Previous encapsulation response enhances within individual protection against fungal parasite in the mealworm beetle Tenebrio molitor

Effect of Juvenile Hormone on Resistance against Entomopathogenic Fungus Metarhizium robertsii Differs between Sexes

Juvenile hormone has been suggested to be a potential mediator in the trade-off between mating and insects’ immunity. Studies on various insect taxons have found that juvenile hormone interferes with humoral and cellular immunity. Although this was shown experimentally, studies using highly virulent parasites or pathogens are lacking so far. In this study, we tested if juvenile hormone administration affected resistance against entomopathogenic fungi, Metarhizium robertsii, in the mealworm beetle, Tenebrio molitor. In previous studies with T. molitor, juvenile hormone has been found to reduce a major humoral immune effector-system (phenoloxidase) in both sexes and decrease the encapsulation response in males. Here, we found that juvenile hormone administration prolonged survival time after infection with M. robertsii in males but reduced survival time in females. This study indicates that the effects of juvenile hormone on insect immunity might be more complicated than previously considered. We also suggest that there might be a trade-off between specific and non-specific immunity since, in males, juvenile hormone enhances specific immunity but corrupts non-specific immunity. Our study highlights the importance of using real parasites and pathogens in immuno-ecological studies.

Immune Defenses of a Beneficial Pest: The Mealworm Beetle, Tenebrio molitor

The mealworm beetle, Tenebrio molitor, is currently considered as a pest when infesting stored grains or grain products. However, mealworms are now being promoted as a beneficial insect because their high nutrient content makes them a viable food source and because they are capable of degrading polystyrene and plastic waste. These attributes make T. molitor attractive for mass rearing, which may promote disease transmission within the insect colonies. Disease resistance is of paramount importance for both the control and the culture of mealworms, and several biotic and abiotic environmental factors affect the success of their anti-parasitic defenses, both positively and negatively. After providing a detailed description of T. molitor's anti-parasitic defenses, we review the main biotic and abiotic environmental factors that alter their presentation, and we discuss their implications for the purpose of controlling the development and health of this insect.

An Invertebrate Host to Study Fungal Infections, Mycotoxins and Antifungal Drugs: Tenebrio molitor

Faced with ethical conflict and social pressure, researchers have increasingly chosen to use alternative models over vertebrates in their research. Since the innate immune system is evolutionarily conserved in insects, the use of these animals in research is gaining ground. This review discusses Tenebrio molitor as a potential model host for the study of pathogenic fungi. Larvae of T. molitor are known as cereal pests and, in addition, are widely used as animal and human feed. A number of studies on mechanisms of the humoral system, especially in the synthesis of antimicrobial peptides, which have similar characteristics to vertebrates, have been performed. These studies demonstrate the potential of T. molitor larvae as a model host that can be used to study fungal virulence, mycotoxin effects, host immune responses to fungal infection, and the action of antifungal compounds.

Establishment Success of the Beetle Tapeworm Hymenolepis diminuta Depends on Dose and Host Body Condition

Parasite effects on host fitness and immunology are often intensity-dependent. Unfortunately, only few experimental studies on insect-parasite interactions attempt to control the level of infection, which may contribute substantial variation to the fitness or immunological parameters of interest. The tapeworm Hymenolepis diminuta—flour beetle Tenebrio molitor model—has been used extensively for ecological and evolutionary host–parasite studies. Successful establishment of H. diminuta cysticercoids in T. molitor relies on ingestion of viable eggs and penetration of the gut wall by the onchosphere. Like in other insect models, there is a lack of standardization of the infection load of cysticercoids in beetles. The aims of this study were to: (1) quantify the relationship between exposure dose and establishment success across several H. diminuta egg concentrations; and (2) test parasite establishment in beetles while experimentally manipulating host body condition and potential immune response to infection. Different egg concentrations of H. diminuta isolated from infected rat feces were fed to individual beetles 7–10 days after eclosion and beetles were exposed to starvation, wounding, or insertion of a nylon filament one hour prior to infection. We found that the establishment of cysticercoids in relation to exposure dose could be accurately predicted using a power function where establishment success was low at three lowest doses and higher at the two highest doses tested. Long-term starvation had a negative effect on cysticercoid establishment success, while insertion of a nylon filament and wounding the beetles did not have any effect compared to control treatment. Thus, our results show that parasite load may be predicted from the exposure dose within the observed range, and that the relationship between dose and parasite establishment success is able to withstand some changes in host body condition.

A dark cuticle allows higher investment in immunity, longevity and fecundity in a beetle upon a simulated parasite attack

Cuticle melanism in insects is linked to a number of life history traits: a positive relationship is hypothesized between melanism, immune function, fecundity and lifespan. However, it is not clear how activation of the immune system affects trade-offs between life history traits in female mealworm beetles (Tenebrio molitor) differing in cuticle melanization. The females with tan, brown and black cuticles examined in the present study did not differ in the intensity of encapsulation response, fecundity and longevity when their immune system was not activated. However, we found that immune activation and cuticle melanization have a significant effect on life history traits. Offspring number and lifespan decreased in females with tan and brown cuticles, while the fecundity and lifespan of black females were not affected. Importantly, we inserted the implants again and found a significant decrease in the strength of encapsulation response in females with tan and brown cuticles. In contrast, black females increased melanotic reactions against the nylon implant, suggesting immunological priming. The results show that cuticle melanization plays an important adaptive role under the risk of being infected, while the lack of these benefits before the insertion of nylon monofilaments suggests that there are costs associated with an activated immunity system.

Early-life temperature modifies adult encapsulation response in an invasive ectoparasite

Immunity of parasites has been studied amazingly little, in spite of the fact that parasitic organisms, especially the arthropod parasites, need immunity to survive their own infections to successfully complete life cycles. Long-term effects of challenging environmental temperatures on immunity have remained unstudied in insects and parasites. Our study species, the deer ked (Lipoptena cervi; Linnaeus 1758), is an invasive, blood-feeding parasitic fly of cervids. Here, it was studied whether thermal stress during the pupal diapause stage could modify adult immunity (encapsulation capacity) in L. cervi. The effect of either a low temperature or high temperature peak, experienced during winter dormancy, on encapsulation response of active adult was tested. It was found that low temperature exposure during diapause, as long as the temperature is not too harsh, had a favourable effect on adult immunity. An abnormal, high temperature peak during pupal winter diapause significantly deteriorated the encapsulation capacity of emerged adults. The frequency and intensity of extreme weather events such as high temperature fluctuations are likely to increase with climate change. Thus, the climate change might have previously unknown influence on host-ectoparasite interactions, by affecting ectoparasite's immune defence and survival.

Effects of food quality on trade-offs among growth, immunity and survival in the greater wax moth Galleria mellonella

The resources available to an individual in any given environment are finite, and variation in life history traits reflect differential allocation of these resources to competing life functions. Nutritional quality of food is of particular importance in these life history decisions. In this study, we tested trade-offs among growth, immunity and survival in 3 groups of greater wax moth (Galleria mellonella) larvae fed on diets of high and average nutritional quality. We found rapid growth and weak immunity (as measured by encapsulation response) in the larvae of the high-energy food group. It took longer to develop on food of average nutritional quality. However, encapsulation response was stronger in this group. The larvae grew longer in the low-energy food group, and had the strongest encapsulation response. We observed the highest survival rates in larvae of the low-energy food group, while the highest mortality rates were observed in the high-energy food group. A significant negative correlation between body mass and the strength of encapsulation response was found only in the high-energy food group revealing significant competition between growth and immunity only at the highest rates of growth. The results of this study help to establish relationships between types of food, its nutritional value and life history traits of G. mellonella larvae.

Resource availability as a proxy for terminal investment in a beetle

Terminal investment hypothesis is a longstanding theoretical idea that organisms should increase their reproductive effort as their prospects for survival and reproduction decline. However, numerous attempts to test the terminal investment in reproduction have yielded contradictory results. This study reports an experimental confirmation of the terminal investment hypothesis. It was predicted that immune-challenged yellow mealworm beetles (Tenebrio molitor) are more likely to follow terminal investment strategy when their food resources are limited. Our results suggest the key role of food resources while making decisions to follow a terminal investment strategy. We found that male individuals invested in their sexual attractiveness at the expense of immune response and survival when food was not available. In contrast, the beetles did not decrease their lifespan and did not invest in the attractiveness of their sex odours under conditions of food ad libitum. Our results show the importance of food availability and quality in understanding the evolution of reproductive strategies.