Canadian Grain Commission
Symbol of the Government of Canada

Protection of farm-stored grains, oilseeds and pulses from insects, mites and moulds

5. Detecting problems

Inspect grain and oilseed stocks regularly to detect the first signs of infestation or spoilage. Sample bulks every 2 weeks to check for insects and heating. To detect insects, warm the sampled grain or oilseed in a screened funnel for several hours (Fig. 4). Insects and mites move away from the gradually drying grain and heat and fall into a collecting bottle.

Apparatus for extracting insects and mites from grains and oilseeds

Fig. 4 Apparatus for extracting insects and mites from grains and oilseeds: A, light bulb; B, metal funnel; C, metal screen soldered to funnel wall; D, glass jar; E, 200 g of grain; and F, 50 ml of 70% alcohol, or water.

Another way to check for insects in cereals is to screen surface samples using a No. 10 sieve (2.0-mm aperture). For the smaller canola seeds use a No. 20 sieve (0.85-mm aperture). Use a sampling probe to obtain deep samples. Warm the siftings for a few minutes and then examine them for insect movement. Check grains and oilseeds for heating by feeling the bulk's surface or a metal rod after it has been inserted for 1 h within the bulk.

To check for mites, sift grain or oilseed samples through a No. 20 or 30 mesh sieve (0.595-mm aperture). Warm the dust and screening to room temperature and examine them through a magnifying glass. Large numbers of mites in siftings look like clumps of moving dust. Smaller numbers that look like specks of dust are hard to see.

Insect-detection devices used to trap insects consist of probes - plastic tubes perforated with small holes that exclude grain kernels but allow insects to drop into, but not to escape from the trap (Fig. 5). Traps are generally not used in oilseeds where insects are usually not a problem. When cleaning samples in a dockage tester, free-living insects may be detected in the aspirator pan.

As a monitoring device, traps can detect infestations early so that producers or elevator managers can act before the grain has deteriorated to the point at which serious losses occur. Traps can be used in granaries, elevators, rail cars and ships to monitor grain at all stages of storage and transport. Push traps into the centre of a grain mass where insects generally accumulate because of warmth and higher moisture. Leave them for about 1 week (adult insects of some cold-hardy species continue to be captured down to 10°C) and remove by pulling on an attached rope. Take care to identify captured insects because grain-feeding pests require chemical control measures, whereas fungus-feeding insects indicate the grain is going out of condition and, therefore, should be moved into another bin or dried.

Table 1 – Percentage moisture content of tough or damp cereal grains, oilseeds, and pulses
Crop Moisture content (wet weight)
Tough (%) Damp (%)
Source: Canadian Grain Commission, 2001.
Wheat 14.6-17.0 >17.0
Amber durum 14.6-17.0 >17.0
Buckwheat 16.1-18.0 >18.0
Oats 13.6-17.0 >17.0
Barley 14.9-17.0 >17.0
Flaxseed 10.1-13.5 >13.5
Canola (Rapeseed) 10.1-12.5 >12.5
Mustard seed 9.6-12.5 >12.5
Rye 14.1-17.0 >17.0
Peas 16.1-18.0 >18.0
Corn 15.6-17.5 17.6-21.0
Soybeans 14.1-16.0 16.1-18.0
Sunflower 9.6-13.5 13.6-17.0
Plastic trap for detecting stored-product insects.

Fig. 5 Plastic trap for detecting stored-product insects.

Spoilage and infestations

Development of storage fungi and insects is favoured by moisture and moderate-to-high temperatures. Continuing growth of these organisms in grains and oilseeds results in spoilage, heating and insect infestations.

Even when grains and oilseeds have been stored dry, moist spots may develop by moisture migration within the bulk or by rain or snow getting in through roof vents and other openings.

Grain along the inner perimeter and roof of the bin cools as the outside-air temperature decreases during fall and winter. Grain in the centre of large, unaerated bins (6-m diameter and larger) remains near the harvest temperature into mid-to-late winter. This temperature difference within the bulk causes air to move up through the warm bulk at the centre. As warm, moist air moves upward, cooler grain at the top of the bulk absorbs moisture from the air. Moisture in this rising air may also condense or freeze on the underside of cold roofs (Fig. 1).

Each crop has its own particular storage characteristics and safe storage depends largely on its moisture content, its temperature and the length and time of storage. By knowing the moisture content and temperature of a crop at binning it is possible to predict its future storability. In Fig. 6, moisture-temperature combinations are shown for storage of canola, which result in either spoilage or no spoilage over 5 months. To move a crop from spoilage in the upper part of the chart to no spoilage below, either dry the canola or cool it by aeration.

To store wheat safely for up to 6 months, temperature and moisture content combinations may be used for prediction (Fig. 7).

Canola storage time chart based on seed moisture and temperature at binning.

Fig. 6 Canola storage time chart based on seed moisture and temperature at binning. At moisture contents above 13% even cool grain deteriorates rapidly.

Wheat storage time chart showing zones in which spoilage occurs in less than 10 days, within 10-30 days, within 1-3 months, and no spoilage for at least 6 months.

Fig. 7 Wheat storage time chart showing zones in which spoilage occurs in less than 10 days, within 10-30 days, within 1-3 months, and no spoilage for at least 6 months.

Bin monitoring

Need for monitoring

Stored grains are living organisms. The economic value of grains in storage can drop rapidly when they are allowed to deteriorate. Successful in-bin drying requires daily monitoring of bulks and an understanding of the drying and spoilage processes. Thus, to maintain the value of grains during storage, measure their moisture and temperature conditions regularly, so that remedial action can be taken if it appears that they will spoil before drying takes place.

Monitoring of bulks can be done by entering the bins, observing the condition and odour of the surface grain, and, with a probe, removing samples from deep in the bulk. This activity can be time-consuming, difficult and dangerous and is therefore frequently neglected. Active spoilage is indicated by changes in temperatures, concentrations of carbon dioxide or both. Off-odour (musty), visible mould (green, blue, yellow, white) or clumped grain may be present.

Bulk temperature

The most common and readily available method of monitoring for spoilage is to measure temperatures throughout the bulk with permanently or temporarily installed electrical sensors (Fig. 8). One such system consists of a cable and a hand-held, battery-operated monitor. The cable hangs down the centre of the bin with temperature points every 1.2 m and hangs down the outside of the bin with the connector at eye level. The top temperature point should be about 0.3 to 0.6 m below the top grain surface (after the grain settles). The monitor is plugged into the connector and the bulk temperatures are read off and recorded.

Bin temperature monitoring system of four sensing cables A-D suspended from the roof

Fig. 8 Bin temperature monitoring system of four sensing cables A-D suspended from the roof. Cables A, B and D are located halfway between the wall and bin centre and C is located close to the centre. Note: Cables longer than 8-12 m require support brackets to prevent the roof being pulled down by the cables (McKenzie et al. 1980). The cables should be attached to the floor, otherwise grain will push them sideways providing false readings.

Measuring temperatures regularly throughout the bulk during aeration can locate the cooling front. Turn the fan off when the bulk temperature is cooled to the outside temperature and turn it on again when the outside temperature drops about 5°C below the bulk temperature.

When grain spoils from the growth of moulds or insects, oxygen is consumed while heat, carbon dioxide and water are produced. The heat can cause the temperature of spoiling grain to rise. Thus, in an unventilated bulk, temperature measurements may be useful in detecting deterioration. But difficulties may arise in using and interpreting temperature results.

  • Temperatures of large grain or oilseed bulks change slowly. For example, at the centre of a 6-m diameter bin the temperature can be highest in winter and lowest in summer.
  • When a small pocket of grain spoils, the temperature at the centre of the pocket may reach 65°C whereas only 50 cm away the grain may be 10°C. To detect small pockets, temperatures must be measured at many points or at least where spoilage is most probable.
  • Low bulk temperatures do not necessarily indicate safe storage conditions. At -5°C, some moulds can begin to grow slowly; above 10°C both moulds and mites can flourish. Most insects, however, require bulk temperatures above 20°C to reproduce rapidly.
  • Bulk temperatures above outside air temperatures do not necessarily indicate the occurrence of spoilage. Straight-grade crops can be harvested and placed into storage in excellent condition at warm temperatures. But, when the crop is harvested and stored on a hot day, insects flying in from outside and those from the walls and floor debris in an unclean bin may start an infestation by multiplying rapidly. Such grain should be cooled by turning or by aeration to prevent insects from breeding. If the crops are stored dry in large, unventilated storage the temperatures near the centre can remain relatively high throughout the cold winter.

Carbon dioxide concentration

A second method of detecting active spoilage caused by either moulds or insects is to measure the concentration of carbon dioxide (CO2) in the intergranular air. The usual biological deterioration process occurring in stored grain and oilseeds consumes oxygen and produces carbon dioxide. The concentration of CO2 in outside air is about 0.03-0.04% (300-400 ppm). Concentrations above this level in a bin indicate that biological activity (moulds, insects, mites, or grain respiration) is causing the stored crop to deteriorate.

As CO2 usually spreads into the surrounding bulk, gas sampling points need not be right in the spoilage pockets. But it is preferable to sample at locations where spoilage usually occurs, such as at the centre of the bulk about 1-2 m below the top surface. Occasionally, spoilage will occur around doors because of leaking gaskets or ill-fitting covers, and on the floor by the bin wall due to bin sweating and condensation, or water leaks through roof vents. These localized areas can produce elevated CO2 levels that may be detected throughout the bin. It is advisable to have several additional sampling locations to determine if elevated levels are localized or throughout the bulk.

Carbon dioxide sampling equipment

Air samples can be withdrawn through small-diameter plastic tubes temporarily or permanently located within the bulk, using a hand pump, syringe, or electric pump. The concentration of the CO2 can be measured with an electronic detector.

A less expensive alternative is to use gas-analyzer tubes, which change colour according to the amount of CO2 passed through them (Fig. 9). The tubes can only be used once. Tubes cost approximately $5.00 each (in 2000) and can be obtained from most safety equipment outlets.

Device for detecting grain and oilseed spoilage by carbon dioxide measurements

Fig. 9 Device for detecting grain and oilseed spoilage by carbon dioxide measurements.