Weevil nuisance!

Insects are the most diverse species of animals living on earth. They are undoubtedly the most adaptable form of life as their total numbers far exceed that of any other animal category. It has been estimated that between one quarter and one-third of the world grain crop is lost each year during storage. Much of this is due to insect attack. In addition, grain which is not lost is severely reduced in quality by insect damage. Many grain pests preferentially eat out grain embryos, thereby reducing the protein content of feed grain and lowering the percentage of seeds which germinate. Herbivorous insects are said to be responsible for destroying one-fifth of the world’s total crop production annually. Some important stored grain pests include rice weevil, maize weevils, grain borers and rust-red flour beetle.

A complex of weevils, the rice (Sitophilus oryza), granary (Sitophilus granarius), and maize (Sitophilus zeamais) weevils, are among the most destructive pests of grains, seeds, and grain products stored in elevators and bins. These weevils are pests of grain throughout the world.

Rice Weevil: The rice weevil is a small snout beetle which varies in size, but it averages about three thirty-second inch in length. It varies from a dull red-brown to black and is usually marked on the back with four light red to yellow spots. The rice weevil has fully developed wings beneath its wing covers and can fly readily. The larval stage of this insect is a soft, white, legless, fleshy grub which feeds on the interior of the grain kernel. When mature, the grub changes to a naked white pupa and later emerges as an adult beetle.

Maize Weevil: The maize weevil, known in the United States as the greater rice weevil, is a species of beetle in the family Curculionidae. It is a major pest of maize. This species attacks both standing crops and stored cereal products, including wheat, rice, sorghum, oats, barley, peas, and cottonseed. The maize weevil also infests other types of stored, processed cereal products such as pasta, cassava, and various coarse, milled grains. It has even been known to attack fruit while in storage, such as apples.

Granary Weevil: Also known as the wheat weevil or the grain weevil, the granary weevil is a common pantry pest found in temperate climates across the globe. They are closely related to the rice and maize weevils. The granary weevil is most often found wherever grain and wheat products are stored, as they are the main sources of food for both larvae and adults. A large population of granary weevil can cause a great deal of damage. Granary weevils are known for destroying the grains and seeds it uses to eat, lay eggs and develop into an adult.

Of the three, the rice weevil is probably the most insidious, owing largely to the ability of flight. All three weevils develop as larvae within the grain kernels. They frequently cause the almost complete destruction of grain in elevators or bins, where conditions are favorable and the grain is undisturbed for some length of time. Infested grain are usually found heating at the surface, sometimes to such an extent that sprouting occurs. Wheat, corn, macaroni, oats, barley, sorghum, kaffir seed, and buckwheat are just some of the grains and products on which these weevils feed. The weevil chews a small hole in the seed and lays an egg in the resulting cavity. The larva bores throughout the seed and pupates there. Corn is a favorite host of the maize weevil and can become infested in the field as well as in storage.

Let us look at some news articles pertaining to the damage caused by weevils.

Scientists caution farmers of banana weevil attack

March 23 2015, The Hindu, India

Scientists of the Agricultural Research Station (ARS) and Krishi Vigyan Kendra (KVK), Virinjipuram have cautioned banana growers in Vellore to keep an eye on banana pseudostem weevil attack on the plantation. A team of scientists had recently spotted high incidence of pseudostem weevil attack on a banana field at Kavasampattu.

R. Rajendiran, director of Tamil Nadu Rice Research Institute, Aduthurai, along with M. Pandiyan, professor and head of ARS and KVK and scientists from ARS, Virinjipuram spotted the pest attack during a visit to a field recently.
The banana pseudostem weevil is a serious attack on bananas. The pest affects varieties of bananas such as “karpuravalli”, “mondan”, “rashthali” that is cultivated in large areas of the district, officials said.
“At least 4,000 hectares of land are under banana cultivation in the district. The incidence of this pest attack is found high in poorly maintained fields. It also spreads to nearby fields,” Mr. Pandiyan said.
Scientists said that being a monophagous pest, the weevils multiply in area where banana is cultivated continuously. This was the case with the farmer at Kavasampattu who has been continuously raising bananas on one acre of land, he added.

Both larvae and adult pest can cause severe damage. As adults are strong fliers, it can move from plant to plant. In the grub stage, it can make extensive tunnels in the leaf sheaths and bore into the pseudostem.

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