Although there are several possibilities, the potential for a stored product pest infestation, in particular the presence of flour beetles, should not be overlooked.
The confused flour beetle and the red flour beetle are very similar in appearance. The best way to tell them apart is by examining their antennae. RFB antennae take on a club-shaped shape with three segments at the end, while CFB antennae gradually enlarge towards the tip, ending in a four-segmented club.
Another difference between the two beetles is that the RFB (found mostly in the southern states) is a high flier, while the CFB (mostly a northern pest) is flightless.
As adults, both beetles have shiny, reddish-brown bodies that are about 1/8-inch long, flattened, and oval. They have a very wide range of foods that includes flour, rice, cereals, grains, spices, grain products, shelled nuts, dried fruits, chocolate, beans, and other similar materials.
The average lifespan of both the confused beetle and the red flour beetle is between 1 and 3.5 years. They have four life cycle stages, which include the egg, larva, pupa, and adult. It is important to note that all four life cycle stages can be found in infested grain products at the same time.
When female beetles of either species lay their small white eggs, they do so on flour or other food. The eggs, which are coated in a sticky secretion, become covered in the product and easily stick to the sides of sacks, boxes, and other containers.
In the larval stage, both species are small, slender, and worm-like in appearance. When fully developed, the larva is 3/16 inch long and white, tinged with yellow.
As the confused flour and red flour beetles transform into a small pupa, they gradually change from white to yellow to brown. Shortly after turning brown, they transform into an adult beetle.
Now that we have identified the pest, the next question is:
Why is a pest that feeds on flour, rice and other grain products in my bathroom instead of my kitchen?
The answer to the question above is very easy: you have a food source in your bathroom.
To eliminate the pest problem, you just have to eliminate the food source. Thoroughly inspect your cabinets, drawers, and closets for rodent poisons, septic system treatments (in powder or flake form), hidden pet treats, rice bag heating pads, prescription drugs, etc. It can be a frustrating task, but with a bit of persistence, you should be able to locate the source of your problem.