Pest infestation in stored grain

by Vaughn Entwistle, Managing Editor, Milling and Grain

The world population is projected to grow to between 9 to 10 billion people by the year 2050. This means that humanity faces a major dilemma as food production must increase by 70 percent from current levels to feed the increased population. Approximately one-third of the food produced (about 1.3 billion tonnes), worth about US $1 trillion, is lost globally during postharvest operations every year. Some of this is due to moulds and mycotoxins, but a high percentage of food is lost due to pest infestations.

An infestation is often hard to detect in the early stages, when there are still very low concentrations of insects. However, even a single insect in every kilogram of wheat will mean that a truck load of grain has the potential to introduce tens of thousands of pests into a silo, storage bin or grain warehouse.

For the farmer, pest infestations lead directly to revenue loss. A mill might reject an entire shipment of grain because of the presence of a single insect. In countries such as Australia the potential impact on exports could prove catastrophic, so the country has a "nil tolerance" policy.


Aeration and grain cooling

One favoured option to prevent/limit insect infestation is increased air circulation to cool the grain mass while in the silo—especially when the grain is first introduced. Newly stored grain is "alive" and continues to increase in temperature inside the silo due to respiration where the grain essentially "breathes," and "sweats", taking in oxygen while releasing carbon dioxide, heat, and water vapour (and therefore weight).

One common solution is the use of aeration fans, which forces cooling air through the grain bulk, up into the silo head space, and out through vents in the top of the silo. Aeration dries the grain and rapidly drops the temperature, preventing condensation from forming in the top of the silo, which can then drip down and moisten the top of the grain, providing ideal conditions for mould growth. Cooling the grain through aeration also reduces the chances of insects hatching.

Grain coolers take this concept one step further. Using the same technology as refrigerators, grain coolers blow chilled air up through the grain bulk. These units can typically cool grain faster and to a much lower temperature than aeration fans alone: 30°C down to the low 20s°C (55-50°F). What"s more, grain temperatures below 15°C (60°F) inhibit most insect activity, while temperatures above that allow insects to grow and breed. As insects cannot control their body temperature, they are inactive at low temperatures (below 8°C for insects and 3°C for mites). Moisture content of grain below 13 percent stops the growth of most moulds and mites. Moisture content below 10 percent limits the development of most stored grain insects and pests. In addition to actual moisture content of the grain, the volume of stored grain also affects the rate of cooling.

In all cases, it is important not to overfill a silo. The silo is correctly filled when the top of the grain is level with the sides of the silo walls. This leaves a so-called "head space", a large airspace above the grain which allows warm, humid air to rise out of the grain mass and then be vented out the top of silo.


Monitoring

It is important to monitor stored grain. Physical inspections are especially critical after initial storage, with continual inspections until the grain has cooled to 10 to 13°C (50-55°F). Many silos are now equipped with sensors that monitor temperature and humidity throughout the grain mass. Temperature probes hung inside the silo can register "hot spots" and alert the operator to take action to avoid loss of quality and commercial value, or even spoilage.

Stored grain should be carefully monitored for the following:

• Grain quality

• Grain temperature

• Insects and insect density

• Mould growth

• Bad odours

A localised increase in temperature or "hot spot" can often indicate the presence of moisture or insects in the grain mass. Grain probes pushed into the grain mass can take samples at various levels to verify the presence of pests in the grain. An alternative is the use of pitfall probe traps that remain in the grain. If checked weekly, these traps allow users to determine the numbers and species of pests in the grain. This then allows the selection of the most appropriate type of fumigant to employ.


The usual suspects

The following is a list of the most commonly found insect pests in stored grain. Note: this is just a partial list. Obviously, specific insects will vary by geographical region.


Primary pests

These insects are capable of penetrating and infesting intact grain kernels. They often have an immature stage that can readily develop inside a grain kernel.

Lesser grain borer (Rhyzopertha dominica)

Granary weevil (Sitophilus granaries)


Secondary pests

These insects cannot infest intact grain, but can feed on broken kernels, grain debris, and high moisture weed seeds, as well as grain already damaged by primary insect pests.

• Rusty grain beetle (Crptolestes ferrugineus)

• Red flour beetle (Triboleum castaneum)

• Confused flour beetle (Tribolium confusum)

• Saw-toothed grain beetle (Oryzaephilus surinamensis)

• Flat grain beetle (Cryptolestes spp.)

• Warehouse moth (Ephestia spp)

• Indian meal moth (Plodia interpunctella)

• Warehouse beetle (Trogoderma variable)

• (Other secondary pests include mites, booklice, and various moths.)


Fumigation

Grain fumigation can provide the answer at an early point in the insect"s life cycle, eliminating even very low initial levels of pest infestation. Fumigation can be used in grain stacks and silos, grain bins, and in shipping containers. The major advantage of fumigation with gases is that insects can be controlled without needing to move the grain. In port silos, both fumigation and insecticide spraying are applied. Intensity of treatment can be lower in the ports than in the silo because of higher turnover of grain mass in the bins.


Phosphine

One of the most common fumigants is phosphine gas, generated from aluminium phosphide or magnesium phosphide. It is available as pellets or tablets that are placed within the grain and which release phosphine gas when exposed to moisture in the air. The stored grain is first covered with a plastic sheet, and then gas is allowed to permeate through the commodity for around 14-28 days (with exact duration dependent on temperature).

The fumigant must be kept in contact with the insects for at least seven days to kill all stages of the insect"s life cycle that usually exist in stored grain. This means that silos and storage bins must be sealed gas-tight to maintain a sufficient concentration of phosphine gas. When all of these conditions are successfully achieved, not only are adult pests killed, but also their eggs. Because of this, grain fumigation is a very effective way to counteract weevils, moths, beetles, mites and any other stored food pests – including sawtooth grain beetle, foreign grain beetle, fungus beetle and flour beetle.

Phosphine can be used for the safe fumigation of foods, without leaving behind harmful residues. The Phosphine fumigant leaves minimal residues, which is acceptable to most markets. Another advantage is that phosphine can also successfully permeate packaging, such as cardboard, while leaving the packaging intact.

In many parts of the world, phosphine can only be sold and used by certified pesticide applicators.


How fumigants kill

As far as is known at present, fumigants enter the insect mainly by way of the respiratory system. The entrance to this system in larvae, pupae and adults is through the spiracles, which are respiratory openings found on the thorax and abdomen of insects. The spiracles are connected to the trachea - tubes within the insect"s body. Air enters the trachea via the spiracles and the oxygen then diffuses into the insect"s body. To enter insect eggs, gases diffuse through the shell (chorion) of the egg or through specialised "respiratory channels". It has been shown that some gases may diffuse through the integument of insects, but at present the comparative importance of this route for the entry of fumigants is not known.

An important variable in the poisoning of an insect by a fumigant is influenced by the rate of respiration of that insect; any factor that increases the rate of respiration tends to make the insect more susceptible. Conversely, low temperatures, such as those created by aeration, and especially grain coolers, slow insect respiration, so that the grain coolers may be required to be shut down and grain temperatures allowed to rise so the fumigants can take effect.


Adding carbon dioxide

Carbon dioxide, in certain concentrations, may stimulate the respiratory movements and opening of spiracles in insects. With different fumigant gases acting on different insects, there seems to be an optimum amount of carbon dioxide needed to provide the best insecticidal results. Excessive amounts of carbon dioxide tend to exclude oxygen from insects and thus interfere with the action of the fumigants.

When employing fumigants such as ethylene oxide and methyl formate, the addition of carbon dioxide may work to advantage both by reducing the fire or explosion hazards and by increasing the susceptibility of the insects. On the other hand, with fumigants that are non-flammable, the advantages of adding carbon dioxide may be offset by the extra cost and work required to handle the additional weight of containers.

There are a variety of fumigants currently being used around the world. Most rely upon silo, grain bin and warehouse structures being adequately sealed against air leaks, or through the use of plastic sheeting/tarpaulins to contain the gas sufficiently for a period of days while the fumigant does its job. This is very important to maintain the efficacy of the gas. It is vital that the level of phosphine remain greater than 200 ppm for as long as possible, with a recommended minimum of 100 hours to kill all life stages of the pest insects. When fumigations fail, it is often because of air leaks or insufficient time. This contributes to insect resistance to phosphine, which been detected in China, India, the Dominican Republic and Australia. It takes many years and great expense to develop and test new compounds. Therefore it is important that insecticide resistance is prevented from spreading.

With certain fumigants, such as ethylene oxide and methyl formate, the addition of carbon dioxide may work to advantage both by reducing the fire or explosion hazards and by increasing the susceptibility of the insects. On the other hand, with fumigants that are non-flammable, the advantages of adding carbon dioxide may be offset by the extra cost and work required to handle the additional weight of containers.


Dangers of fumigants

Many pesticides and fumigants can cause mild or severe poisoning. Long-term exposure to them can cause cancer. Workers most commonly absorb pesticides and fumigants through their skin. The chemicals can also be breathed in or swallowed.

Only qualified professionals should mix, load, or apply pesticides. Equipment should be inspected before each use, checking for leaks. Containers or buildings should never be entered if fumigants might be present unless a hazard assessment is conducted. If fumigants are found, workers should leave the area immediately and notify a supervisor. In many parts of the world, phosphine can only be sold and used by certified pesticide applicators.

There are other issues with the use of these gases. Methyl bromide is known to be an ozone-depleting gas, and thus may be the target of increasing regulation. ProFume

(sulfuryl fluoride) is known to be a potent greenhouse gas. Lately, countries such as Australia have reported a rise in phosphine-resistant pests. As a result, new formulations of gas, such as a mixture of phosphine and carbon dioxide, are being experimented with.


Alternative fumigants—controlled atmospheres

The use of phosphide is proven and well regulated. However, due to regulatory changes, and the need for organic-labelled grains to remain chemical free, the use of controlled or modified atmospheres (CAs) have recently come into vogue. This refers to the creation of a localised atmosphere within the silo or grain bin by the introduction of various gases such as carbon dioxide, nitrogen, or ozone.

Although not currently in widespread use, Nitrogen has several advantages over other gases: nitrogen constitutes 78 percent of "air", thus air is a rich, free source of nitrogen; nitrogen is not toxic; the use of nitrogen would greatly reduce occupational health and safety and environmental risks; nitrogen provides a residue-free grain; nitrogen has no known resistance problems; nitrogen does not react with construction materials; and finally, there is no need for ventilation before grain can be marketed.

The following articles in this special report have been provided by companies/academics that are highly expert in the application of their chosen gas and should hopefully provide Milling & Grain readers with an informed discussion on the advantages and drawbacks of each gas.

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