Factors that Determine the Dispersion of Insects in Grain

Size of the Insect and Body Characteristics

Moth types such as the tropical warehouse moth and the Indian flour moth are much bigger than most types of beetles that are pests in stored grain. Their bodies are also softer than that of weevils, even in the larval stage. For the moth types it is thus more difficult to creep through the grain kernels deep into a volume of grain. This is one of the reasons why moth types infest grain near the surface of the grain volume, while most of the beetle types penetrate deep into the grain volume.

The Life Span of the Insect Type and its Mature Stage

The moth types and very few beetle types live only about a week in their mature stage – just long enough for the female to find a partner, mate and lay eggs. Infestation by these types of insects is therefore limited to the grain near the surface of the grain volume. Most of the beetle types, on the other hand, live quite a few months in the mature stage, mating takes place in the grain and the females lay eggs at a relatively slow pace of around ten per day. These beetle types have a lots of time to penetrate the grain volume deeply and to infest deeper layers.

The Place Where the Larvae Develop

Insect types of which the larva develop inside the grain kernel, can infest the grain on the land already and be taken in with the grain at the storage site. These insects are inside the grain kernel and well protected against fatal injuries during harvesting and handling. Insects of which the larval stage do not develop inside a grain kernel, can also infest the grain on the land already, but handling of the grain during the harvesting and the intake process kills most of them, especially if they are still in the larval stage. Consequently, these insects are less often taken in with the grain from the land into the storage site. These insects penetrate the grain in storage later and the earliest infestation takes place somewhere on the upper surface of the grain volume. Insects that develop inside the kernels and enter the silo with the grain from the land, can occur anywhere in the grain volume and the infestation will then spread further. These insects can also infest the grain from outside at a later stage and then the earliest infestation will also take place on the upper surface of the grain volume.

Consignments of Grain Which are Warmer than the Rest

In the normal course of grain intake at the silo, grain consignments of which the temperatures differ drastically are received, depending on the time of day when the grain was harvested. Loads of warm grain then lie between the cooler grain in the silo bin and because of the poor conductivity of the grain, it takes weeks before the grain temperature in the bin becomes more uniform. If insects are taken in with the grain, or were in the bin before, they will be lured to the warmer areas, where they will develop quicker. This can also be the start of dry grain heating.

Consignments of Grain that are Wetter than the Rest

Exactly the same happens with grain that is wetter than the rest. Wetter grain is attractive to insects and they will move in the silo to the areas of grain that contain the most moisture. If the moisture content of the grain in the area is higher than 15%, wet grain will start heating. If not, the concentration of insects in the wetter area will start the process of dry grain heating. If fungi start to grow on the wetter grain, the fungi will attract fungus beetles, even at a stage when the fungal growth cannot even be seen with the naked eye.