Mites in Stored Grain
Mites are often a problem in stored grain, normally when the temperature and humidity
are high and when the moisture content of the grain is between about 12% and 18%.
In general, the best conditions for most insect pests is a temperature around 30°C, with a
relative humidity of 40 to 80 percent. Above 40°C, most species will eventually be
killed, while reproduction ceases at temperatures below about 20°C, with dormancy
induced at temperatures below 10°C. Reproduction is inhibited at less than about 40
percent humidity.
Mites (class Arachnida) of stored seed and grain are very small, 0.2–1 mm long, and
unlike other insects have four pairs of legs (insects have three) and an apparently
unsegmented body, with no wings or antennae. A hand lens is normally needed to see
them. They are mainly whitish in colour, and they move rather slowly.
Some species of mites are beneficial, and eat the eggs of moths and other mites,
smaller mites tend to be more harmful than larger ones.
Control of mites is possible with phosphine (online information available from
http://phosphine.com)) or with an acaricide that is proven to be both effective against
mites and safe for use with stored food and/or seed. Lindane is highly effective but has
wisely been banned, or has seriously restricted use, in most developed countries.
If the moisture content of legume seed is between about 9% and 14%, the two following
guidelines can be applied independently of each other:
- if the storage temperature is reduced by 5°C, the life of the seed (ie the length of
time it remains viable) will be approximately doubled. - if the moisture content of the seed is reduced by 1%, the life of the seed will
similarly be about twice as long.
The most important storage fungi are: Alternaria, Aspergillus, Mucor, Penicillium,
Streptomyces, Cladosporium and Sporendoma. They can cause reduced germination,
discolouration, heating, biochemical changes and loss in grain weight.
Seed which has become very “mouldy” ie infected with fungi, should be destroyed
(burned or buried) and not used as either animal or human food.
Fungi in stored seed/grain is mainly controlled by reducing the temperature,
humidity and seed moisture content; chemical control is very limited.
Clean, unbroken seed is less liable to be damaged by fungi than seed that is broken,
damaged or dirty.
Rodents, birds and insects (various beetles, weevils and mites) are the main pests.
Damage by birds is mainly caused by spoiling the seed with their faeces, as well as by
birds eating it.
although it is difficult to tell the difference between the species. As a very rough guide,
101
- Storage of Grain Legumes for Seed
- Storage Fungi
- Storage Pests