Darkling Beetles

Have your new seedlings been chewed off at ground level? It may be darkling beetles.

Darkling beetle is the common name for all 20,000 species within the Tenebrionidae family. Some of the more famous California darklings include Blapstinus, Coelus., and Eleodes..

'Blapstinus' darkling beetle (xpda) CC BY-SA 4.0

‘Eleodes hispilabris’ desert stink beetle (Judy Gallagher) CC BY 2.0

Darkling beetle identification

Darkling beetles are dull black, blueish-black, or rusty brown, depending on the species. Unlike other beetles, the wings of darkling beetles are fused together. Some species have longitudinal ridges on their wing covers, but most are smooth, without markings of any kind. One exception is the wooly darkling beetle (Eleodes osculans), which is covered with tiny hairs. Darkling beetles can range from 1/8 to 1-1/2 of an inch long. The antennae are well developed and often have knobs at the ends (clubbed).

Coleoptera darkling beetle (Mick Talbot) CC BY 2.0

Predaceous ground beetle (gailhampshire) CC BY 2.0

Darkling beetles are frequently confused with predaceous ground beetles (Carabidae), but ground beetles are shiny and their antennae are more delicate. There are other differences, but you get the idea. Darkling beetle larva look like pale yellow or dark brown wireworms. They are less than 1/3 of an inch long and are often called false wireworms. You may know one particular branch of this family as mealworms (Tenebrio molitor).

Mealworms in a bowl (Pengo) CC BY-SA 3.0

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Did you know that mealworms can digest styrofoam?
Stanford researcher Craig Criddle discovered that the bacteria found in a mealworm’s gut can transform polystyrene into “carbon dioxide and recyclable organic waste.” How cool is that?!!? (UCANR)

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Darkling beetle behavior

These pests tend to walk with their heads down and their abdomens somewhat lifted. Very often, they will stand completely still, in a headstand position, if frightened. While not dangerous, they can squirt you with a nasty smelling secretion from that rear end that does not wash off easily. This behavior is what gives some darkling beetles the name Skunk Beetle or Stink Beetle, not to be mistaken with stinkbugs. For the most part, these pests only come out at night, but you may catch one running across the patio or lawn during daylight hours. Normally, they hide under dirt clods, fallen tree bark, rocks, boards, wood chips, and in other dark places during the day. Often, the only way you know you have a darkling beetle problem is because of the damage they cause.

​Darkling beetle damage

In addition to attacking seedlings at ground level, darkling beetles feed on the foliage of many garden plants, such as lettuce, chicory, cabbage, broccoli, and other brassicas, beans, melons, pumpkins, tomatoes, and squash. This feeding can be seen along leaf edges and on the underside of leaves. Darkling beetles also feed on the ‘netting’ (raised areas) of melons, the flowers of most cucurbits and pistachio, and on some fruit crops, such as figs.

Some species of darkling beetle also act as vectors for plant diseases and poultry parasites. Some darklings can also cause structural damage to insulation and wooden buildings. They are also attracted to poultry feed, but my guess is that the chickens make short work of that arrangement, as long as the unavailable feed is stored properly, in a metal container.

Darkling beetle control

Commercial farmers use insecticides to kill these pests. They also use a shallow ditch, filled with water, to block these pests from coming out of the weeds and into the fields. If you are an organic grower, as I am, you will have to use cultural controls. You can minimize darkling beetle damage organically when you:

• remove weeds and other plant debris from an area before planting
• speed up the breakdown of organic matter on and in the soil
• do not put seedlings out until they are 5 or 6 inches tall
• raise fruit off the ground with straw or hammocks
• eliminate dirt clods
• apply sticky tape barriers around fig trees
• allow planting beds to go fallow (unused) periodically
• avoid using alfalfa as a green manure in areas affected by darkling beetles (UCANR)

In 2015, a report published by The Coleopterists Bulletin warned that three non-native darkling beetles have been found in California and Nevada. Each of these species is somehow tied to ant or fire ant populations:

• Opatroides punctulatus Brullé, first discovered in the Sacramento area in 2003, now spreading rapidly throughout Nevada and California.
• Gondwanocrypticus platensis, a South American species of Crypticini that has spread widely in southern California
• Gonocephalum sericeum, an African native; this is the first sighting in the Western Hemisphere

I was unable to find any freely available photos, so you will have to track them down on your own. If you see any of these invasive beetles, please capture them and contact your local County Extension Office.

You may be surprised to learn that some people keep darkling beetles as pets.