Red flour beetle

Red flour beetle
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Coleoptera
Family:	Tenebrionidae
Genus:	Tribolium
Species:	T. castaneum
Binomial name
Tribolium castaneum (Herbst, 1797)

The red flour beetle (Tribolium castaneum) is a species of beetle in the family Tenebrionidae, the darkling beetles. It is a worldwide pest of stored products, particularly food grains, and a model organism for ethological and food safety research.

The red flour beetle attacks stored grain and other food products, including flour, cereals, pasta, biscuits, beans, and nuts, causing loss and damage. It may cause an allergic response but is not known to spread disease or cause damage to structures or furniture. The United Nations, in a recent post-harvest compendium, estimated that Tribolium castaneum & Tribolium confusum, the confused flour beetle, are “the two most common secondary pests of all plant commodities in store throughout the world.”

The red flour beetle is of Indo-Australian origin and less able to survive outdoors than the closely related species Tribolium confusum. It has, as a consequence, a more southern distribution, though both species are worldwide in heated environments. The adult is long-lived, sometimes living more than three years. Although previously regarded as a relatively sedentary insect, it has been shown in molecular and ecological research to disperse considerable distances by flight.

Adult

This species closely resembles the confused flour beetle, except with three clubs at the end of each of its antennae.

Female red flour beetles are polyandrous in mating behavior. Within a single copulation period, a single female will mate with multiple different males. Female red flour beetles engage in polyandrous mating behavior in order to increase their fertility assurance. By mating with an increased number of males, female beetles obtain a greater amount of sperm. Obtaining a greater amount of sperm is especially important since many sexually active male red flour beetles are non-virgins and may be sperm-depleted. It is important to note that red flour beetles engage in polyandry to obtain a greater amount of sperm from males, not to increase the likelihood of finding genetically compatible sperm.

Patterns of Reproductive Fitness and Variation in the Red Flour Beetle

Variation in Polyandrous Behavior

Red Flour beetles engage in polyandrous mating behavior; males and females often mate with multiple individuals within a single copulation period. There is a significant amount of variation in polyandrous mating behavior present both between geographically isolated populations and within individuals in a single population.

Variation between Different Populations

Females of different geographic regions—and subsequently, different genetic backgrounds—often show great variation in mating behavior. Certain strains of females avoid multiple mating events while other strains of female engage in higher degrees of polyandry. This variation in different populations suggests that polyandrous behavior presents fitness advantages to certain populations in a geographic region and fitness disadvantages to other populations in a different region.

Variation Within the same Population: Females and Males

Female Beetles

Females within the same population often display variation in male choice. Females that possess physical mechanisms to facilitate sperm storage, have the ability to mate with multiple males and select which sperm to use for fertilization. Such physical traits may include multiple sperm storage compartments in the female’s reproductive gland. Therefore, female choice can greatly influence which traits are passed on to the offspring, which can subsequently affect the fitness of the offspring.

Male Beetles

Males within the same population of beetles also display variation in their ability to differentiate female maturity. Males are extremely selective in their mate choice; they prefer to mate with mature, virgin females or with females that have already been inseminated by another male (as opposed to repeatedly mating with the same female). As a result of phenotypic variation, some males possess better suited characteristics to detect the maturity and reproductive status of the female, and as such, will preferentially breed with only those females that will have the highest production of offspring. Males are able to differentiate between virgin females and non-virgin females through scent; that is, the wax-like secretions of competitor males could be found on the reproductive glands of non-virgin females, but not on virgin females. Males that possess more advantageous physical traits to detect female reproductive ability—such as an increase in the number of odor receptors—are better able to choose which females to reproduce with and subsequently, increase their fitness.

Potential Fitness Advantages of Polyandrous Mating Behavior

Successful Insemination

Multiple mating events can ensure that females obtain a greater net amount of sperm, resulting in an increased likelihood of successful fertilization. In nature, repeated matings could result in males that have a low sperm count. As such, by mating with multiple males, females can better ensure that they are successfully inseminated.

Although multiple mating events may result in an increased likelihood for finding genetically compatible sperm, genetic compatibility cannot be considered a major fitness advantage for polyandrous behavior. According to an experiment performed by L. Bennett, A. Pai, and G. Yan, the increased viability of embryos—due to increased genetic compatibility—did not significantly increase the number of adult beetles over time, and therefore, did not play a significant role in the fitness of the overall population.

Mate Choice

Male choice in female mating partners also poses significant fitness benefits. Males preferentially mate with mature, virgin females because until that female mates with another male, there is a 100% chance fertilization is from the respective male’s sperm. Therefore, adaptive traits—such as increased presence of odor and touch receptors in antennae to detect the reproductive status and capability of each individual female— can greatly increase a male’s fitness, since he is reproducing with a female that is more likely to produce a larger quantity of his own offspring.

Female choice in sperm can also enhance the fitness of her offspring. By selectively choosing which sperm will be utilized for fertilization—through compartments in her female reproductive gland—the female can better ensure that the best available traits are passed to her offspring.

Potential Fitness Consequences for Polyandrous Behavior

Environmental Detriments

The availability of resources and population size can greatly affect how many matings each individual participates in. Increased population size within a given area with fixed resources can limit how many offspring can survive. As such, in these areas, polyandrous behavior—to increase the number of offspring—may not provide any real benefit to the population, since the environment strongly dictates the number of offspring that can actually survive. In fact, in areas with limited resources, higher rates of cannibalism among competitor males can result in an overall decrease in fitness of the population, since there is a net decrease in offspring production and survival.

Genetic Costs

Polyandrous behavior may not always result in the propagation of adaptive genes. In the red flour beetles, the ability of a male to attract females—through pheromones—is genetically based. That is, males possess variation in their ability to attract females. However, the most attractive males—that is, males producing the most pheromones—may not always be carrying the most adaptive genes. In an experiment performed by R. Christine and B. Boake, it was found that offspring fitness was not related to the ability of the males to attract females. In other words, just because a male reproduced more often due to increased ability to attract females, does not necessarily mean the offspring have inherited the traits that result in increased fitness.

Polygamy in Red flour beetles.

During mating, red flour beetles are known to engage in polygamous behavior. A single female copulates with alot of males especially when there is lower genetic diversity in the population. Male flour beetles have been known to recognize their relatives while the females do not have this capability. This is believed to lead to females being compatible with just any partner during fertilization. Red flour beetles engage in polygamous behavior to avoid inbreeding depression especially when there is competition from other males. There is a higher fertilization success in out bred males when they compete with inbred males to fertilize the same female. Female red flour beetles store sperm after mating. More sperm is stored by the first mating, which leads to less sperm stored in subsequent matings. However, amount of sperm stored does not stop the last male to mate from fertilizing the egg.

In polygamous beetles, the male that last fertilizes the female ends up having a higher fertilization success. Polygamy can thus be seen as an evolutionary result as males compete to be the last to fertilize the females egg and contribute more to the next generation. Sperm precedence is a means of evolutionary competition through which the males try to achieve reproductive success.

In polygamous beetles, females that engage in polygamy produce more offspring than those that are less polygamous. Polygamy is mostly seen in populations that lack genetic diversity. Polygamy in less genetically diverse populations is a means of avoiding fertilization when beetles that are closely related mate since they may be genetically incompatible.

In some studies, it has been noted that there is no fertilization failures observed when related beetles mate. However, it is worth noting that there is a significantly lower number of offspring produced when inbred beetles mate than when the matings are by outbred partners. Lack of fertilization failure leads to some biologists concluding that theres is no inbreeding depression in beetles.

See also

• Home stored product entomology
• Flour beetle

External links

• Tribolium castaneum genome. Beetlebase.
• Tribolium species comparison.
• Confused and red flour beetles. University of Florida IFAS.

Further reading

• Granousky, T. A. 1997. "Stored Product Pests". In: Handbook of Pest Control, 8th Ed. Hedges, S.A. and D. Moreland (editors). Mallis Handbook and Technical Training Company.