Photo 3. Maize weevil, Sitophilus zeamais, clearly showing four light reddish to yellowish spots at the corners of the wing case.
Maize weevil, greater grain weevil, greater rice weevil
Worldwide. In sub-tropical and tropical countries. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from Australia, Fiji1, New Caledonia, Papua New Guinea, Solomon Islands, and Tonga.
Maize, rice, sorghum, wheat, casssava and yam, and also dried stored products, including pasta. CABI says that "Sitophilus zeamais is predominantly found associated with maize grain, whereas Sitophilus oryzae is associated with wheat".
A very destructive weevil, with the adults attacking sound grain, and the adults and the larvae feeding inside them, leaving large cavities and emergence holes. The maize weevil is very similar to the rice weevil, Sitophilus oryzae (see Fact Sheet no. 328). During large infestations, heat and moisture are produced, leading to colonisation by moulds and mites.
Females lay up to 150 eggs placed in holes chewed into the grain and plugged with a gelatinous substance. Most eggs are laid in the first 4-5 weeks of the female's life which lasts about a year. The eggs hatch into white, legless larvae, which remain inside the grain, and pupate there. Adults are 3-4 mm long, reddish-brown to black (Photos 1&2), with four light reddish to yellowish spots at the corners of the wing cases (Photo 3&4). Development is about 35 days under optimal cnmditions.
The maize weevil has fully developed wings, and is a stronger flyer than the rice weevil, Sitophilus oryzae, and spreads more widely this way. Infestations can start in the field, but most damage occurs in storage. The female has a sex pheromone to attract males.
Commonly, loss of weight is up to 5%, but severe infestations increase the losses up to 40%. Secondary damage is caused by moulds, insects and mites.
It is difficult to detect the weevil unless populations are high as it spends most of its life inside grain. Look for a dull red-brown to nearly black weevil, with faint yellowish or reddish spots on its back, at the corners of the wing cases (Photos 3&4). Look for the characteristic large emergence holes in the grain with irregular edges. Look for the adults, with long snouts (about 1 mm). Note that identification of Sitophilus zeamais is difficult because of its similarity with Sitophilus oryzae, and needs to be done by a taxonomist.
It is most important to dry the grain properly, to keep the storage area clean, and to monitor the grain often and regularly. Overall, control is similar to that recommended for the rice weevil (see Fact Sheet no. 338).
If pesticides are needed, use the following:
Routine hygiene treatments
Treatment of seed for sowing
Treatment of grains for human consumption or for animal feed [make sure the product is labelled for use on rice, maize and small grains (barley, oats, wheat)].
Phosphine: Used as a fumigant; grain is covered by a tarpaulin or in other ways sealed for the duration of the fumigation. The procedure is carried out by certified operators.
ALWAYS CHECK WHETHER THE PRODUCT IS FOR TREATING EQUIPMENT, BINS AND BUILDINGS OR FOR TREATING GRAIN FOR HUMAN USE AND ANIMAL FEED. READ THE INSTRUCTIONS.
AUTHOR Grahame Jackson
1Information from Swaine G (1971) Agricultural Zoology in Fiji. Her Majesty's Stationery Office. London; and CABI (2015) Sitotroga zeamais (greater grain weevil) Crop Protection Compendium (www.cabi.org/cpc); and from BioNET-EAFRINET Keys and Fact Sheets (http://keys.lucidcentral.org/keys/v3/eafrinet/maize_pests/key/maize_pests/Media/Html/Sitophilus_zeamais_Motschulsky_1855_-_Maize_Weevil.htm). Photos 1,2&4 Walker K (2006) rice weevil (Sitophilus zeamais) PaDIL - http://www.padil.gov.au. Photo 2 Georg Goergen, IITA-Benin.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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