Callosobruchus chinensis
Callosobruchus chinensis, also known as the adzuki bean weevil, pulse beetle, Chinese bruchid or cowpea bruchid, is a common species of beetle found in the bean weevil subfamily. Although it is commonly known as the "adzuki bean weevil" it is in fact not a true weevil, belonging instead to the leaf beetle family, Chrysomelidae.
C. chinensis is originally distributed in the tropics and subtropics of Asia. The first recorded sighting and description of C. chinensis was in China, where the species gets its name. They are now spread worldwide due to the international trade of legumes.
C. chinensis is known to be a pest to many stored legumes, including green gram, lentil, cowpea, pigeon pea, chickpea and split pea. The majority of their lifespan is spent on the host plant, such as growth, feed and reproduction. The penetration throughout the legume can lead to severe damage to the plant quality and thus cause huge economic loss. C. chinensis is one of the most damaging crop pests to the stored legume industry due to their generalized legume diets and wide distribution.
This species has a very similar lifestyle and habitat to Callosobruchus maculatus and their identities are often mistaken for each other. It also exhibits reproductive interference with C. maculatus. These beetles can be monandrous or polyandrous, depending on their environment.
Description
Physiology
C. chinensis is a small insect, growing to be about 5 mm in length as an adult. Unlike true weevils, C. chinensis has no snout. Adults are described as being brown in color, with black and grey patches over the body. The abdomen of the female is slightly longer than the elytra, and it is white in colour with two oval black spots on it. This species exhibits some sexual dimorphism, with the female being larger and heavier than the male beetle. The adults are capable of flight and they can disperse to other fields and bean storage sites easily using this method. The larvae are yellowish-white in color, with reduced legs. The pupae are dark brown, and pupation occurs inside the legume. The eggs occur singly and have a yellow coloring which become opaque when hatched. Their eggs become much smaller in areas of high population density to accommodate for competition of resources in the legume, which results in smaller adults and less fit larvae.Flightless vs. flight forms
C. chinensis have two morphologies: active or flight forms and sedentary or flightless forms. These forms have various physical and biological differences. Flightless form adults emerge earlier from dry seeds, and flight form adults emerge later from pods. In addition, flight forms have lighter colored and larger bodies, which might be necessary to fly and blend in with the environment. Flight form females also have larger wings, which could be helpful in finding pods to lay their eggs on. In contrast, flightless forms that emerge from dry seeds in storage have darker and smaller bodies. Flightless forms also show lower pre-maturation periods, lower fecundity, and shorter adult lifespans compared to flight forms. These differences might be adaptations to field conditions where pods grow for longer periods of time compared to storage conditions where flightless forms emerge earlier.Antennae
According to electron microscopy studies, antennae are composed of sections called a scape, a pedicel, and nine flagellomeres, which are used for movement of the antennae. However, the antennae are pectinate, or narrow, in males, while, in females, the antennae are serrate, or thicker and notched. Female antennae are shorter, while male antennae are longer. The antennae are composed of sensilla, which are used for the perception of stimuli. There are sensilla that detect odors, pheromones, and chemicals, and each is located and distributed in a specific location on the antennae.Distribution and habitat
C. chinensis displays a cosmopolitan distribution pattern and has been spotted in most countries due to the commercial export of beans. The beetle's natural ranges are in the tropics and subtropics of Asia, and their population has grown extensively since the cultivation and distribution of legumes. Their distribution is heavily influenced by human production since they only live on legumes suitable for mating and feeding their larvae.Some of their common host plants include green gram, lentil, cowpea, pigeon pea, chickpea and other pea species, though they are known to live on a larger variety of legume hosts. The species' most preferred habitat is in the tropics, on green gram or chickpeas.
C. chinensis reaches the height of egg production and legume infestation in July–August.
Food resources
Both the larvae and the adults feed on the legume. In general, C. chinensis can feed on any legume type that they can live on, including beans, lentils, chickpeas and split peas.Oviposition
Females lay one egg in a single seed. They avoid seeds that already have eggs by detecting physicochemical stimuli. However, when resources are scarce, females may lay multiple eggs on a single seed. Despite this, the distribution of eggs is regular, which indicates that C. chinensis determines how many eggs are in a seed. Laying as few eggs as possible on a seed could reduce competition between larvae in a seed, which is beneficial for the survival of offspring even if looking for empty seeds may require more energy expenditure for the mother. The instar development duration decreases when there are more than two larvae in a seed. A study found that, as the number of eggs on a seed increased, reproduction rate decreased and instar mortality increased.Life history
Adzuki bean weevil females lay their eggs directly on the surface of the legume singly and move on to either a different part of the legume or to a different one depending on bean density and competition among other females. They can lay as many as 90 eggs after a single fertilization. Fecundity is relative to which legume is being used as a host and female fitness. The eggs usually hatch after 3–5 days and the new larvae will burrow into the bean for the rest of development.The larvae chew tunnels through the bean until it is ready to pupate. Mature adults emerge from the bean, biting a neat circular exit from the pod 25 days after hatching. The adult beetles live up to two weeks after emerging from the pupa.
The total life cycle of C. chinensis ranges from 29 to 39 days depending on different pulses they grow in. The incubation period ranges from 4 to 6 days, the larval period from 12 to 20 days, the pupal period from 7 to 10 days, and adult longevity from 7 to 20 days.
Parasitism
C. chinensis are host to various parasites, including mites that prey on eggs and wasp species that are larval parasitoids. A. calandrae is a parasitic wasp that attack C. chinensis by laying their eggs in seeds with C. chinensis eggs. A. calandrae paralyze host larvae so that their own larvae can emerge and develop from the host seed. If C. chinensis lay more than one egg on a single seed, A. calandrae are able to find seeds more efficiently, which suggests why C. chinensis may avoid laying multiple eggs in one seed.Protective behavior
Death feigning
C. chinensis shows death feigning behavior as an anti-predatory technique. Certain stimuli will startle the beetle, and it will roll onto its back and curl its legs up. This is likely used in order to dissuade parasitoid wasps from preying on the beetle.Temperature has been shown to alter this behavior in adult beetles. As temperatures rise, this behavior becomes less common. A larger body size also shows a decline in thanatosis.