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Darkling Beetle Problems: Prevention and Management

Darkling beetles (family Tenebrionidae) are a large and diverse family of beetles with over 20,000 known species worldwide and over 1,500 species in North America. Many home gardeners who raise chickens or ducks may be familiar with mealworms, the larvae form of the darkling beetle, and may have even dabbled in raising these nutrient-dense worms. 

While a darkling beetle may be a nutritious treat for birds or tasty bait for fish, it can also be problematic for gardeners. Some adults and larvae will chew on seedlings and the lower leaves of plants. They leave damage marks that are similar to that of cutworms or related pests, and attack leaves with their chewing mouthparts. Luckily, more established plants over 5-6 inches in height are typically not bothered by the darkling beetle. 

Out of the thousands of types of darklings, there are several species such as the red and the confused flour beetles that attack stored grains. They can be pests in grocery stores and in homes and are a major problem in grain silos and other storage facilities. Luckily, many darkling beetle problems can be resolved using good garden maintenance and hygiene.

Darkling Beetle Overview

Darkling beetle Tenebrionidae spp.
A darkling beetle is part of the Tenebrionidae family of insects. Source: Bernard Dupont

As their name suggests, adult darkling beetles are typically dark brown or black in color. They are sometimes mistaken for predaceous ground beetles (Carabidae), but Tenebrionidae beetles generally have clubbed antennae. However, both families of beetles tend to be most active at night and are commonly flightless.  

Because there is so much diversity in this family of beetles, adults of different genera or subfamilies have varying sizes and appearances. Most are an elongated oval shape, with convex to flattened bodies. Their size ranges from 1-60 mm worldwide with many species within 2.5-20 mm in North America. According to the National Park Service, however, Tenebrionidae beetles found in the Grand Canyon can even reach 1.5 inches (~38 mm) in length! 

Like other insect species, the darkling beetle’s body is divided into the head, the thorax, and the abdomen. For entomology enthusiasts out there, the unique differentiators of this family include emarginate eyes, definite epistomal margin, heteromerous tarsi, closed prothoracic coxal cavities, clubbed or stout antennae, and abdominal defensive glandular reservoirs. This last trait enables the beetles to emit a very foul-smelling fluid as their chemical defenses against predators. 

Life Cycle of Darkling Beetles

The darkling beetle undergoes four developmental stages: egg, larvae, pupa, and adult. Many species of darklings are only active in the spring through fall. Some adult beetles may lay eggs before the onset of winter and these eggs will stay dormant until the following spring to hatch as beetle larvae. 

Each female can lay hundreds of tiny white sticky eggs during its lifetime. Eggs will hatch into light yellow or brown larvae, which are commonly known as mealworms or false wireworms. A mealworm can molt up to 20 times during the 8-10 weeks of its larval stage and gradually grows larger to reach 1.5 inches in length. After their final molt, the darkling beetle larvae become fragile and whitish pupae that take 1-3 weeks to mature into darkling beetle adults. 

Common Habitats

In the wild, adults and larvae are found or near the surface of the soil. They take shelter in weed-filled environments, leaf litter, under rocks or stones, or in decomposing logs. There are a few specialized species that are adapted to live in caves or in arid dunes. Some specialize in the dwellings of other mammals and bugs. Rodents, termites, and birds all may be involuntarily sharing their homes with these intruders. 

Species such as the red and confused flour beetles specialize in grains and are attracted to grain storage facilities and chicken coops. They can also inhabit places where gardeners or grain producers set out material to dry, such as sheaves of wheat before threshing. These beetles are particularly attracted to grain with high moisture content. 

Darkling beetles don’t typically infest people’s homes. In fact, many people will purchase mealworms to raise at home and even start businesses selling these worms. However, accidental visitors may hang out near the kitchen or the pantry, where there is a food source for them. They tend to be drawn to dark, cool, and protected locations.

What Do Darkling Beetles Eat?

Both the adult and larval forms typically feed on plant debris, dead insects, and decomposing plant material like rotting wood. A few are minor agricultural pests, although they seldom cause severe damage to crops as they eat, but may become a vector for disease transfer.

Because of the diversity of this family of beetles, there are species that specialize in different food sources. In forests, for example, some darkling beetles are surface eaters that graze on bark, lichen, and fungi. In deserts and arid climates, they are capable scavengers that feed on whatever they can find including animal dung or carrion.

How to Control Darkling Beetles

Isomira spp. darkling beetle
Isomira species darkling beetles are very rounded. Source: treegrow

While the damage they cause may not be too severe at the onset, darklings can be vectors of different diseases that might exacerbate the damage to plants. A darkling beetle infestation can also cause issues in raising poultry because they can also be a carrier of avian diseases and bacteria such as Salmonella.  

Organic or Chemical Control

Because insects like the darkling beetle are a great source of nutrients for other animals including wild birds, chickens, ducks, etc., it is not advisable to use chemical sprays to control them. In organic poultry operations, farmers may use pyrethrum-based insecticides to help manage their population. 

Environmental Control And Prevention

Environmental control and prevention are the most effective techniques to manage darkling beetle populations and deter them from taking hold in your home or garden. Remove weeds that can shelter beetles and check for any leaks in your grain storage. 

Seedlings are the most vulnerable plants in the garden. Make sure to inspect your young plants frequently for any damage and look around the plants and soil to check for the source of the damage. You may find darkling beetles or other insects like cutworms as the culprit. You can add a floating row cover overtop as an extra barrier for protection. 

If you have an infestation on hand, first remove the source of the infestation. Consider how you store all of your grain and cereal products including pet food, birds’ seeds, and pantry items.  Locate all infested material and place it in a tightly sealed container before disposing of it. You can also freeze the infested material for at least five days to kill off the adults and eggs. 

Additionally, some darkling beetles and their larval forms are attracted to animal feces. Remove any from your property so as to not accidentally create a breeding ground for them. 

Frequently Asked Questions

Ecnolagia grandis darkling beetle
Ecnolagia grandis is a species that tends towards brown in coloration. Source: m9rt1n

Q: Are darkling beetles harmful?

A: There are thousands of species of darkling beetles around the world. Some species can cause harm to young plants by chewing on the developing leaves. Several species also feed on grains and can be a pest in silos, chicken coops, or even inside the home. While they are not able to sting or bite humans, they can be a vector for plant and avian diseases. 

Q: Are darkling beetles invasive?

A: Darkling beetles are a common and diverse family of insects found around the world. There are both invasive and native darkling beetle species. The US Department of Agriculture has been working on projects that monitor their population and movement due to an increased risk of invasive species from global trade.