Crop: Stored grain, especially maize
Why is it a problem? Grain weevils are the most important stored grain (post-harvest) pests in Bhutan. The attack whole grains, especially of maize.
Where and when is it a problem? They can cause high losses below about 1,200 m asl, intermediate losses to 1,700 m asl and don’t pose a threat above about 1,700 m asl. Maize can be stored safely for many years above 2,400 m asl. Most losses have been seen in maize. An FAO-funded project is currently quantifying those impacts across Bhutan (2017).
Weevil are beetles that have characteristic “snouts”. They can often be seen walking over the grains. There are two species of grain weevil that look very similar. Both can fly.
Rice weevil (Sitophilus oryzae): Adults are 3 mm long. They are reddish brown with four red to yellow spots on their elytra (hard forewings) and small round pits on their thorax (the segment between the head and abdomen).
Maize weevil (Sitophilus zeamais): Adults are slightly larger than the rice weevil. They have similar colouration but their thorax is densely pitted with irregular punctures.
Grain weevils are primary pests of stored grain, which means that they can penetrate and feed within whole, undamaged grains. The presence of larvae cannot normally be seen from outside. However, adults leave a characteristic, large emergence hole after completing their development within the seed.
Confusion with other pests: A range of pests can damage stored grain, sometimes at the same time. However, adult weevils are distinctive. The other major stored grain pest of maize is the grain moth.
Lifecycle: Adults are long-lived, from several months up to a year. Eggs are laid individually in small holes chewed into the grain by the female. Egg-laying can start in the field once the grain is mature and moisture content is less than about 20%. Eggs are protected by a waxy “egg-plug” so are not visible. Upon hatching larvae feed inside the grain, excavating a tunnel as they develop. Larvae pupate within the seed and adults chew their way out. Total development can take about 35 days under ideal conditions.
Dispersal: Adults can fly, so are able to penetrate grain stores. Infestations may begin in the field, allowing infested seeds to enter otherwise insect-proof grain stores. Grain weevils can also penetrate a bulk of grain, not just the upper layers.
When can damage be expected? Damage is greatest below about 1200 m asl, but can occur up to about 1500 m asl. The grain weevil appears to be present in most farm stores under 1200 m asl. However, greatest damage can be expected when seeds are not sufficiently dried, there are poor hygiene practices and maize remain unprotected.
Hosts: Grains weevils feed within a wide range of cereal. In Bhutan they are mainly a pests of maize, but can also cause problems in wheat and rice. Sitophilus zeamais is the dominant species in maize and Sitophilus oryzae is dominant in wheat.
Seed losses to pest insects within storage can be minimized through good management practices, including the use of modern storage techniques.
Monitoring throughout the storage period is necessary to ensure that pest insect populations, and moulds, are not building up to unacceptable levels. Inspect a random sample of seeds or cobs for evidence of seed damage and pest presence. Note that immature weevil stages won’t be evident as development occurs entirely within maize grains.
Effect of variety
Differences in susceptibility between maize varieties is suspected in Bhutan. The new white variety is thought to be more susceptible when stored as cobs as they don’t have a protective sheath. However, this is currently being tested through field survey (2017). Previous observation suggested that hybrid variety 410 was much more susceptible than were local varieties.
- Keeping areas clean within and around the storage area is the first step to minimizing losses. Storage areas should be cleaned of grain at least once a year, ideally just prior to storing the new harvest. Left-over stock should be thoroughly checked for insect damage, and removed if contaminated.
- Don’t leave mature cobs in the field as they will be attacked.
- All produce should be very dry (< 12 % moisture content) prior to storage to prevent insect attack and mould development. Seed dryness can be assessed using a moisture meter if available, or using the bite test (seeds can’t be broken if sufficiently dry).
- Sunning: Exposing the infested products to strong sunlight can remove insects and is already widely practiced by farmers. This may also help dry the seed.
- Airtight storage: insects require oxygen to survive. Storage of dried seeds in specially designed plastic storage bags (superbags) is therefore a guaranteed way to prevent losses to insects. Contact NPPC for advice on procuring superbags.
- Ensure grains are properly dried (< 12 % moisture content)
- Place the Super Bag inside another bag such as a jute bag or polypropylene bag.
- Fill the Super Bag with dried seed or grain.
- Remove air from bag completely by pressing the side of the bag, then twist the free plastic to fold it into two.
- Tie off the twist with a strong rubber band or an adhesive tape.
- Close the outer bag by tying or sewing. Make sure not to puncture the Super Bag.
- Bagged produce should be stacked with free space between walls and ceilings to allow sufficient space to allow checking and control of infestations and the rotation of stock.
- Smoking: By hanging maize cobs above the fire, the heat might reduce the moisture content or the smoke may deter the insects to lay eggs.
- Rough grinding: This technique is followed by some farmers where grains are coarsely ground into 3-4 pieces. It deprives grain weevils and grain moths from the whole grains needed for development and appears to keep them free of attack for up to a year.
- Admixture of ashes: Ash provides a coating on the grain which prevents insect attack. The efficacy of this has not been tested in Bhutan.
- Shelling: Most farmers store their maize on the cob until it is required for consumption. However, shelling can provide greater options for managing weevils.
Image acknowledgements: AgResearch Magazine – USDA & Joseph Berger, Bugwood.org