The presence of insects and pests in stored grain can be an issue in New Zealand, especially due to long storage periods and New Zealand grain is often harvested at higher moisture contents than in many other grain growing countries. These pests have the potential to damage grain significantly; by either feeding on the embryo of the grain, or through external damage allowing fungi to cause greater deterioration.

From a silo survey conducted by FAR last year, samples of stored wheat and barley, mainly from Canterbury, were gathered, tested, and preliminary results found. Of the 60 samples, two thirds had received one or more insecticide treatment; however, approximately 75% were found to be infested with one or more insect or mite species. The predominant species that were found were saw tooth grain beetles and flat grain beetles. These species’ in both larval and adult form are able to damage the grain. Flying beetles have been found to be able to infest grain 2km away so make sure that waste grain is not dumped close to uninfected grain. A single bag of infested seed/grain can produce more than one million insects in a year; so ideally, this grain should be burned or buried to remove the risk of spreading of infestation.

To prevent insect and pest infestation, the following can be undertaken:

• Ensure silos are able to be closed properly and are in good condition to avoid insects entering. Then seal them properly;
• Clean not only the silos, but also the surrounding area, of old seed/grain. If previously stored grain was infested, a suitable insecticide should be sprayed inside the silo inside prior to use;
• Maintain clean storage premises with occasional residual spraying of critical areas.
• Check that grain is free of insects before storing;
• Only store dried seed, as insects cannot develop if seed moisture content is too low. Alongside this, storage fungi growth occurs at moisture contents of >14.5%, which results in faster grain or seed deterioration; again resulting in a need for dry seed/grain in storage.

Good quality, dry and cool grain or seed can be safely stored for many months without any insecticide application.

Monitoring grain

Remember to monitor grain regularly during storage. Setting up a regular monitoring and recording system, e.g. monthly, enables any changes in the temperature or moisture content of grain to be identified prior to a major problem developing and allows appropriate action to be taken. Insect and pest populations, and their damage, are likely to be less apparent in the early stages of storage, becoming more prevalent later.

• Inspect stores for insects in the late afternoon, when they are more active. The insects will accumulate near or on the grain surface;
• Monitor temperature in storage. Temperatures above 21°C favour insect activity, and above 27°C almost certainly indicates active insects. This monitoring can be done through the use of a temperature probe. Sample at the same points each time - the center and the sides of the silo, just below the surface of the grain;
• Grain moisture monitoring is also a good way to detect pests. Sample grain from just below the surface in the center, and at the side of the silo, ensuring that the sampling points are the same each time. An increase in the moisture content suggests insect activity or fungal growth;
• If the temperature or moisture start to increase, aerating the silo with cool dry air should be the first action;
• In a situation where there is an increase in temperature and moisture, but no visible insects, there is a likelihood of mite or fungal infestation.