1. What is a bean weevil?
Bean weevils, commonly referred to as bean beetles or wheat beetles, are insects belonging to the Bruchinae subfamily of Coleoptera. Over 1,400 species of bean weevils exist worldwide, with more than 40 identified within our country.
2. What is the habitat of bean weevils?
Most bean weevil species inhabit the wild, while some reside in warehouse. They are capable of year-round reproduction in warmer regions and warehouses, leading to significant losses in legumes. In mid-to-late April of each year, adult weevils become active. During the flowering and podding stage of beans, they migrate to the fields to feed on pollen and nectar, mate, and oviposit on pods. Upon hatching, larvae bore into the beans for sustenance. After the beans mature, the weevils move into the warehouse with the beans.
3. What damages are caused by bean weevils?
Despite their small size, bean weevils pose significant threats. They are formidable predators to leguminous plants, primarily harming the seeds of these plants. By drilling and consuming them as larvae, they can quickly damage and hollow out the beans. The infestation rate of affected beans generally ranges from 10% to 50%, with the maximum rate reaching as high as 80% to 90%, leading to overall losses of 15% to 50%. After being infested, beans are unsuitable for use as seeds, significantly reducing their market value and severely impacting the production, processing, and sales of legumes.
Bean weevils are particularly detrimental to cultivated soybeans, peas, cowpeas, lentils, red beans, kidney beans, and certain wild legumes. Taking the callosobruchus maculatus as an example, under conditions of 30°C temperature and 70% relative humidity, a female can lay over 100 eggs in her lifetime, with a maximum of 157, and a hatching rate between 94% and 99%. In Guangdong, the annual generational occurrences of the callosobruchus maculatus are 11 to 12 per year on average. In regions with higher temperatures and within warehouses, reproduction and infestation can occur year-round, posing significant challenges for prevention and control.
4. What are the common types of bean weevils?
1. Bruchus pisorum
The bruchus pisorum is also called the pea weevil or pea beetle. Adult bruchus pisorums are oval-shaped and grayish-brown. The antennae are serrated; the elytra display 10 longitudinal grooves, with the central tips of each side of the pronotum extending backward, and the central rear edge is marked by an oval white hair patch. The proximal end of the hind femur features a distinct long pointed spine; the mid tibiae in males have a spike at the tip, which is absent in females. The larvae undergo complete metamorphosis, passing through 4 instars. The pupae are round and feature 5 dark brown spots on the elytra. The primary hosts include peas, wild peas, and lentils. Adults feed on pea nectar, pollen, petals, or leaves. After mating and laying eggs, both sexes die shortly after. Each female can lay approximately 800 eggs. The peak laying period typically occurs in mid-to-late May, with eggs dispersed on both sides of the pea pods, predominantly on the pods located in the middle section of the plant.
Bruchus pisorum larvae bore into pea seeds, creating slightly raised brown spots on the exterior of the seed coat. After harvesting, larvae infest the beans, causing the centers to become hollow. When the infestation is severe, damaged beans lose their edibility and market value.
2. Callosobruchus chinensis
The callosobruchus chinensis is also known as the Chinese bean weevil and small bean weevil. Adult callosobruchus chinensis exhibit variable body colors and can be categorized into "light-colored" and "dark-colored" forms. Adult callosobruchus chinensis are 2 to 3.5 mm in length and 1.3 to 2 mm in width, oval-shaped, and dark brown. Their heads are densely punctuated, featuring a longitudinal ridge on the forehead. Male beetles have comb-like antennae, while females have serrated antennae. The rear of the pronotum is broad, tapering toward the front with a narrow anterior. The surface bears fine markings and yellow-brown and gray-white hairs. The posterior margin's central lobe features a pair of knobby protrusions covered in white hairs, with each side of the middle section having one gray-white hair patch. The scutellum is covered in grayish-white hairs. The base of the elytra is broader than the pronotum, bearing dense punctuations and patterns formed by gray-white and yellow-brown hairs, with two outward-slanting stripes present at the midpoint front and rear. The pygidium is covered in grayish-white hairs, featuring four brown spots near the central and terminal sides. The inner margin of the apex of the hind femur bears a long and straight tooth, accompanied by a terminal tooth on its outer edge. The ventral apex of the hind tibia possesses one pointed inner tooth and one pointed outer tooth. The elytra are densely marked with fine punctures and display patches of intermingled grayish-white and yellowish-brown setae. This insect is primarily parasitic on hosts including mung beans, cowpeas, red beans, soybeans, common beans, peanuts, and lotus seeds.
3. Callosobruchus maculatus
The callosobruchus maculatus, an essential quarantine pest in China, exhibits high reproductive capabilities, severely affecting legumes such as soybeans, mung beans, and cowpeas, and is recognized as a globally significant storage pest. It measures 2.5 to 3.5 mm in body length and 1.4 to 1.6 mm in body width. The tumor-like prominence at the median posterior margin of the pronotum is indistinct; the inner margin of the hind femur has large, sharply pointed triangular teeth and one tooth located at the proximal outer margin. The cuticle ranges from dark reddish-brown to black. The elytra are marked with punctures and covered with short, light brown and grayish-white hairs interspersed with patches of light and dark brown hairs, with variations in pattern depending on the species. Each elytron has three dark spots located at the shoulder, middle, and tip. The shoulder spots are relatively small, and the light-colored areas of the two elytra together often form an "X" pattern.
5. How can bean weevils be prevented and controlled scientifically?
1. Maintain a clean and hygienic environment
Before storing grain, remove pests and impurities from the grain to ensure it is dry. Eliminate dust, leftover grains, and insect eggs from the granary. Disinfect storage containers made of wood, bamboo, cotton, and hemp using methods such as sunlight exposure, hot water scalding, or chemical treatment.
2. Physical control
This includes methods like high-temperature extermination, low-temperature extermination, radiation treatment, laser usage, light traps, sunlight exposure, freezing, lime-based humidity control, and packaging sealing, which are economical, simple, and efficient.
3. Chemical control
Fumigants are currently the most widely used type of pesticide. Common fumigants include aluminum phosphide, methyl bromide, carbon disulfide, sulfuryl fluoride, and ethylene oxide. Advantages include strong pest control capabilities, effective results, and quick action. Drawbacks include toxicity to humans and animals, potential residue on beans causing contamination, and the possibility of developing resistance.
4. Biological control
Natural enemies, such as parasitic or predatory insects, can be introduced to regulate the population of bean weevils; similarly, microorganisms with pesticidal effects, such as Bacillus species, can be sprayed on leguminous plants to limit the reproduction of these pests.
VI. Customs notice
Throughout the year, international ships navigate worldwide, potentially harboring harmful organisms in their kitchens, storage areas, shelves, floors, and corner crevices, presenting a risk of transboundary pest transmission. Persons responsible for international voyage ships, agency companies, and related personnel must adhere to national laws and regulations, rigorously preventing the spread of hazardous pests that threaten agricultural production and ecological environments, thereby safeguarding national biosecurity together.
(Meng Hui)
Disclaimer：The above content is translated from Chinese version of China Inspection and Quarantine Times. The China Inspection and Quarantine Times version shall prevail.
