When inspecting stored rice, one may find that insect numbers increase dramatically in short periods of time. This sudden explosion of insects may lead you to believe that they all came crawling or flying into the bin from a nearby location, such as a field, weed patch or the neighbor’s bins. Most likely these insects originated in your own bin. The insects that infest stored rice can survive in grain residue inside the bin or around the storage area. Later, when bins are filled, these insects, extremely well adapted to the temperature and moisture conditions of stored grain, will reproduce exponentially and may cause problems.

Initial infestation of farm stored rice can originate outside grain bins. A few insects might take refuge and survive in weeds, other nearby bins, even rice fields. Several stored rice insects are strong flyers and can find their way to a filled bin. More often, however, stored grain insects survive inside farm bins when there is no grain in the bin, that is, when we think there is no grain inside the bin. Very small amounts of grain residue, such as spilled grain, broken kernels, or grain dust can be used by these insects to survive between harvests.

An almost undetectable number of insects can live in these residues, but because of their high reproductive capacity, a few insects when bins are filled with grain may result in large infestations later on. For example, female rice weevils can lay between 300 and 400 eggs. Under favorable conditions, larvae will emerge from these eggs and go through four larval instars and a pupal stage, after which they will become adults, all in just 35 days. These new adults will mate and females lay eggs, repeating the cycle. Soon, you will have a large weevil infestation.

Another insect, the confused flour beetle, starting from a single pair, can reach a population of over 1 million in just five months. These figures demonstrate how easily grain can be infested when a few insects survive inside an empty bin.

Internal versus external feeders

Insects infesting stored rice can be classified as internal or external feeders. Internal feeders develop inside the kernel and feed on the embryo and endosperm; these are also referred to as “hidden infesters” because they can’t be easily seen when inspecting the rice. They are considered important because they directly affect the grain; milling quality is reduced, milled rice can be contaminated, and, in the case of seed, germination can be affected.

External feeders develop outside the kernels and use brokens, dust, fines, bran and other small particles as food. The activity of large numbers of both internal and external feeders produces heat, which in turn can promote spoilage. A special group within the external feeders is one consisting of fungus (mold) feeding beetles. These do not damage the grain directly, but their presence indicates that mold is developing on the grain. Additionally, these beetles can reach high numbers and cause a load of rice to be classified as “infested” when delivered to the mill, or they might be confused with more damaging pests and trigger an unnecessary fumigation. Either way, it is important to monitor the grain constantly to detect any signs of infestation early on so that management measures can be taken.

Mold feeders

A recent survey of farm-stored rice has shown that mold feeding insects predominate during the first few months of storage. The survey found that the most common insects were silken fungus beetles, the hairy fungus beetle, minute brown scavenger beetles and the foreign grain beetle. When “panning” your rice, you will probably find high numbers of these beetles, especially right after harvest. It is possible that these beetles were picked from the field during harvest. For example, if rice lodged and panicles came in contact with water, there might be mold growing on the grains at the time of harvest. Fungus beetles can be found feeding on this mold in the field and can be picked up with the combine. Or, as mentioned earlier, they might have been feeding on mold growing on grain residue inside the empty bin.

If during storage, grain is aerated and dried properly, mold will be eliminated and the insect infestation will be drastically reduced. Fungus feeding beetles will have to move somewhere else where they can find food. During the survey, proper aeration caused the insect population to drop from 3 to less than 0.5 insects/trap/day in 3 months.

Other damaging insects of stored rice, such as the Angoumois grain moth, lesser grain borer or rice weevil, were not found during the farm survey. These insects become active in stored rice when temperatures start increasing early in the spring and thrive during the hot summer months. If you store rice during the summer, increase your monitoring efforts and keep an eye out for these insects.