The yellow mealworm, Tenebrio molitor L. (Coleoptera: Tenebrionidae), is a critically important insect species with both beneficial and detrimental role for humans; it can therefore be described as a "beneficial pest" (Vigneron et al. 2019). As a pest species, it infests stored cereal products (Bjørge et al. 2018), but it is also one of the most valuable and high-nutritional food insects; it's used in the animal and human food industries and consequently is currently also being promoted as a beneficial insect. It's capable of degrading polystyrene and plastic waste (Bjørge et al. 2018; Vigneron et al. 2019), and it's also used as a model organism in scientific research (Martynov and Brygadyrenko 2018; Brai et al. 2023). The high-nutritional aspect of T. molitor makes it an attractive option for the mass production of food. However, the high-density rearing conditions associated with mass production may increase the risk of disease and infections by parasitic symbionts and promote their spread within these insect colonies. These, in turn, may be passed to humans and animals consuming the mealworms as food (Siemianowska et al. 2013; Vigneron et al. 2019; Slowik et al. 2023).

Gregarines are protozoans belonging to the phylum Apicomplexa and are parasites of many different groups of invertebrates. The effects of gregarines on their hosts have been the subject of numerous experimental studies and have been shown to be both detrimental and beneficial. Their negative effects include arresting development, causing mortality, and inhibiting reproduction in their hosts (Lazurska and Brygadyrenko 2024). Gregarines can also be beneficial, increasing host growth, fitness, and longevity (Slowik et al. 2023). Multiple gregarine species have been recorded in the digestive tract of T. molitor.

As T. molitor is both harmful to products in storage and beneficial in the mass production of a primary protein source for human and animal nutrition, it is regarded as a target organism for the study of gregarine pathogens in different geographic populations. In this study, we investigated the presence of gregarines in two populations of T. molitor obtained from Georgia and Türkiye.

Samples of adult and larval yellow mealworms, T. molitor, were provided from insect-rearing facilities in Tbilisi, Georgia, and Antalya, Türkiye. Forty samples of both larval and adult individuals were dissected in a physiological solution, and their intestines were examined under a microscope for protozoan parasites at a magnification of 10x to 1000x (Yaman, 2020). When a parasitic infection was observed, part of the material was used to prepare Giemsa-stained smears. For this, the slides were air-dried and fixed with methanol, then stained with a freshly prepared 5% solution of Giemsa stain and re-examined under the microscope. The parasite's life stages detected by light microscopy were measured and photographed using a microscope with a digital camera and a Soft Imaging System. The following structures were measured: length of epimerite (LE), length of deutomerite (LD), length of protomerite (LP), total length (TL), width of deutomerite (WD), width of protomerite (WP), the ratio of the length of the protomerite to the total length (LP:TL), and the ratio of the width of the protomerite to the width of the deutomerite (WP:WD).

During the study, gregarine infection was only observed in the Tbilisi (Georgia) population of T. molitor. Several life stages of the gregarine parasite were found in the midgut lumens of both the larvae and adults of the yellow mealworm (Figs 1, 2, 3). Each trophozoite was elongate with a bud-type epimerite (5.5 µm) (Fig. 1). The largest trophozoite that could be observed was 67.9 µm. The average measurements of the trophozoite are given in Table 1.

Sporadins found in the midgut lumen of the host were solitary (Fig. 2). The septum between the protomerite and the deutomerite was clearly distinct. The deutomerite was ovoid to ellipsoid, with a spherical nucleus, 10.3 µm in average diameter. The average measurements of the sporadins are given in Table 1.

Sporonts were biassociative and ovoid to ellipsoid. The primite had a conical to hemispherical protomerite. The septum was clearly distinct. The deutomerite was ovoid (Fig. 3). The measurements of the associations are given in Table 1.

As seen in Table 2, gregarine infection rates in T. molitor larvae were higher than in the adults. Twenty-nine of the 40 examined larvae were found to be infected, an infection rate of 72.5%. However, only seven of the 40 examined adults were found to be infected, an infection rate of 17.5%. The results indicate that T. molitor larvae are more susceptible to the gregarine parasite than the adults. In contrast, there was no gregarine infection in either the larvae or adults of T. molitor from the Antalya, Türkiye, population.

According to Clopton et al. (1991) and Slowik et al. (2023), members of the subclass Gregarinasina, commonly known as gregarines, including eugregarines and neogregarines, are a well-studied protist group infecting T. molitor. In a recent review on the harmful and beneficial symbionts of T. molitor by Slowik et al. (2023), amongst the eugregarines, four species are known to infect this species: Gregarina cuneata, G. polymorpha, G. niphandrodes, and G. steini, of which G. cuneata appears to be the most impressive and complex among them.

A well-documented comparative study by Clopton et al. (1991) of gregarine species infecting T. molitor has described their morphology, general appearance, morphological measurements, and ratios. Using these descriptions, the gregarine parasites from the T. molitor in our study are similar to the descriptions given of both G. cuneata and G. niphandrodes in terms of their body morphology and general appearance. However, they differ from the description in both morphological measurements and ratios.

Clopton et al. (1991) also report that G. polymorpha and G. steini are only found in the larvae of T. molitor, whereas G. niphandrodes was reported only from adults. Gregarina cuneata, in contrast, is reported to infect both larvae and adults of T. molitor (Stein, 1848; Schneider, 1875).

As the gregarine parasites found in this study were in both larvae and adults of T. molitor, our results suggest it is possible that more than one gregarine species might be infecting the Georgian population of this mealworm.