Signs of Pest Infestation

The effect that insects have on the environment can be used to determine the presence of insects deep in a large volume of grain in an early stage of infestation, without large scale sampling. The most important effect in this regard is the heating of grain by the insects and the symptoms that accompany it.

Grain that is wet, in other words that contains more than 15% moisture, will start to warm up and can reach a temperature as high as 64°C. This heating is caused by fungal spores that are always present in grain, which germinate and lead to fungal growth. This is called wet grain heating. Grain that is dry, for example contains less than 15% moisture, will, however, also heat up if it contains insect infestation, but the maximum temperature that could be caused by insects in grain is 42°C. This is called dry grain heating.

Usually the process starts with an infestation by an insect type that is able to continue its development at the initial grain temperature. As the temperature rises, this pest type develops more rapidly, but at the same time the temperature is becoming more suitable for other types that can only develop in warmer conditions. Eventually it becomes too hot for the insect type that started the whole process and they start to move to cooler extremities of the hot spot. In this way the hot spot is enlarged so the process continues with one insect type after the other, until the hot spot becomes too warm for even the insect that prefers the highest temperature conditions. The whole process is therefore characterized by a succession of one insect type after the other.

The fact that there is an area in the grain that is warmer than the rest of the grain, has further consequences:

• Because the inter-granular air in the hot spot is warmer than the inter-granular air in the rest of the grain, it can absorb more water vapour from the grain as well as water that is excreted by the insects.
• This warm, moist air starts to rise as it moves through the cooler grain to the top and then starts cooling off. When it reaches the upper grain surface, the humidity of this air is very high as a result of the cooling off. The grain in this area now absorbs the water from the air again, because the water content always maintains a balance with the water content of the surrounding air. The water content of the grain on the surface therefore rises and it could easily exceed the limit of 15% that is needed for the germination of fungal spores.

The moment this happens, the process of wet grain heating commences, which may result in a temperature as high as 62°C.