Crop: Stored grain, especially maize
Why is it a problem? The grain moth is an important pest of whole cereal grains, especially maize. It destroys the inside of grains, resulting in weight loss.
Where and when is it a problem? Grain moths are less widespread than grain weevils. They tend to be most prevalent above 900 m asl and below 1,700 m. Maize can be safely stored for many years above 2,400 m asl.
Adult moths are small (5-7 mm long, wingspan 10–16 mm) and pale brown. Larvae are rarely seen as they complete their development within a single grain. The final-instar larva spins a silken cocoon, in which it turns into a reddish-brown pupa.
Feeding damage is hidden within the grain. However, just before pupation, the larva constructs a chamber just below the grain seed coat, forming a small, circular, translucent “window.” On emergence, the adult pushes the window open, leaving a small but characteristic round hole, usually in the crown end of the grain. Part of the window often remains at the edge of the hole in the form of a “trapdoor” or shallow cone.
Confusion with other pests: A range of pests can damage stored grain, sometimes at the same time. The adult grain moths and the form of the emergence holes are characteristic.
Lifecycle: Attacks on grains can start in the field. Eggs are laid on the outside of grains, in cracks, grooves, or holes made by other insects. Adults live for up to 15 days. On hatching, larvae bore into the grain, completing their development within a single grain. Just before pupation, the larva will extend a chamber to just beneath the surface of the grain, forming a small circular “window” of translucent seed coat. This is the first visible sign of an infestation. In very small grains (e.g., some sorghum grains), pupation may occur between two or more grains held together by the silken threads of a thin cocoon. Two or three larvae can complete their life cycle within a single maize grain. Several generations can be completed in a year under the right conditions. In temperate areas, they overwinter as larvae.
Dispersal: Adults are strong flyers, and cross-infestation occurs easily. They cannot, however, penetrate deeply into densely packed grain. As larvae also stay in the same grain throughout their development, infestations in bulk grain are usually confined to the outermost exposed layers.
When can damage be expected? The grain moth tends to be most prevalent between about 900 m asl. and 1,700 m asl. It is generally less common than grain weevils.
Hosts: The grain moth has been reported from a wide range of cereals, as well as other crops. However, in Bhutan it is only recognized as a serious pest of maize.
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
Varietal differences are significant for some crops, but this has not been studied 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 and safer storage options available, such as Super Grain Bags (see brochure).
Image acknowledgements: Clemson University-USDA Cooperatives Extension Slide Series, Bugwood.org