Crop: Stored grain, especially maize
Why is it a problem? Grain weevils are the most important storage pests in Bhutan, particularly 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.
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 https://pestsofbhutan.nppc.gov.bt/crop-and-pest-identification/insects/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 hermetic storage methods such as the Super Grain Bag.
Monitoring throughout the storage period is necessary to ensure that pest insect populations and mold are not building. Inspect a random sample seeds or cobs for evidence of seed damage and pest presence. Note that larval stages won’t be evident as development occurs entirely within the grain.
Effect of variety
Differences in susceptibility between maize varieties is suspected in Bhutan.
Good practices should be adopted and followed throughout the maize post-harvest system, starting from harvesting to storage, to minimize losses due to storage pests. Refer to https://www.nppc.gov.bt/download/maize-harvesting-and-drying/ and https://www.nppc.gov.bt/download/manual-on-good-praticies-to-minimize-postharvest-losses-in-maize/.
Insecticides are not needed as there are more effective, safer options available such as Super grain bags (see brochure).
Image acknowledgements: AgResearch Magazine – USDA & Joseph Berger, Bugwood.org