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IN FOCUS: STORED PRODUCT BEETLES

A helpful overview of the eight stored product beetles that pest managers are most likely to encounter.

 

There are a large number of stored product pests. Correct identification can be a challenge due their small size and the fact that most pest managers will only come across these pests infrequently. Whilst there are a wide range of moths and a couple of mites that are considered stored product pests, here we focus on eight of the key beetle pests of stored products.

The first three beetles can be grouped together as they are generalist feeders, not only eating a wide range of grains and dried vegetable material, but also a range of animal materials. They have similar oval-shaped bodies and the adults are good fliers.

The drugstore beetle (Stegobium paniceum) and cigarette beetle (Lasioderma serricorne) are from the Anobiidae family, which includes the common furniture beetle and powderpost beetle. They are both small brown beetles 2.5-3.0 mm long. However, the drugstore beetle is more of a reddish brown, has distinctive three-clubbed antennae and the elytra (wing covers) have a grooved appearance, whereas the cigarette beetle is a light brown, has a uniform serrated antennae with 11 segments and smooth elytra. The larvae of both are small white ‘curl grubs’ up to 4.0 mm long.

 

 

Both beetles are very common and attack a wide range of foodstuffs, including rodent baits. The cigarette beetle (also known as the tobacco or cigar beetle) gets its name as it also attacks tobacco, while the drugstore beetle has been observed damaging prescription drugs! However, the notable difference in feeding behaviour is that both the adult and larva of the cigarette beetle feed and cause damage, whereas only the larvae cause damage in the drugstore beetle.

The warehouse beetle has a similar shape to the drugstore and cigarette beetle but is a mottled dark brown colour with three faint, lighter bands across the wing covers. It is a member of the Dermestidae family, which also includes the khapra beetle, considered the “world’s worst stored grain pest”, but which is not currently present in Australia. They are sometimes confused with carpet beetles as their larvae are very similar (‘woolly bears’) and the adults can be mistaken for variegated carpet beetles.

 

Warehouse beetle (Trogoderma variabile)

 

Although warehouse beetles eat a range of seed, grains and packaged dry foods, like their relatives the carpet beetles, they will also eat a range of animal material including bee and wasp nests, rodent and insect carcasses and droppings.

The next group of five beetles can be grouped together as specialist stored product pests, with a more limited diet. They are more varied in body form and therefore easier to identify.

The rice weevil (Sitophilus oryzae) is probably the easiest to identify as it has the unmistakable snout of a weevil. However, it can be confused with the generally larger maize weevil (far less common). The rice weevil feeds mostly on intact grains (not just rice) with the adult boring a hole into the grain to lay a single egg. The larvae then consumes the grain from the inside out.

 

Rice weevil (Sitophilus oryzae)

 

The saw-toothed grain beetle (Oryzaephilus surinamensis) is also easy to identify with its six saw-like projections on either side of the thorax. Both the adults and larvae feed on and cause damage to a variety of grains, seeds, cereals and other dried food, although they do not attack intact grain. They are commonly found in infested pet food, bird seed and rodent bait.

 

Saw-toothed grain beetle (Oryzaephilus surinamensis)

 

The lesser grain borer (Rhyzopertha dominica) is also reasonably easy to identify. Although it has a brown elongated body like a number of the stored product pest beetles, as with other borers it holds its head under its body, so it isn’t visible from above. They are probably the biggest pest of stored grain in Australia, although they lay their eggs outside the grain and the larvae burrow inside. They will also feed on processed grains such as cereals, biscuits and spaghetti.

 

Lesser grain borer (Rhyzopertha dominica)

 

The last two beetles can be confused with each other and indeed each is actually a group of beetles rather than a single species. Firstly, there are the flour beetles (Tribolium spp.) – primarily the confused flour beetle and red-rust flour beetle – which are reddish brown, slender beetles around 4.0 mm long. Both feed on a wide range of stored products and dried food. They are considered a secondary pest of grain as they will only attack damaged grain. The easiest way to distinguish between the two beetles is that the red-rust flour beetle has a tri-segmented club at the end of the antennae and is a strong flyer, whereas the confused flour beetle has gradually thickening antennae and cannot fly.

 

A flour beetle, the red-rust flour beetle (Tribolium castaneum)

 

Secondly, there are the flat grain beetles (Crypolestes spp.), which can only be identified at the species level by genital dissection. However, they are all very small, brown, slender and fast moving beetles only 2.0 mm long. Their size and speed of movement make them ant-like in appearance.

 

A flat grain beetle, the rusty grain beetle (Cryptolestes ferrugineus)

 

Correct identification demonstrates your skill as a pest professional and is important for ensuring that the appropriate treatment is carried out. However, the reality is that in most domestic situations the treatment process is the same – throw out any infested food, clean up any spills and if necessary apply surface sprays to food storage cupboards. Treatment in commercial situations is more complicated and correct identification becomes critical. Certainly removal of infested material and improved hygiene becomes important, but the use of pheromone systems and fumigation options (requires separate licensing) may also need to be considered, as well the need for ongoing monitoring.

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