Effect of parental age and temperature

Offspring produced by older beetles have shorter larval stages than those produced by younger beetles. Larvae from the older beetles also show a rapid weight increase at an earlier age than those from young parents. At 25°C, the larval stage was shortened, the number of larval molts decreased, and the durations of adult life decreased when parental age increased compared to the beetles at 30°C.^[9] Another study found that at 20°, 25°, and 30°C, parental age does not have any effect on the duration of the egg stage or the weights of the eggs.

However, the amount of hatched eggs decreased when parental age increased. When eggs were laid during the first two months after emergence, approximately 90% of the eggs hatched, but when they were laid after four months, only about 50% hatched.^[9] It was also found that larvae from young parents grow at a slower rate compared to larvae produced by the same parents nine weeks earlier. At 30°C, there were no other effects of parental age on the larvae. However, at 20° and 25°C, the larvae from young parents required significantly more time to complete development and had more molts compared to the larvae from the same parents after they had aged one more or longer. Additionally, the duration of adult life decreased when parental age increased.^[9]

Immunocompetence

Evidence suggests that in many animal species, secondary sexual traits reflect male immunocompetence, the ability of an individual’s immune system to resist and control pathogens or parasites. A study found that a single parasite-like immunological challenge, created via a nylon monofilament implant in the beetle, significantly reduced the sexual attractiveness and locomotor activity of males, it did not negatively affect their survival. When the inserts were removed, the majority of the males showed greater encapsulation responses of the implant, though some of the males seemed to have already chosen a terminal reproductive investment strategy.^[12] And thus, the majority of males invest in their immune system after the first challenge.

A second immune challenge increased their attractiveness, but was found to significantly reduce locomotor activity of the males and increase their mortality. This represents a trade-off between pheromone production and energy required for activities such as immune system recovery and locomotor activity.

When there was a third challenge (implantation) in the same males, there was a lower encapsulation rate of the nylon implants in more attractive males than the less attractive, showing how the males made no attempts to boost their immune system.^[12] The results suggest that the males who become sexually attractive after the second immune challenge have a trade-off where they sacrifice locomotor activity and do not energetically invest in immune system recovery. This shows how the female mealworm beetles consistently preferred males who invested significantly less in immune system recovery and that males are not able to allocate resources simultaneously to both improving their health, or in this case, recovery of their immune system, and to increasing their sexual attractiveness.^[12]

Nutritional condition

Pheromones are chemical signals that function as mate attractors and relay important information to prospective mates. For a reliable signal, it must be costly to produce, which means it is likely to have condition dependent expression. A study found that female preference of the pheromones was dependent on the nutritional condition of the males, shown by how they spent significantly more time with males who received constant food than males who received no food.^[13] It was also found that phenoloxidase activity was dependent on the nutritional condition of the males, with phenoloxidase activity being two to six times higher in males with constant food than in males who received no food. However, nutritional conditions had no effect on the encapsulation rate of the males.^[13]

When receiving constant food, male initial body mass had no correlations with phenoloxidase activity or encapsulation rate. This shows that pheromone mediated attractiveness and the immunocompetence in terms of phenoloxidase activity of males were condition-dependent, as both decreased with nutritional stress. This suggests that there is a trade-off between allocation of resources and energy into the production of pheromones and immunocompetence and that the production of pheromones are condition dependent sexual traits.^[13]

Behavioral immunity

When the mealworms feed on infected rodent feces, they may consume the eggs of the tapeworm parasite Hymenolepis diminuta. Infected male beetles pay a higher reproductive cost than the female beetles. T. molitor displays behavioral immunity when exposed to H. diminuta, shown by how the infected males develop an avoidance behavior for feces that harbor the tapeworm, which decreases their probability of coming into contact with the tapeworm in the future.^[14] The female mealworm beetles develop a qualitative resistance through mate choice, as they are able to evaluate male immunocompetence through pheromone signaling, allowing them to choose the more immunologically fit males as mates. This also reduces the probability of the females being infected by their mates and may cause them to pass on an enhanced level of immunocompetence to their offspring.^[14]

Another way mealworms may display behavioral immunity is how they may tolerate infections by limiting negative effects on their reproductive success. For example, mealworm beetles tolerate a high number of cysticercoids of H. diminuta at the expense of their own fitness and longevity. But the males produce improved spermatophores that contain superior nuptial gifts that will be passed to their mating females, increasing female fecundity and causing a greater number of eggs to be fertilized.^[14]

Cuticular color

Cuticular color, a heritable component, of the mealworm beetle varies from tan to black. In the mealworm beetle, evidence suggests that population level variation in cuticular color is linked to pathogen resistance in that darker individuals are more resistant to pathogens. A study found that two immune parameters related to resistance, haemocyte density and pre-immune challenge activity of phenoloxidase, were significantly higher in selection lines of black beetles compared to tan lines. Higher haemocyte density is likely indicative of a heightened immune response.^[15]

There were no effects of gender on the immune traits. Cuticular color is dependent on melanin production, which requires phenoloxidase, an enzyme that is present in its inactive form inside haemocytes. This shows why darker insects have a heightened immune response and are more resistant to pathogens that invade the hemocoel via the cuticle. However, there was no significant difference in haemolymph antibacterial activity between black and tan lines, explained by how antimicrobial peptides are produced by haemocytes but are not involved in cuticular darkening.^[15]

In T. molitor, the degree of cuticular melanization is a strong indicator of resistance to the entomopathogenic fungus Metarhizium anisopliae, which could be explained by the thicker and less porous cuticle displayed by darker insects compared to lighter ones. However, there seems to be underlying trade-offs that prevent the fixation of the darker phenotype, shown by how the plasticity of melanization phenotypes in response to population density may contribute to the absence of predominance of darker individuals among T. molitor populations.^[14]

Food restriction

Because immune defenses against parasites and pathogens require metabolic resources, food restriction can impair immune function of T. molitor. For adult T. molitor beetles, phenoloxidase activity activity can be reduced by half during short-term food deprivation but returns rapidly to original levels when the beetles are given access to food again.^[14] Additionally, T. molitor larvae can eat five times more food per day than usual following an immune challenge to compensate for the caloric loss from the immune response. These immune challenged larvae also show significant weight loss when fed with either protein or carbohydrate rich diets but show stable weights when given both protein and carbohydrate-rich diets.^[14]

Healthy T. molitor larvae usually prefer diets with a lower protein to carbohydrate ratio but shift toward food with higher protein contents after an immune challenge with bacteria. This causes enhanced hemocyte circulation and antibacterial activity in the hemolymph, which likely maximizes resistance against bacterial infection. However, phenoloxidase activity is not affected by this shift in diet choice.^[14]

A study found that the effects of nutritional imbalance on body composition were buffered by the subsequent selection of complementary foods. This demonstrates that the mealworm beetles are capable of compensating for nutritional imbalances and that the way nutritional balance is restored depends on the nutrient that is initially deficient in their food.^[16] For example, if the beetles were previously fed a protein-rich, carbohydrate-deficient diet, they would prefer carbohydrates to protein, whereas beetles fed a carbohydrate-rich, protein-deficient diet, they would strongly prefer a protein-rich diet. They found that self-selecting T. molitor beetles recovered from carbohydrate or protein deficiency within six days by selecting the complementary diet.^[16]

As feed and pet food

Mealworms are typically used as a pet food for captive reptiles, fish, birds, and some small mammals. They are also provided to wild birds in bird feeders, particularly during the nesting season. Mealworms are useful for their high protein content. They are also used as fishing bait.^[18]

They are commercially available in bulk and are typically available in containers with bran or oatmeal for food. Commercial growers incorporate a juvenile hormone into the feeding process to keep the mealworm in the larval stage and achieve an abnormal length of 2 cm or greater.^[19]

As food

Mealworms are edible for humans, and processed into several insect food items available in food retail such as insect burgers.^[20]

Mealworms have historically been consumed in many Asian countries, particularly in Southeast Asia. There, they are commonly found in food markets and sold as street food alongside other edible insects. Baked or fried mealworms have been marketed as a healthy snack food in recent history, though the consumption of mealworms goes back centuries.^[4]

In May 2017, mealworms were approved as food in Switzerland.^[21] In June 2021, dried mealworms were authorized as novel food in the European Union,^[22] after the European Food Safety Authority assessed the larvae as safe for human consumption.^[23][24]

Mealworm larvae contain significant nutrient content.^[18] For every 100 grams of raw mealworm larvae, 206 kilocalories and anywhere from 14 to 25 grams of protein are contained.^[25] Mealworm larvae contain levels of potassium, copper, sodium, selenium, iron and zinc that rival those of beef. Mealworms contain essential linoleic acids as well. They also have greater vitamin content by weight compared to beef, B12 not included.^[25][26]

Mealworms may be easily reared on fresh oats, wheat bran or grain, with sliced potato, carrots, or apple as a moisture source. The small amount of space required to raise mealworms has made them relevant for scalable industrialized mass production.^[27]

In waste disposal

In 2015, it was discovered that 100 mealworms can degrade polystyrene into usable organic matter at a rate of about 34–39 milligrams per day. Additionally, no difference was found between mealworms fed only Styrofoam and the mealworms fed conventional foods, during the one-month duration of the experiment.^[28] Microorganisms inside the mealworm's gut are responsible for degrading the polystyrene, with mealworms given the antibiotic gentamicin showing no signs of degradation.^[29] Isolated colonies of the mealworm's gut microbes, however, have proven less efficient at degradation than the bacteria within the gut.^[29]