Drugstore beetle
The drugstore beetle, also known as the bread beetle, biscuit beetle, and misnamed as the biscuit weevil, is a small brown beetle that infests a wide variety of dried plant products, where it is among the most common non-weevils. It is the only living member of the genus Stegobium. It belongs to the family Ptinidae, which also includes the deathwatch beetle, furniture beetle and cigarette beetle. A notable characteristic of this species is the symbiotic relationship the beetles have with a yeast-like fungus which is transmitted from female to larvae through the oviduct.
The drugstore beetle is distributed worldwide with higher prevalence in warmer climates. It is commonly confused with the cigarette beetle, as they have a similar size and coloring. Adults possess antennae ending in 3-segmented clubs, while cigarette beetles have serrated antennae. Their bodies are lined with grooves running longitudinally along the elytra, whereas the cigarette beetle is smooth.
Description
Adult drugstore beetles are between long and have a reddish-brown colour with a cylindrical body. Larvae are habitually curled. The pupa is proportionally more slender than that of the cigarette beetle.Image:Drugstore beetle 02.jpg|thumb|right|Drugstore beetle on human fingertipBoth males and females appear nearly identical except that males have a sexually dimorphic slot-like structure on the tarsal claws which can only be observed under a microscope. During the pupal stage, males and females can be differentiated as the female genital papillae are bulging outwards and divergent, while the male genital papillae are not protruding and are not as pronounced. As adults carry their genitals within their body, there is no reliable way to distinguish sexes in live specimen beyond the pupal stage.
Food resources
Drugstore beetles primarily feed and cause damage during the larval stage. The drugstore beetle attacks a wide variety of food sources including dried herbs and medicinal plants, earning its name. This species is known for feeding on both food and non-food materials. The beetle eats bread, grain, coffee beans, powdered milk, sweets like cookies and chocolates, spices and herbs, dried fruit, seeds, and more. They also consume museum specimens, wool, hair, leather, books, upholstery, and manuscripts. Adults gnaw through packaging forming large holes in the material, leading to their deterioration, contamination, and loss of structural integrity.Life history
The life cycle of this beetle transitions from the egg, to 4-6 larval instars, the construction of a cocoon for the pupa, and finally, the adult stage. The female can lay up to 75 eggs, depositing them in or near sources of food. Approximately 80% of her eggs end up being fertile. The egg stage lasts around 7–20 days at a temperature between 20 °C and 27.5 °C. The larvae then form a cocoon of silk and food material which they use to pupate. The pupa stays in the cocoon for varying amounts of time before the adult emerges. Between temperatures 20 °C and 27.5 °C, the pupal stage typically lasts from 5–12 days. Temperature and relative humidity play an important role in the growth and development of drugstore beetles, with the optimal range being between 15 °C and 35 °C, and an ideal temperature of 30 °C. Studies have found that increasing the relative humidity or temperature shortened the incubation period for these eggs and the pupal period. At peak conditions, it takes the eggs around 40 days to develop into adults. At the end of this cycle, adults live for about 85 days at 17.5 °C. The specific length of each stage depends on the temperature and food sources available to the beetles.Taxonomy and evolution
The drugstore beetle was originally described by Carl Linnaeus in 1758 as part of his Systema Naturae under the name Dermestes paniceus. It was assigned to the newly created genus Stegobium by Victor Motschulsky in 1860. The drugstore beetle is the only living species in the genus Stegobium. The oldest known member of the genus is Stegobium raritanensis from the Late Cretaceous aged New Jersey amber.Genetics
As a stored product pest, the evolutionary history of the drugstore beetle suggests that it evolved from a wood-feeding ancestor. Studies found that the beetle has 8 autosomal chromosomes and 1 sex chromosome forming a diploid set of 18 chromosomes. The sex determination system for Stegobium paniceum is based on XX-X0 as they lack the Y chromosome. The genome size of this species is estimated to be around 238 to 345 Mb which is significantly smaller than the median genome size of 760 Mb for the Coleoptera order.Mating
Male and female S. paniceum use sex pheromones to attract a mate. Female drugstore beetles utilize stegobinone, a volatile compound, to elicit a pheromonal response that communicates their presence and availability to males. This compound makes it much easier for the drugstore beetle to search for mates, indicates their willingness to mate, and facilitates the mating process by exciting and attracting males. They produce copious amounts of sex pheromones, reaching as much as 50 to 200 ng, allowing the females to maintain the signal for greater distances. It was found that male response to females peaked 5 to 12 days after becoming an adult. For females, their pheromone levels increased after 1 day, levelled off after 5, and lasted at least 14 days. The pheromone is also specific to this species, ensuring the propagation of the drugstore beetle's genes.The proper orientation of the male and female during the mating process follows two experimentally determined phases called pre-mounting and post-mounting. The first phase involves the female proactively directing pre-mounting behaviors such as mate finding and courtship. The latter component involves the male detecting tactile cues by coming into contact with the female's dorsal setae, which are sensory hairs located on her back. The male uses physical touch to obtain information regarding the female's condition and her capacity to mate, which allows him to better engage in mating behaviors. The tarsal claw slots are secondary sex features of males that play a major role in mating behavior, especially in species that do not engage in parental care. The male drugstore beetle uses the claw slots found at the end of their legs to trap the setae on the elytra or forewing of the female. The hairs on the female beetle gets inserted and trapped into the claw slots of the males. This behavior is supported by evidence that when the male dismounts from the female, there is a time lag caused by the male trying to remove its claws from the female. This interaction improves the positioning of the male during mating, preventing him from falling off of the female prematurely, thereby prolonging the duration of mating and enhancing the overall mating process.
Mutualism with fungi
Stegobium paniceum possess large cells with symbionts located in the mycetome and lumen of the intestine. Mycetomes, an organ part of the digestive system, has four lobes in drugstore beetle larvae. As the beetle develops into an adult, six tubular appendages appear from each lobe. Two of the six appendages may exist to remove waste products produced by the symbionts. They also serve to provide nitrogenous waste products to the yeast to promote their growth. The larvae lack symbionts immediately after hatching until they take in the yeast cells orally, infecting the mycetocytes of the midgut mycetome organ. The yeast continue to accumulate throughout the pupal stage. The symbiotic yeast cells found in this species are elongated with a bud attached to one end. The cells were named S. anobii and were classified under the genus Saccharomyces temporarily, but this taxonomic categorization needs further study. The size of the yeast cells ranges from 1.5 to 3.5 micrometers in width and 3 to 6 micrometers in length.The beetles obtain sterols, necessary for their growth, from their diet as well as from the yeast-like fungi they harbor. The main function of the yeast is to provide their host with B-complex vitamins. Multiple theories exist to explain how the drugstore beetle obtains the vitamins from the yeast. The prevailing theory is that as the larvae develops, the yeast cells leave the mycetomes, allowing the host to obtain the essential vitamins by digesting the yeast cells. Another theory claims that the vitamins enter the host by diffusing from the yeast cells to the cytoplasm of the mycetome cells. Experiments discovered that when the yeast cells were eliminated, no larval growth occurred, indicating that the vitamins are essential for host survival. Drugstore beetles rely on the symbiotic relationship they have with the yeast cells housed within their bodies to survive. While rearing their young, females use their oviducts to place the yeast cells on their eggs which are then consumed by larvae after they hatch. The B vitamins that the yeast produce are essential for the survival of the larvae, making it possible for them to exist in areas with food of poorer nutritional value. This symbiotic relationship increases the chance of survival for the larvae even in environments with scarce resources. As the beetle's lifespan increases, the yeast also live for longer. The yeast cells are able to propagate themselves using the beetles, spreading through the beetle's offspring. The drugstore beetle act as a source of protection for the yeast as well. Both groups benefit greatly from their coexistence in this symbiotic relationship.
Pest control
The oldest records of the modern drugstore beetle as a pest are known from the Bronze Age of Akrotiri, Santorini, Greece around 1500 BC where it was found associated with stored pulses.Efficacy of essential oils
The cons of insecticides, increasing resistance to compounds, and high costs were incentives that drove researchers to discover alternate means of pursuing pest control. In the search for decent alternatives, scientists have found that carbon dioxide gas and essential oils from plants could be used to safely regulate Drugstore Beetle numbers. This transition away from synthetic insecticides to plant based insecticides was meant to resolve some of the health concerns associated with synthetic compounds. Studies were conducted to investigate the usefulness of an essential oil from Z. bungeanum Maxim on Drugstore Beetle larvae and adults. Results showed that beetles that were given higher doses of these oils had higher rates of mortality. The benefit of using essential oils is that they exhibit low mammalian toxicity which makes it a promising alternative to synthetic insecticides. Z. bungeanum Maxim was shown to have high repellent activity which points to its promise as an effective means of pest control.Drugstore Beetles are a pest that severely impacts Chinese medicinal materials. The essential oil from Z. bungeanum Maxim provides a safer alternative for pest control. It has the capacity to prolong the time it takes for larvae to develop and can even keep adults from laying eggs successfully, effectively controlling pest numbers. Both adults and larvae were impacted by the oils and any nuances in their responses was attributed to differences in morphology and behavioral response. The essential oil can be used as a natural pest control agent with healthier and safer implications compared to other means of pest management.