Profile

The rice weevil originates from southern countries and is always spread with rice or cereals. Rice weevils can infest healthy, undamaged cereal grains as primary pests and are typical pests of stored grain in warehouses and farms.

Biology

The rice weevil belongs to the weevil family (Curculionidae). It is closely related to the grain weevil and very similar to it in appearance and lifestyle. Unlike the grain weevil, the rice weevil is capable of flight. The females can lay up to 300 whitish shiny eggs and gnaw a hole in a grain kernel, lay an egg inside and close the cavity with a secretion drop. The hatching beetle larvae spend their entire life in the cereal grain. They pass through four larval stages and finally pupate in the grain. Development from oviposition to the finished insect takes 90 days at 18°C, 33 days at 26°C, and no growth takes place above 34°C. The optimum temperature for the beetles is 18°C. The optimum temperature is 30 °C.

The life span of adult beetles is three to four months at normal Central European temperatures. The survival of the beetles without food is strongly dependent on the ambient temperature and is one to two weeks at room temperature, and correspondingly longer at cold temperatures. All developmental stages are able to survive temperatures around freezing for about a week, so overwintering in unheated storage rooms at low temperatures is usually possible.

Host plants

Larval development is only possible in the whole grain and can therefore be excluded in loose substrates, such as flour. The adult beetles also feed on cereal products, such as flour, semolina and pasta.

Distribution

Due to globalization and worldwide trade in agricultural products, the rice weevil has spread to practically all continents. In Central Europe, the beetles survive exclusively in heated buildings. They are found in warehouses and stores of all kinds and can also occur in the household.

Propagation and transmission

Rice weevils live in the open in the tropics and infest rice grains in the fields there. They regularly fly out of the stores into the rice field and land directly on the stalks. After harvesting, they return from there to the stores.

Economic importance

In many countries, the rice weevil is one of the most important pests of stored grain. Often, they also pave the way for secondary pests, which take advantage of already damaged cereal grains.

Prevention and control

General hygiene measures

• Periodic and thorough cleaning of storage facilities
• Pest monitoring
• Temperature measurement in the stored goods
• Cooling (10-13 °C) and drying of the stored goods
• Disposal of junk and cleaning waste
• Destruction of already infested supplies
• mechanical prevention of beetles by sealing windows and doors (insect screens)
• Insulation of ceilings, walls and pipes (prevention of condensation)
• Plastering of crevices, cracks and porous walls
• Thorough cleaning of harvesting and transport machinery and storage rooms before storing the harvest
• Empty space treatment with a plant protection product (see list of plant protection products approved in Austria) or heat treatment

Proof

• Periodic temperature measurement: a rise in temperature in the stored goods provides information about the presence of storage pests
• Sampling with a grain sampler
• Sieving of samples with a continuous sieve
• Float test: infested, hollowed-out cereal grains float on the surface of the water and can be examined for rice beetle larvae
• Noise measurement: detection of larval feeding sounds by inserting a sensitive microphone into stored supplies

Control

• Control of the rice weevil and its larval stages requires increased effort due to protected larval development inside the grain kernel.
• Inert dusts (diatomaceous earth) and contact insecticides allow successful control of adult beetles when distributed appropriately in stored grain. However, larvae and eggs deposited in the grain are not captured and can continue to develop unhindered inside the grain.
• Safe killing of all stages is achieved with gaseous agents (hydrogen phosphide, see list of plant protection products approved in Austria) including inert gases (nitrogen, carbon dioxide).
• Thermal treatments (60 °C for three minutes) or cold treatments (below -10 °C) are possible, but only effective for smaller batches due to the energy required.
• Biological control by release of the pea wasp Lariophagus distinguendus (see list of plant protection products approved in Austria). This insect finds the rice weevil larvae and pupae living hidden in the grain thanks to its good sense of smell.

Tips for household use

• Store food only in well-sealing storage containers with rubber seals or screw caps
• Dispose of all infested food directly in the garbage can outside the house
• Remove bugs sitting around with a vacuum cleaner
• Fly screens on (roof) windows, doors and skylights reduce the risk of infestation
• If you are repeatedly struggling with rice beetles, wasps can help. These are natural enemies of the beetles and are simply released in the kitchen(Lariophagus distinguendus, see list of plant protection products approved in Austria).

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