出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/05/12 15:42:01」(JST)
| Callosobruchus maculatus | |
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Order: | Coleoptera |
| Family: | Chrysomelidae |
| Genus: | Callosobruchus |
| Species: | C. maculatus |
| Binomial name | |
| Callosobruchus maculatus (Fabricius, 1775) |
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Callosobruchus maculatus, despite being commonly known as a cowpea weevil, is not a true weevil, but instead a species of bruchid beetle of the family Chrysomelidae. This common stored-legume pest is found on every continent except Antarctica, and causes problems for farmers everywhere it goes.[1] The beetle most likely originated in West Africa, and moved across the globe with the trade of legumes and other crops.[2] As only small number of beetles were moved in each trade, the populations that have invaded various parts of the globe have likely gone through multiple bottlenecks. Despite these bottlenecks and the subsequent rounds of inbreeding, these populations persist. This ability to withstand a high degree of inbreeding has likely contributed to this species’ prevalence as a pest.[2]
It is used as a model organism in many biological laboratories due to its quick generation time and ease of maintenance.
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Although C. maculatus is commonly referred to as the cowpea weevil, they lack the distinctive snout of the weevil and belong to a different family, the bruchid beetlefamily. It can be distinguished from other bruchid beetles by its more elongated body and red-brown coloration as an adult. The beetle's elytra is short in comparison to the rest of its body and ends leaving the last segment of the abdomen exposed.[3] The average body length of an adult beetle is 4–6 mm, though beetles that experience restricted nutrients in their larval stages can be much shorter. The average body mass of an adult is 4–6 mg, with female beetles generally being slightly larger than the males.[4]
The beetles are sexually dimorphic, which means their sex is easily determined by the naked eye. Males tend to be smaller and possess a more rotund shape than the female.[1] Additionally, females have dark stripes on each side of the dorsal abdomen. This gives the impression of a white stripe down the middle of the abdomen.[4]
The eggs of C. maculatus are translucent while the larva has yet to hatch and are 0.6mm in length. The eggs are small, flat and off-white once the larva have hatched from the egg.[1] The egg is translucent while the larva has yet to hatch and are 0.6mm in length.[4] The larva of C. maculatus is curved, white, and has a small head.[3]
There are two morphs of C. maculatus: the dispersal form and the sedentary form. Both morphs are capable of flight, but the dispersal form is more likely to take flight than the sedentary form. While both forms are still sexually dimorphic, the visual differences between the sexes tend to be less obvious in the dispersal morph. The dispersal morph develops when there is a high concentration of larvae in one area. The dispersal morph lives longer than the sedentary morph and has lower fecundity; specifically, it lays fewer eggs and of the eggs it lays, fewer reach adulthood. Scientists speculate that the dispersal morph continuously colonizes new areas with more abundant resources than the previous habitat, while the sedentary morph is content to remain in its current environment.[5]
A female C. maculatus beetle can lay over a hundred eggs during her adult life, and most of them will hatch. She lays these eggs on the surface of the bean (the type of bean is variable), where they remain. The larvae hatch out of the egg and burrows straight into the bean about 4–8 days after oviposition (the laying of the eggs).[6] During maturation, the larvae chew near the surface of bean, leaving behind thin, window-like layers in the seed coat.[3] The larvae emerge from this thin window after 20–35 days from first burrowing into the bean.[4]
Larval crowding can get quite impressive as 8-10 larvae can develop within the same bean. The larval crowding limits the amount of resources available to each individual. This results in longer development time, higher larval mortality, lower fecundity in survivors, and smaller adult size.[5]
Once the beetles emerge from the bean as adults, they mature within 36 hours.[4] Adults have a mean lifespan 7 days in a lab environment, but some have been able to live up to 14 days.[5] C. maculatus adults do not need to feed, and spend their time as adults mating and ovipositing or egg laying. Despite the beetles not needing to eat, if offered sugar water, yeast, or plain water, adult beetles will eat. This eating replenishes spent resources and affects how many eggs the female will lay.[6] In a study, females were provided with water, sugar, or nothing. The females with access to sugar lived longer and laid more eggs than those without that access.
C. maculatus can live in a range of humidity levels and temperatures, which has made its invasions of multiple continents successful, but there are optimal conditions. Variation in developmental time can be affected by a number of factors, including: humidity, environment temperature, type of legume, number of larvae in the same bean, and amount of inbreeding experienced by the family.[5][7] A bean that is too dry will be impossible for the larva to bore into, and a bean that is too wet will have too much fungal growth for the larva to survive. When experimenters kept temperature constant at 30°C and varied the humidity the beetles were exposed to (0-3%, 21%, 44%, 63%, 80% and 91%) they found a wide variance in acceptable humidity ranges. In general, a humidity range of 25% to 80% was acceptable, but each life stage had a different optimal humidity level. The most eggs hatched at 44% and 63% humidity, which is a wide range, but 21% and 80% humidity caused significantly fewer eggs to hatch. Larval and pupal development could not be studied separately, as they both take place inside the bean, but 44% humidity led to most surviving both stages. However, the adults lived much longer at higher humidity levels (81% and 90%) than juvenile stages.[8] In another experiment, researchers manipulated the temperature (17°C, 27°C and 37°C) while keeping humidity constant. While larvae developed in a range of temperatures, 17°C and 37°C stressed the organism. The ideal temperature range is 24-28°C.[7]
The age of the mother also significantly affects the development and survival of the offspring. When mothers lay eggs later in life, the eggs are less likely to hatch, the larvae will take longer to develop, and there will be higher larval mortality.[9]
C. maculatus mating is traumatic for the female. The male has developed penile spines, and these spines significantly damage the female reproductive tract. As a result, the females kick the males during copulation. To determine the reason for this response, researchers amputated the hind legs of females before their first mating. They found that when females are prevented from kicking the males during copulation, the mating continues for a longer period of time and the injuries sustained are more serious. However, the removal of the hind legs does not affect the amount of time before the female would again be receptive to mating or the rate of oviposition. The removal of the hind legs does appear to reduce the lifetime fecundity of the female. Males seem to gain nothing from the presence of the penile spines (in the form of remating or otherwise). Experimenters speculate that the spines are a side effect of a separate male adaptation and provide no benefit to either sex.[10]
Once the female has mated with a male, whose donation of spermatophores (capsule containing nourishment and sperm) can be up to 20% of his body weight, she glues single eggs to the chosen bean.[4][5] Females generally lay fewer eggs when there are fewer hosts than when there are many hosts. In an experimental set-up, females that were presented with three large beans laid more eggs than females presented with three small beans. However, they occasionally dump many eggs onto nonviable surfaces if there are no or very few hosts available. This leads to a larger mortality rate in eggs and potential larvae, but it may also lead to host expansion in the long term.[11]
The conditions found in most labs are not different from the conditions in stores of legumes; there is temperature control, humidity control and a high density of beans.[11] In both storage and in laboratory settings C. maculatus lays its eggs directly on the seed of the legume. In the field, C. maculatus lays eggs on the nearly mature pod of the preferred legume, and the next generation emerges after the harvest or complete maturation of the legume.[1] While the specimens in the field tend to move around a fair amount, there does not seem to be any negative effects suffered by the specimens that remain in storage areas and laboratories.[3] Because they breed continuously in storage and laboratories, while breeding in the field is more limited, there may be more inbreeding in laboratories and storage facilities than in the wild; however, inbreeding experiments have been unaffected by this so far.[9]
C. maculatus parasitizes a wide range of legumes, but in general it prefers members of the Virgna genus.[1] Among its most preferred hosts is the cowpea, Virgna unguiculata.[3] The type of legume from which individual beetles emerge is the single largest factor in determining what legume the individual beetle will parasitize. C. maculatus prefers to lay eggs on the same type of bean in which it developed. However if natal beans already have eggs, when given the choice between the natal bean and an alternative, the alternative is chosen. If other legumes are more abundant, they will also be used, but preference is for the natal bean type. Interestingly, the preference can be changed for a population. Researchers have done this by only having one bean type available to a population that prefers a different bean. After many generations, when presented with the original beans and the ones that have been available more recently, females lay more eggs on the second host than their predecessors did.[12]
Like several other species of beetle (Broscus cephalotes and other ground beetles), C. maculatus have fully functional wings. Both the dispersal and the sedentary morph can fly. However, both morphs, especially the sedentary, tend to walk instead of fly when moving, unless threatened.[4]
As mentioned in the life cycle section, the adults of C. maculatus do not need to feed or drink. However, if sugar water or yeast is available, they will take advantage of it. Females are more likely to exhibit this behavior and thus utilize more resources than the males. Scientists tested the hypothesis that access to nutritional resources would affect the frequency of second matings. After their first mating, females were isolated for 24 hours with sugar, water, or nothing available. They were then presented with another male. Those that had access to sugar are significantly less likely to remate, though a high percentage do. Being able to feed makes the female less receptive to advances from males for mating. C. maculatus males present a nuptial gift as part of the mating process. The nuptial gift consists of nutritional content and is integrated into ejaculate content. If the female has access to nutrition elsewhere, then she is less likely to want or need anything from the males and can thus afford to be choosier in mate selection.[13]
Females without this additional nutrient source are significantly less picky. Most of the time females do not appear to apply any criteria toward choosing a mate. Most species avoid mating with closely related relatives to avoid the negative effects of inbreeding. However, C. maculatus does not avoid mating with siblings. Researchers have tested this by placing virgin females with virgin males (either siblings or unrelated). Females are just as likely to mate with their brothers as with unrelated males of the same age. In remating trials, females still do not avoid mating with brothers, even if their previous mate was also a brother.[14] The species does suffer from inbreeding depression, but it does not seem to take behavioral action to avoid it.[7][9]
Once the female has mated and is ready to lay the eggs, she must determine egg location and distribution. Females prefer to lay their eggs on the smooth “cheek” of the bean. They avoid the wrinkled tops of the bean, and they avoid legumes that do not have smooth surfaces. .[15] C. maculatus females also have some unknown method to determine the mass of each bean they encounter. When the beetles are presented with a combination of bean sizes (a mix of small and large), they distribute their eggs so that each larva has access to roughly the same amount of nutrients. Their method is not based on surface area, but rather on the mass of the bean and the number of eggs already present.[11]
Despite the mother’s efforts, there will likely be more than one egg on each bean. The larva within the bean form a pupal cell and passages through the bean. The pupal cell is formed as the larva presses feces into the wall of the cell. As the larva eats its way through the bean, it will encounter other larvae. Rather than fight, both retreat and create walls of feces. If the wall is removed, the two larvae will fight to the death. The reasons or mechanisms behind this reaction are currently unknown.[16]
The predators of C. maculatus are mostly parasitoid wasps. There are likely other generalist predators that would eat C. maculatus if the opportunity presented, but Dinarmus species, Anisopteromalus calandrae and Uscana mukerjii all specifically target C. maculatus, as well as other species in the Callosobruchus genus.[17][18][19]
Dinarmus basalis parasitizes the early larval stage of C. maculatus and prevents the larva from developing. This limits the amount of damage C. maculatus can do to the bean, though the larva is still left inside the bean, as is the parasitoid which must also emerge. The parasitoid does not consume the bean, but gains its nutrition from the beetle larva. This leads to less damage to the bean than if the larva had developed to maturation. However, the bean is likely not viable for the next season and probably will not be used for human consumption.[17]
Uscana mukerjii is an egg parasite and prevents the egg from hatching, thereby indirectly protecting the egg from damage.[19]
C. maculatus is medically harmless to humans. There are no documented ill effects from inadvertently eating one of them, directly or through an infested bean.[3] However, these beetles still limit production of beans.[20]
A single larva of C. maculatus can diminish the weight of a cowpea by 3%, and multiple larvae will infest one cowpea in most storage infestations.[1] Other legumes are not as weighty as the cowpea and will lose more of their percentage body mass due to one larva. Additionally, even slight damage can destroy a seed’s viability for the next planting season.[1]
Due to the prevalence of this agricultural pest, many have been researching control options. In many developing countries where this beetle often causes the most damage to small scale farmers, people use the crushed leaves of ''Cassia ocidentalis mixed with the beans to deter C. maculatus.[21] Many Cassia species are used as a deterrent as the genus is abundant in the tropics and is mildly effective at driving away C. maculatus. While the powdered form of the leaves is more effective against other pests than against C. maculatus, a warm water extract or the seed oil of Cassia is much more effective than the powdered leaves could ever be.[21][22] The seed oil will not stop the adults from laying eggs, but it will increase the mortality of the eggs and of the first instar of the larvae.[21] The warm water extract will deter the C. maculatus adults from laying the eggs at all.[22] A combination of the two would likely be the most effective.
Other plant based controls include neem oil, nishinda, eucalyptus, and bankalmi. All of which kill.[23]
Another option for control is biocontrol. D. basalis is effective at controlling the C. maculatus population in black gram in laboratory setting. Within a few generations there is no emergence of C. maculatus and no more emergence of D. basalis either, as they have killed all the available hosts.[19] Anisopteromalus calandrae and Uscana mukerjii are also candidates for biocontrol. Both are effective at reducing the number of C. maculatus alive and emerging within a few generations.[18][19]
Freezing the whole storage area will also control C. maculatus. Between 6–24 hours at -18°C will kill all adults and implanted larvae. If the cooling is slow, then a longer time might be required for all pests to die as they will instead acclimatize to the cold.[24] It is not as elegant of a method, and it is not viable in developing countries where farmers might not have the access to enough electricity or the funds to sustain a cold room constantly .
| Wikimedia Commons has media related to: Callosobruchus maculatus |
Male
The penis of a Callosobruchus maculatus beetle
Dorsal view of female Callosobruchus maculatus
Female Callosobruchus maculatus
Eggs laid on cowpea and azuki
| リンク元 | 「ヨツモンマメゾウムシ」 |