What Do Mealworms Eat? Diet and Feed Substrates Explained

Quick Answer

Mealworms (the larvae of Tenebrio molitor) mainly eat grain-based plant material such as wheat bran, oat flakes and flour, plus a small share of moist vegetables like carrot or potato for water. Research points to an intake of roughly 71% carbohydrate, 23% protein and 6% lipid for good growth. Producers also use food by-products, but EU rules prohibit manure and catering waste as insect feed.

What Are Mealworms and Why Does Their Diet Matter?

"Mealworm" usually refers to the larval stage of the yellow mealworm beetle, Tenebrio molitor. The larva is the feeding and growth stage, so it is the form that is reared at scale for animal feed and food use.

Definition: Rearing substrate. The substrate is the dry feed bed in which mealworm larvae live and eat. In farming it serves as both food and habitat, and it is usually a blend of bran, flour or by-products, topped up with a moisture source.

Diet matters for three practical reasons. It controls how fast larvae reach harvest weight, how efficiently feed is converted into biomass, and what the larvae contain once harvested. A comparison of agro-industrial by-products found that the substrate affects larval growth, feed conversion and macro-composition alike.

Note that the lesser mealworm (Alphitobius diaperinus) is a different species with its own feeding data. This article covers the yellow mealworm unless stated otherwise.

What Do Mealworms Eat in the Wild?

In natural and storage settings, mealworms live in dry, decaying plant material. They are commonly found in stored grain, flour, bran, and spoiled or damp cereal products, which is why they are classed as a pest of stored products. They also consume dead insects and other organic debris when available.

This scavenging habit is the reason they can be farmed on low-value plant by-products. It is also the reason an unmanaged feed source is a quality and safety risk, as covered later in this article.

What Do Mealworms Eat in Farming and Captivity?

Commercial and laboratory colonies are fed a dry, grain-based substrate and a separate water source. A self-selection study of 25 substrates found that larvae preferred grain-based diets. The most suitable ingredients were wheat bran and flour, maize hulls, oat bran and flakes, rice flour, lupine flour and potato flakes.

Typical components of a farm substrate include:

• Wheat bran: the standard base, inexpensive and widely available.

• Oat flakes or oat bran: a common alternative or supplement.

• Brewer's spent grain, bread by-product, tomato pomace: by-product options studied for circular production.

• Moist vegetables (carrot, potato, cabbage): used mainly as a water source rather than a primary feed.

• Protein supplements: dried egg white, soy protein or similar, used in some trials to speed development.

Dry matter intake is only half of the picture. Studies of apparent digestibility show that larvae do not extract everything from bran. One trial on wheat bran and oat flakes reported digestibility of about 0.50 for dry matter and 0.33 for crude protein in wheat bran, which is why feed conversion differs between substrates.

Best Mealworm Feed Ingredients Compared

The table summarizes findings from published studies. Results come from laboratory or pilot trials, so commercial performance will vary with temperature, humidity, stocking density and larval age.

Particle size is an overlooked variable. The same particle-size study found that particles under 2 mm improved larval growth on all feeds except alfalfa pellets, and that maximum larval weight on ground corn rose by up to 70% with finer particles.

Nutrient Balance: What Self-Selection Research Shows

Self-selection experiments let larvae choose among ingredients so researchers can see which macronutrient mix they pick. In a trial using 20 ingredients, the best-performing treatments showed intake ratios of about 0.06 lipid, 0.23 protein and 0.71 carbohydrate.

Ingredients with high relative consumption in that study included cabbage, potato, wheat bran, rice bran, corn dried distillers' grain, spent brewery grain, canola meal and sunflower meal.

Three practical lessons follow:

1. Carbohydrate is the bulk of the diet. Grain-based bases supply most of it.

2. Protein needs rise with age. The age-based self-selection study reported that protein and fat requirements grow as larvae mature.

3. Low protein can slow growth. Research on maize and barley substrates found that a low nitrogen content could impair growth, an effect that eased once the larvae's protein needs were met.

Adding protein can shorten development. In an older trial using dry potato as the carbohydrate base, adding protein to the supplement shortened development time and improved feed conversion and fecundity. Adding lipid gave no significant improvement.

Water and Moisture: Do Mealworms Need Vegetables?

Mealworms are often kept on dry substrate and receive water through moist plant pieces. Colonies in published studies were maintained on wheat bran with fresh vegetables as a water source, and carrots were placed in the center of the multiple-choice arena in the age-based trial.

There is a trade-off to manage:

• Too little moisture slows growth and can reduce survival.

• Too much moisture promotes mold and ammonia, and spoils the substrate.

Moisture sources also change larval composition. In one study, a vegetable mix raised the crude ash, fat and protein content of the feed compared with an agar control. The same work noted that arsenic accumulated in larvae while cadmium and lead did not, which shows why the source of vegetables and by-products must be controlled.

What Do Adult Beetles and Pupae Eat?

Mealworms pass through four stages: egg, larva, pupa and adult beetle.

• Larvae are the main feeders and consume the bulk of the substrate.

• Pupae are a non-feeding stage. They are soft and vulnerable, and in crowded or underfed colonies larvae and beetles may gnaw them, so pupae are normally separated.

• Adult beetles eat the same bran-based substrate and also need a moisture source. Published protocols provide water to adults, for example twice weekly in one entomology study, while larvae drew moisture from their feed.

Breeding colonies are often fed richer diets than growout colonies because protein supports egg production.

Can Mealworms Eat Plastic?

Yes, in laboratory conditions. A Stanford research group found that yellow mealworms can eat polystyrene, supported by microorganisms in their gut. In a study of polystyrene containing the flame retardant HBCD, the larvae excreted about half of the polymer as partially degraded fragments and the other half as carbon dioxide. Worms fed HBCD-laden polystyrene remained as healthy as controls, and shrimp fed those worms were also healthy.

This research is valuable for plastic waste science, but it has limits for the feed industry:

• The work addresses degradation, not approval of plastic-fed larvae as a feed ingredient.

• A substrate that is not an authorized feed material cannot simply be used because larvae survive on it.

• Researchers described the egested HBCD as still an environmental concern.

For commercial insect production, treat plastic degradation and feed-grade production as separate applications.

What Mealworms Should Not Eat

Mealworms can survive on a wide range of material, but surviving is not the same as producing safe, consistent feed ingredients. Avoid:

• Moldy or rancid grain. Mycotoxins and off-flavors can carry into the product.

• Substrates high in antinutritional compounds. One by-product trial linked thistle cake to poorer growth.

• Unverified by-products. Heavy metal and contaminant levels in the substrate can influence what ends up in larvae.

• Excess lipid or sugar. Imbalanced rice bran diets reduced growth.

EU rules on insect feed substrates

In the EU, farmed insects are subject to the animal feed rules. Regulation (EU) 2017/893 states that the use of ruminant proteins, catering waste, meat-and-bone meal and manure as feed for insects is prohibited.

Some research papers use poultry litter as an experimental substrate. One poultry litter study found that a litter-only diet lowered flour crude protein by only 8%. Such results illustrate biological flexibility, not a compliant commercial practice in the EU. Always check the current consolidated legislation and the rules of your destination market before choosing substrates.

How Diet Shapes Mealworm Nutrition for Feed Buyers

Larvae composition depends on both the substrate and the processing. Reported values for dried larvae vary:

Diet explains part of this spread. Higher protein intake increased larval protein content in the age-based trial, and a wheat bran versus sow feed comparison found that larvae on wheat bran had higher crude protein and lower crude lipid. Oil inputs in the feed also changed fatty acid composition in that work.

For buyers, this means the same species can yield different products. Whole dried mealworms and dried mealworms for poultry and pet feed retain the natural fat, while mealworm powder and mealworm meal for aquafeed formulation are ground forms used as ingredients. Where a higher protein concentration is needed, defatted mealworm protein powder removes part of the fat, which can be separated as mealworm oil for use as a lipid ingredient in feed formulations.

Buyers comparing insect ingredients may also review dried black soldier fly larvae, black soldier fly larvae specifications and BSF protein powder, which differ in species, fat profile and substrate rules.

For food use, EFSA's 2021 opinion on dried yellow mealworm found the toxicity studies submitted raised no safety concerns, and noted that the proteins may cause allergic reactions in people allergic to crustaceans and dust mites.

Buyer Checklist: Questions to Ask About Mealworm Feed Substrates

Ask your supplier for written answers to the following:

1. Which substrate ingredients are used? Request the bran, grain or by-product list.

2. Are all substrate materials authorized feed materials? Confirm that no manure, catering waste or ruminant protein is used.

3. How are moisture sources sourced and controlled? Vegetables should come from traceable suppliers.

4. Are substrates tested for mycotoxins and heavy metals? Ask for test frequency and methods.

5. What are the typical protein and fat values of the finished product? Request batch-specific analysis, not only typical values.

6. How consistent is the diet between batches? Substrate changes can shift larval composition.

7. What documentation supports your target market? Confirm certificates and regulatory status for the destination country.

FAQ

What do mealworms eat as their main food?

Mealworms mainly eat grain-based plant material. Wheat bran is the most common farm substrate, with oat flakes, flour, potato flakes and cereal by-products also used. They get moisture from small pieces of vegetables such as carrot or potato.

What do baby mealworms eat?

Young larvae eat the same fine, grain-based substrate as older larvae. Particle size matters: a particle-size trial found that particles smaller than 2 mm improved growth on most feeds. Protein needs increase as larvae mature.

Do mealworms need water?

Mealworms need a moisture source, but they usually get it from moist vegetables rather than open water. Excess moisture causes mold, so only small amounts of fresh vegetable should be added and removed before they spoil.

Can mealworms eat meat or dead insects?

They are omnivorous scavengers and will eat dead insects and animal matter in natural settings. In regulated feed production, however, animal-derived substrates are restricted, and the EU prohibits catering waste and ruminant proteins as insect feed.

Can mealworms eat plastic and Styrofoam?

Laboratory studies show that yellow mealworms can degrade polystyrene foam with the help of gut microorganisms. This is a research finding, not an approved way of producing feed-grade larvae.

Does what mealworms eat change their protein content?

Yes. Higher protein intake raised larval protein in one self-selection study, and wheat bran produced larvae with more protein and less fat than sow feed in another. Larvae protein is commonly reported between about 49% and 56% of dry matter, depending on the study.

References

1. Self-selection of feeding substrates by Tenebrio molitor larvae of different ages (Insects, 2022). PMC9323919

2. Self-selection of agricultural by-products and food ingredients by Tenebrio molitor (Insects, 2020). PMC7760134

3. Growth performances and nutritional values of Tenebrio molitor larvae: influence of different agro-industrial by-product diets. PMC12840792

4. Valorisation potential of using organic side streams as feed for Tenebrio molitor, Acheta domesticus and Locusta migratoria (Insects, 2021). PMC8467494

5. Effects of particle size of four feeds on larval growth of Tenebrio molitor (European Journal of Entomology, 2022). DOAJ record

6. Self-selection of two diet components by Tenebrio molitor larvae and its impact on fitness (Environmental Entomology). BioOne

7. Use of nutrient self-selection as a diet refining tool in Tenebrio molitor (Journal of Entomological Science). BioOne

8. Bread by-product and maize silage as alternative feeds for Tenebrio molitor larvae. DOAJ record

9. Growth performance and digestibility of mealworm fed wheat bran and oat flakes. Scindeks

10. Nutritional profile of yellow mealworm reared with supplementation of organic side streams. PMC10353995

11. Development of Tenebrio molitor on poultry litter-based diets. PMC7820700

12. Commission Regulation (EU) 2017/893 of 24 May 2017. EUR-Lex

13. EFSA NDA Panel. Safety of dried yellow mealworm (Tenebrio molitor larva) as a novel food. EFSA Journal 2021;19(1):6343. doi:10.2903/j.efsa.2021.6343

14. Stanford University. Mealworms and polystyrene degradation research. Stanford News

15. Nutritional composition of mealworm larvae and crickets with potential usage in feed. SciELO

16. Chemical composition of mealworm larvae reared in Serbia (Contemporary Agriculture, 2019). Reference Global

17. Tetlow E. Manipulation of the nutritional composition of Tenebrio molitor larvae for aquaculture (University of Nottingham, 2019). Nottingham ePrints