the body of the pests through the spiracles. Examples of the fumigants include carbon tetra chloride, carbon disulphide, methyl bromide, ethylene oxide, chloropicrin, sulphur dioxide and trichloroacetonitrate.