Flour Beetles or Weevils: Facts, Pest Control, and Fiction
What Are Flour Beetles or Flour Weevils?
Flour beetles are annoying insects that infest flour and make it unusable. They are often known as flour weevils. Technically this term is incorrect, however, since weevils belong to a different group of insects. An examination of infested flour often reveals little brown or dark red insects crawling through the flour. The sight is not a pretty one. Flour beetles aren't poisonous, but they do produce an unpleasant odour and can cause flour to become grey. Although much of our food is contaminated by insects and their secretions, the thought of eating flour beetles or their eggs is unappealing for most people.
The most common beetles that infest flour are the confused flour beetle and the red flour beetle. Sometimes the pest in flour is the larva a moth instead of an adult beetle, however. The larva of the Indian meal moth is often detected by the silk strands that it creates in flour. These produce a structure that resembles a cobweb and traps grains of flour.
In this article I'll focus on the confused flour beetle, which is common in the cooler, more northern part of North America where I live. The red flour beetle generally lives further south but has a very similar appearance and biology to the confused flour beetle. The Indian meal moth has a wide distribution in North America.
Several weevil species infest grains and are sometimes kitchen pests, including the rice weevil shown above. The female inserts an egg into an intact grain kernel and then seals the hole. The new beetle develops within the grain. Since flour beetles are a kitchen pest, infest a grain product, and look somewhat like grain weevils, they are often mistakenly called "flour weevils."
The Confused Flour Beetle
Although I'm not fond of the insect, I love the common name of the confused flour beetle. It makes me think of a little insect scurrying through a bag of flour in a panic as it tries to find its way out. In reality, the name (apparently) refers to the fact that the beetle is easily confused with the red flour beetle.
The scientific name of the confused flour beetle is Tribolium confusum, while that of the red flour beetle is Tribolium castaneum. The two insects sometimes live in the same area. For many people, distinguishing between the species isn't important. They both produce the same unpleasant effects and they can both be removed by the same methods. For beetle fans and entomologists, however, identifying the species is useful.
All beetles belong to the class Insecta and the order Coleoptera. True weevils, such as grain weevils, belong to the family Curculionidae in the order Coleoptera. Confused and red flour beetles belong to the family Tenebrionidae.
How to Recognize a Confused Flour Beetle
The confused flour beetle is a shiny, red-brown insect with a flattened body. It's one eighth to one quarter of an inch in length. While its species may not be recognized without magnification, the beetle is not microscopic and can definitely be seen. Its presence in foods such as flour is one clue to its identity. The beetle has the following features.
- Like other insects, the body of the confused flour beetle is divided into three sections—head, thorax, and abdomen.
- Two segmented antennae are attached to the head.
- The ends of the antennae have a clubbed appearance. There are four segments to each club. The segments gradually increase in size from the beginning to the tip of the club. The club of the red flour beetle contains only three segments and is more distinct.
- When the beetle is viewed from the side, an observer can see that the large compound eyes are notched.
- Three pairs of legs are attached to the thorax. The legs are jointed, as in other insects.
- There are two pairs of wings. The outer pair are known as the elytra. The elytra are tough and protect the membranous inner wings, which in most beetles are used for flying. The confused flour beetle generally doesn't fly, however, while the red flour beetle does.
Confused Flour Beetles and Larvae (No Sound)
The colour of confused and red flour beetles depends on factors such as lighting and whether an insect is living or dead. It isn't a reliable feature for distinguishing the two species.
During mating, the male confused flour beetle inserts a package of sperm known as a spermatophore into the female's body. The female stores the sperm until she is ready for the sperm to fertilize her eggs. Once they are fertilized, the eggs are laid. The female lays several hundred eggs, but not at the same time. The process takes months. The eggs are covered with a sticky material which enables them to stick to flour grains.
The eggs of the confused flour beetle are white in colour. Unlike the adult beetles, they are microscopic. An egg hatches into an elongated and segmented larva which is yellow or light brown in colour. The larva eventually turns into a pupa. The pupa undergoes metamorphosis and changes into an adult beetle.
Confused flour beetles chew, but they don't bite or sting humans. They can't break through intact grain but may be found around damaged grain where there is grain dust. They can easily break through paper and cardboard food packaging, however. In addition to feeding on flour, they eat foods such as cake and pancake mixes, cereals, crackers, spices, chocolate, and even dry dog and cat food. The good news is that the beetles don't damage furniture or buildings and aren't dangerous to humans even if they are eaten.
A Red Flour Beetle Feeding (No Sound)
The larvae of Indian meal moths (or pantry moths) are also common pests in flour. The adults don't feed and don't live for long.
If you experience a flour beetle infestation, the source of the problem may not be your home. Confused flour beetles enter flour in warehouses and grocery stores as well as homes. They are common pests. Purchased flour may already contain beetle eggs.
Once beetles are visible in flour, the flour generally needs to be thrown away. The insects could be killed and/or removed instead if a person prefers to do this. If there are lots of dead beetles in the flour they will be hard to remove, however. In addition, they may have changed the taste and colour of the flour and may have encouraged the growth of mold. Another thing to keep in mind is that the infested flour will contain beetle feces and larval cases as well as eggs and intact insects. Personally, I would rather discard an obviously infested bag of flour than try to clean the flour.
Some people actually want to develop a Tribolium confusum culture, as shown in the video below. The animals are reared as food for some types of pets.
How to Control and Prevent a Flour Beetle Infestation
I've experienced a flour beetle infestation. Discarding the flour, cleaning the cupboard shelves thoroughly, and putting new flour in a screw-top canister solved the problem for me. Other steps than can help, however. The following ones are often recommended by pest control agencies and entomologists.
- When new flour is purchased, transfer it from the bag to a glass or heavy duty plastic canister that has a secure lid. Don't leave the flour in its bag for a long time before you transfer it.
- Buy flour in small quantities so that it can be used up quickly before an infestation begins.
- Keep shelves of food storage cupboards clean.
- If you're recovering from a flour beetle invasion, clear shelves with a vacuum cleaner to remove insects and then wash the shelves thoroughly. Pay special attention to cracks and corners as you clean. Dry the shelves well after washing them.
- Remove shelf paper before cleaning, if you use it, and cover the shelves with new paper once they are dry.
- Dispose of infested products outside the home to prevent reinfestation.
- Inspect other food packages for beetles before putting them back in the cupboard in case the beetles have spread.
- Consider subjecting flour to a freezing temperature after purchase. This will kill eggs and beetles. (It won't make them disappear, though.) Maintaining a deep-freeze temperature of 0ºF for four days is recommended.
- Heating the flour can also kill insects. A temperature of 130ºF and a time period of thirty minutes are recommended. I have neither frozen nor heated my flour before use, so I don't know how the temperature change affects its taste. In addition, heating whole grain flours before use may damage them and shorten their lifespan.
- If you find beetles in flour and remember where you bought the product, it might be a good idea to buy your flour somewhere else for a while.
I'm fascinated by nature. As I investigate plants and animals—including weevils—I often find inspiration for my creative writing, including my stories. One of these stories is shown below.
Bread Flour With Weevils: A Flash Fiction Story
Rebecca collided with the bakery door as the wind and driving rain pushed her into the building. "Five pounds of bread flour with weevils, please," she gasped to the shopkeeper, trying to catch her breath.
"None of our flour has weevils. We run a clean establishment and follow all the health rules," the shopkeeper announced, simultaneously smiling at the other customers.
"I promised my children that they could play with some weevils today," Rebecca complained. "A bakery that mills so much flour ought to have weevils!"
"All of our products are in pristine condition," the shopkeeper said, giving Rebecca the evil eye. "We would never allow insects on these premises!"
"I have lots of weevils in my house," said a friendly voice. Rebecca turned to see an elderly woman drinking tea at a table. "Of course, they only enter my flour with permission," the woman laughed. "One lump or two?" she asked with a smile.
Now this sounds promising, thought Rebecca. "No sugar for me," she said, sitting down at the table. "I don't suppose you have lemon?"
"Well, you are a woman after my own heart!" the old lady exclaimed. "I always carry a lemon with me for emergencies like these, as well as my little pot of honey. It's a great combination for a cold."
"I don't have a cold," Rebecca said, just before she sneezed loudly. The two women burst into laughter. The shopkeeper brought Rebecca some tissues, slamming the box on the table.
"I really don't know why weevils are so unpopular," the elderly woman lamented, stirring lemon juice and honey into Rebecca's tea. "They have much to offer us. It's wonderful to find someone else who likes them!"
"It's my youngest, really. She's never seen a weevil before," Rebecca explained as she drank some tea. She was surprised and embarrassed to feel tears welling up in her eyes.
"Poor thing! Weevils are so enticing. Your daughter deserves to meet one," said the woman. "I'd love to give her some of mine."
"That's very kind of you. Do you live nearby?" Rebecca asked, patting her eyes with a tissue.
"If you don't mind, I'd prefer to go to your home. I don't have the opportunity to visit people very often these days. Would four o'clock be a good time to arrive?" she inquired.
How sad, thought Rebecca. "Of course. I live about a forty minute walk away, though," she said, considering the women's age and frail appearance, "and the weather is terrible. There's no bus, either."
"Oh, the trip's no problem. I have my own form of transport and I enjoy riding in a storm," the woman said, smiling as she sipped her tea. "Meeting your children would be delightful. We evils love a chance to visit!"
Information about the confused flour beetle from the Canadian Grain Commission
Confused and red flour beetle facts from the University of Florida
Facts and control methods related to the confused and red flour beetles from Penn State College of Agricultural Sciences
Red flour beetle information from Iowa State University
© 2015 Linda Crampton