Bad Bugs: Flour Beetle

Tribolium castaneum / T. confusum
Flour Beetle

By Alain VanRyckeghem, BCE
Technical Director

The red and confused flour beetles (Tribolium castaneum and T. confusum) are beetles that have a long life as an adult beetle. Typically they can live 6–12 months and sometimes longer. Because of this biology they produce a pheromone called an aggregation pheromone. They do not produce a sex pheromone like stored food moths.

Stored food moths only live for a few days to a week or so. As a result, the use of a sex pheromone is important. Males will respond to female very quickly and will fly long
distances to follow the pheromone trail. Typically, monitoring traps can be set 25–50 feet apart and still detect the presence of moths within that area.

The flour beetles, however, do not have a strong attraction to the aggregation pheromone. They live long enough that they can encounter another male or female beetle during its random wandering in search for food. They are mostly food oriented
rather than solely motivated by pheromones.

The distance of attraction for flour beetles to a pheromone trap is greatly reduced. Typically the range of attraction for flour beetles to a trap is less than 10 feet. It will rarely be caught on traps further than this distance from an infested food source.
If pallets are stored above ground and the traps are below on the floor, these traps will not attract the beetles. In fact, because the beetle is happily feeding on a high quality food product (like flour or milk powder) it may not be attracted to a trap that is within 3 feet of the source. This makes flour beetles one of the most difficult stored food pests to detect and monitor.

Flour beetles will enter into packages with defects; it cannot penetrate into intact packaging (with the exception of very thin paper). Once they have entered the package, reproduction will occur with a cycle being completed in about 30 days
at 90°F. Red flour beetles have optimum development temperatures about 5°F higher than confused flour beetles. Females can lay 200–400 eggs over a period of 2 months. This infestation will continue to produce new generations until at some point the package is overcrowded and food becomes limited. The beetles then produce another pheromone called benzoquinones that repel each other (anti-aggregation) and direct beetles to leave the package.

It is only after product is heavily infested (several months) that we may discover their activity. It is during this time the beetles are exiting infested products that they may become caught in shrink wrap. This is a good spot to use flashlight inspections of pallets to discover infestations that cannot be picked up by pheromone traps. When
beetles are wandering around floors they can be in search of food and pheromone to direct them to new food sources. This is when the traps become useful. The detection range of food odors is limited to perhaps 15–20 feet. Sanitation within the warehouse is an important factor as the spilled foods or crevices with food product will compete with pheromone traps on the floor reducing their trapping ability. These spilled foods can be a source of infestations for other products in the warehouse.

Flour beetle traps should be ‘targeted’ toward these potential patches of available food. A grid system of flour beetle traps is an expensive and inefficient use of this
type of device due to the short range of attraction. The percentage of the population that is actually in the adult form is about 10% at any given time. Repeated lab tests
show that traps placed within 2 feet of a food and harborage site typically attract 10-25 % of the adult beetles present. At best a trapping system in a facility is sampling about 2% of the total population so this can give you an estimate of the infestation level for that area.

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3 thoughts on “Bad Bugs: Flour Beetle

  1. Alain, thanks for the article. As a PCO servicing a very large commercial bakery, my primary concern is the origin of these pests. If CFB are coming in with the bulk flour delivery, what is the best way to inspect for them before they enter the building, or actually the silos? The problem is they move from the silos, to the sifters, and then they’re in the building. Anyway to be proactive? Thanks for any information.

    • In most cases the flour arriving in tanker trucks is sifted with a rebolt sifter before loading into tankers and also before loading into the receiving silos. If not, it could be requested that loading of the silos use a rebolt sifting with at least a 50 mesh screen as this will remove eggs as well as adults and larvae. Inspection of the rebolt after unloading could reveal if the sources are being delivered…but this would be rare. Once the flour is in the closed system, another rebolt sifter is used before delivery to production. Again this could be set to 50 mesh screen or smaller. Adult beetles collected in the interior rebolt system means there are beetles reproducing in the system; possibly the tank but likely in other locations (assuming a rebolt sifter was used during loading of Silos). I am also sure that entoleters are used in most mills, so in system contamination is usually filtered out before production use.

      Unfortunately most of the cases of flour beetles in the mills and even in the equipment are self-propagating, and do not arrive or get refreshed by new deliveries of flour. The exception to this may be bagged flour that has been in storage and delivered to your warehouse. The flour beetles can exist within equipment at slide gates, and dead end piping or just above pressure vents and in dust collecting systems; anywhere where there are product hang-ups. In the past we could manage this by using ‘spot fumigants’, which are no longer available to us. The answer to this problems is a system shutdown and extensive cleaning…which rarely happens… or a fumigation where the system is open to fumigant entry.

      Within the structure a treatment program with insect growth regulators such as Nyguard can have dramatic effects over the long run. Appropriate fogging when available can distribute the growth regulator to ledges, crack and crevices that are hard to reach. The use of a low odor vapona (does not have xylene as a solvent) could also aid in killing beetles and larvae in the structure. This is not considered a fumigation with a gas, so this has limited effects because it cannot penetrate walls or cracks and crevices very far.

      I hope some of this information is helpful.
      Alain VanRyckeghem, BCE

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