Keeping and Raising Mealworms Easy for Busy People

Mealworms are a popular feeder insect for bearded dragons, known for their ease of care. Whether you’re breeding them at home or simply keeping pre-purchased mealworms alive and healthy for feeding, this guide will help streamline the process for busy pet owners.

Mealworms are not worms, they are the larval stage of the darkling beetle, Tenebrio molitor. This is an important distinction because larvae have a high fat content designed to take them through metamorphosis.

Mealworms are scavengers and omnivores (Dellinger and Day, 2018) eating pretty much anything. In fact, they will even eat polystyrene and plastics (Brandon et al., 2018) which is being researched for its benefits from an environment perspective.

In the wild their diet will include decaying vegetation such as leaves and grasses, feces and grains; decomposing animals and even feces. Some darkling beetles are considered pests. They are very prolific breeders but they have an insurmountable number of predators such as lizards, birds, spiders, rodents and humans to keep them in check.

Key Points

• Mealworms are omnivores and scavengers (Dellinger and Day, 2018).
• Mealworms thrive in high-density populations.
• Mealworm larvae, pupae, and adult beetles can all be used as food for reptiles.
• Mealworms can eat various substances, including organic waste and even plastic (Brandon et al., 2018).
• To keep them nutritious for your reptiles, proper food, hydration, and environment are essential.
• Just as with bearded dragons, mealworms are ectotherms, so they depend on their environment for heat.
• Darkling beetles are prolific breeders.

Mealworm Life cycle

Mealworms go through metamorphosis from eggs to larvae to pupae and eventually hatch as beetles.

Eggs

• Eggs are small, bean-shaped, and usually hatch within 4 to 19 days, depending on the temperature.
• At an optimal temperature of 25°C (77°F), eggs typically hatch in 7 to 8 days (Park et al., 2014) to as long as 14 days (Ghaly and Alkoaik, 2009).

Larvae

Mealworms develop a darker, more durable chitinous exoskeleton as they mature. A few days before pupating mealworms usually move to the surface of their substrate.

• Larvae grow from a tiny size to about 2-3.5 cm in length where it will go into pupae stage weighing about 0.2 grams (Ghaly and Alkoaik, 2009; Ghaly and Alkoaik, 2009).
  • Lifespan: At 25°C (77°F), the typical lifespan of larvae is about 5 months (Tracey cited by Fiore, 1960) but it can up to 8 months under optimal conditions. With less favorable environments, it can extend to 2 years (Ghaly and Alkoaik, 2009).
  • Instars: Larvae undergo several molts, called instars. At 25°C, larvae typically experience 14 to 20 instars, with most pupating between the 15th and 17th instar (Park et al., 2014). Higher temperatures, like 30°C, result in more instars, averaging 19.1 instars (Ludwig, 1956).
  • Factors Impacting Instars:
    • Parental age: Older parents shorten the larval stage (Ludwig, 1956; Tracey, 1958).
    • Population density: Higher density promotes faster development (Andersen et al., 2017).
    • Humidity and temperature: Both significantly affect growth, with optimal conditions leading to fewer instars (Murray, 1968; Ludwig, 1956).
    • Food quality: Poor-quality food can delay development (Stellwaag-Kittler, 1954).

Pupa

• When the mealworm is ready to enter pupal stage it moves to the surface (Singh, 1975).
• The pupal stage typically lasts about 7 days at 25°C (77°F) (Andersen et al, 2017), though it can be as long as 9 months if it is subjected to winter conditions.
• The pupa has a light casing which does not provide a great deal of protection. The pupa will rotate its abdomen when touched. This little wiggle is a defense mechanism against the colony eating it. When Ichikawa and Toshiaki (2009) blocked the nerve enabling that movement the majority of the pupa were killed within 3 days by the colony.

Beetle

• The adult darkling beetle measures 1.25 to 1.8 cm.
• Beetles start with a light-color and darken over the first week to a black or brown.
• Beetles live for several months and are capable of breeding prolifically.

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Breeding Mealworms

Breeding mealworms for a steady supply of feeders requires the right conditions to ensure healthy reproduction.

The Mealworm Colony

• Population Density: Mealworms thrive in high-density setups even though it can reduce the number of young (Morales-Ramos et al., 2012).
  The recommended density of adult beetles to attain maximum reproduction output is 8.4 adults per dm² (Morales-Ramos, 2012). This is around 1,700 beetles or 200 grams of beetles (weighing 0.13 g each) in a container 40 x 60 cm (Andersen et al., 2017).
  High density can promote faster growth, though it may lead to egg cannibalism (Morales-Ramos et al., 2012).
• Mating: Beetles are ready to mate by the 2nd or 3rd day after emerging from pupae and are most productive at 2-3 weeks of age (Gerber, 1973; Morales-Ramos et al., 2012).
  Mating with different males increases larvae production (Worden and Parker, 2001).
  The male transfers a packet of semen into the female. In a few day the female will burrow and lay her eggs. The eggs are covered in a sticky secretion which the substrate sticks to (Singh, 1975).
  Worden and Parker (2001) observed that mating with different males almost doubled larvae production compared to those mating with the same male.
• Egg Production: Females lay 5 to 8 eggs per day, producing 300 eggs over their lifetime (Andersen et al., 2017). Other research shows 250 to 1,000 eggs, depending on factors like environmental conditions and egg cannibalism (Ghaly and Alkoaik, 2009).
  In high-density environments, beetles may eat their own eggs, which reduces the number of offspring (Morales-Ramos, 2012). However, a high population is still recommended for optimal reproduction.

Mealworm Cuticule Color & Pruning Out the Weaker Colors

The color of darkling beetles’ cuticle is a strong indicator of their ability to defend against pathogens and parasites, especially fungal infections. Darker cuticles are thicker and provide better protection (Silva et al., 2016). Research shows that darker beetles have stronger defenses against pathogens (Evison, 2017; Armitage and Siva-Jothy, 2005; Barnes and Siva-Jothy, 2000).

Because cuticle color is hereditary (Armitage and Siva-Jothy, 2005), you can improve your colony’s resilience by removing lighter-colored beetles from your breeding stock. Keeping only beetles that mature to the typical black or brown color can help strengthen the colony’s overall defenses.

Mealworm Inbreeding

Inbreeding can cause smaller beetles and slow down their development. It also weakens their immune system (Rantala et al., 2011). To prevent this, regularly introduce new beetles into the colony if it is to be kept over multiple generations. Females are naturally more attracted to pheromones from outbred males, which helps keep the colony healthy (Polkki, 2012).

For small colonies, such as those kept by most pet owners, it might be easier to let the colony run its course and then start fresh. However, if you’re keeping a colony long-term, bringing in new stock will help avoid the negative effects of inbreeding.

Interestingly, while females prefer outbred males, males show no preference between inbred or outbred females (Polkki, 2012). Males are more attracted to the pheromones of mature, virgin females (Carazo et al., 2004, cited in Polkki et al., 2012).

Mealworm Colony Housing

Container Size:

• Use a 36 to 52 litre container. 1,700 beetles comfortable fit into a 36 litre container (Andersen et al., 2017). As a guage, a 52 litre container will accommodate approximately 1500 to 2000 mealworm larvae with a 2 inches of food substrate if they were at their peak weight of 0.2 grams (adapted from Andersen et al., 2017).
• Keeping high density improves the beetle cuticle and makes them less vulnerable to fungus (Barnes and Siva-Jothy, 2000).

High Density Benefits:

• A high-density population helps larvae grow faster, as they don’t have to move far for water. It also improves their overall health (Andersen et al., 2017).
• Beetles raised in high-density environments develop stronger, darker cuticles, making them more resistant to fungus and other pathogens (Barnes and Siva-Jothy, 2000).

Dark Environment:

• Mealworms prefer darkness. Cover the substrate with layers of paper or a piece of cardboard to block out light. This also encourages them to use all layers of their food substrate.
• Avoid disturbing them unless necessary. When exposed to light, they will quickly scatter.

Temperature & Humidity

Just as with bearded dragons, mealworms are ectotherms, so they depend on their environment. To maximize the health and reproductive potential of the colony provide the right temperature and humidity.

Temperature:

• For busy pet owners who only need a few mealworms at a time: In the temperature conditions that humans typically live in a house a mealworm can also survive in. It does not need special heating unless you live in temperature conditions which a mealworm cannot survive in or you wish to improve the colony productivity. For busy pet owners who simply wish to maintain sufficient feeder insects for 1 or 2 reptiles, you may not need to provide heating. Placing a mealworm colony house on top of the bearded dragons enclosure or similar may provide sufficient heat.
• To maximise the colonies productivity:
  • The ideal temperature range for mealworms is 22°C to 28°C (72°F to 82°F), with optimal conditions at 25°C (77°F) (Punzo, 1975).
  • The lethal upper limit is ≥42°C (107°F). At this temperature, beetles can die within 24 hours (Punzo, 1975).
  • At 7°-8°C (45°F-46°F), mealworms enter a chill-coma, and young larvae won’t survive for long at these low temperatures (Punzo, 1975).
  • For breeding, maintain temperatures between 25-27.5°C (77°F-82°F) for the best reproductive outcomes (Hein and Tracey cited in Fiore, 1960).

Humidity:

• The optimal relative humidity is 75%, but mealworms can cope with a range between 50% and 75% without major issues (Punzo, 1975).
• Maintaining humidity in this range promotes growth and egg-laying while reducing the risk of fungal infections (Andersen et al., 2017).
• High humidity will create conditions that encourage other organisms to infect the environment, not the least being fungi.

Temperature and Immune System:

• Extreme temperatures, whether too high or too low, can stress mealworms and impact their immune systems. This stress can also affect the next generation (Vigneron, 2019).

Feeding & Watering

Watering Mealworms

Mealworms need water a source of water. Water plays a key role in how quickly larvae grow (Ozsoy, 2019; Hopkins, 1973). They get water from both humidity and direct sources like water or moisture-rich foods. They do not need water from a water container such as crickets might.

• Water Use: Mealworms are efficient with water. Their droppings are tiny, dry pellets because they reabsorb fluids before excreting (Jurenka, 2019).
• Humidity: Mealworms can extract moisture from the air (Buxton, 1930; Cloudsley-Thompson, 1964; Mellanby, 1932, 1958 cited in Punzo, 1975) with the water content of the body increasing or decreasing in line with the relative humidity.
  The best conditions for larvae are 25°C (77°F) and 75% relative humidity (Buxton, 1930; Cloudsley-Thompson, 1964).
• Water Needs: The amount of water needed depends on both the humidity and the type of food. Foods like bran require more water for mealworms to grow properly, while wholemeal flour needs less (Murray, 1968). As the bearded dragon is our intended consumer, pollard is the desired substrate for mealworms. Vegetation will need to be provided with pollard.

Providing Fluids:

• Carrots are commonly used however they don’t offer optimal hydration.Vegetables like cucumber, watermelon, and lettuce can be a better source of water.
• There’s a risk of pesticides, so always wash fruits and vegetables before feeding (Spencer and Spencer, 2006).
• Best Practice: Replace fruits and vegetables every couple of days and remove uneaten portions daily to prevent spoilage. For small colonies it is easy to replace provide small quantities of vegetable scraps daily when preparing the dinner at night. If only small amounts are fed there will be little if anything to spoil or clean up by the time you feed them the next day.

Other Watering Methods:

• Vegetation is the typical way to provide water. However, if you have concerns that the level of moistsure being is inadequate then provide moisture by soaking cotton pads and place them on a flat dish. Do not place wet pads directly on the dry food substrate. Keeping mealworms properly hydrated can speed up their growth by up to 2 weeks.

Feeding Mealworms

• Food as Substrate: Mealworms’ food is also their bedding. Fine cereal products work best, especially pollard, as they make it easier to sift out the mealworms compared to larger flakes like oats.
• Pest Prevention: Freezing the substrate for 2 days before use helps kill pests like moths. It’s hard to fix a container contaminated with other insects once it happens, so prevention is key.
• Food Restriction: Limiting food can weaken mealworms’ immune system, but it returns to normal once food is restored (Siva-Jothy and Thompson, 2002). To keep them healthy and productive, avoid restricting food.

Hygiene:

• Keeping the environment clean is essential, especially in dense populations. Replace the substrate every 7 to 10 days to maintain a healthy colony (Spencer and Spencer, 2006).

Can I Keep Mealworms in the Countainer they Arrive In?

Mealworms generally come in a small container. They can continue to live in that container for weeks however:

• It is not adquate for a healthy colony.
• There will not be enough room for the mealworms to thrive.
• The container is likely full of frass (insect excrement).
• The substrate should be changed to the one you choose to feed for your reptile as the consumer.

Keeping Mealworms in the Fridge

With good care, your mealworm colony can grow rapidly, and you might find yourself needing to slow down their production. Refrigeration can be a way to manage their growth, but it’s not ideal for their long-term health. Mealworms should be used within 5 days after refrigeration to avoid significant losses.

• Recommendation: For your reptile’s mealworms, keep them in the fridge for no more than 5 days. After that, take them out, feed and water them for 24 hours, then refrigerate again if needed (Sonmez & Koc, 2019).
• Effects of Refridgeration:
  • Fridge Temperature: Mealworms enter a cold coma at +7°C (45°F) (Punzo & Mutchmor, 1980). Standard fridge temperature is 4°C (40°F) (FDA, 2018). While they can survive short-term in the fridge, extended exposure leads to serious health issues.
  • Long-Term Refrigeration Effects:
    Prolonged time in the fridge causes nervous system failure, muscle damage, and deformities in mealworms. These effects worsen the longer they’re stored (Sonmez & Koc, 2019).
    When mealworms were kept at 2°C (36°F) for 15 days, 75% died, and the survivors had a high rate of deformity (Sonmez & Koc, 2019).
  • Short-Term Refrigeration: At 10°C (50°F), mealworms showed minimal losses for up to 48 hours, but young larvae (under 30 mg) begin to die within 6 to 24 hours (Punzo, 1975).

How to Freeze Mealworms for Bearded Dragon Feed

Freezing mealworms is possible, but they won’t provide the same enrichment as live insects. If you are freezing because you wish to reduce the size of your colony then consider giving them to a neighbour with chooks instead.

• Pre-Freezing: If you want to clear their gut before freezing, keep the mealworms at 15°C (59°F) for 1 day to allow them to pass their gut contents. Skip this step if you’ve already gut-loaded them (Andersen et al., 2017).
• Freezing Process:
  • Freeze mealworms at -18°C (0°F) for 2 days (FDA, 2018).
  • Store them in an airtight bag in the freezer until you’re ready to feed them to your bearded dragon.

References

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