Let's meet the real stored grain weevils

The stored products, mainly grains, are used by numerous insects and mites as food. Among the former, there are Coleoptera, weevils, woodworms and others, and Lepidoptera, moths, which can carry out primary and secondary attacks. The primary attack implies that the pest insect has the ability to deteriorate whole grains, while the secondary attack is carried out by insects that feed on broken grains or flour.

In this note we will describe the true weevils, of the Curculionidae family, responsible for primary damage to a large number of tons of grains worldwide.

Weevils

The three species are distributed throughout the world. Sitophilus granarius tends to predominate in the coldest or temperate areas, while S. oryzae and S. zeamais They predominate in subtropical and tropical regions.

The three weevils are very similar in appearance, with a head fitted with a long proboscis, oblong eyes, and club-shaped antennae. In all three species, the male's trunk is shorter and more wrinkled, while the female's is slightly longer, thinner and with fewer wrinkles. With a little practice, this characteristic can be used to differentiate the male from the female, without having to observe the genitalia.

The three species primarily attack cereal grains. Adults and larvae feed voraciously on grains of wheat, corn, rice, sorghum, barley, oats and rye. They have also been found in some legume grains such as chickpeas, peanuts, and hard industrial products such as noodles and cookies.

The females pierce the grains and deposit an egg in each tiny perforation, which is later covered with a secretion, so its presence goes unnoticed. The larvae are legless, they feed, transform into pupae and finally into adults inside the grain.

The female of S. granarius It lays between 50 and 250 eggs while that of the other two species varies between 300 and 400. The biological cycle takes 4 to 6 weeks depending on the temperature. The adults of S. granarius, according to bibliographic references, can live 7 to 8 months, while those of S. oryzae and S. zeamais, 4 to 5 months.

The optimal temperature for the development of the three species is 26 to 30 °C and relative humidity of 70%. Under these conditions, its biological cycle can last 26 to 30 days. S. granarius develops at temperatures of 11 to 34 °C while S. oryzae and S. zeamais from 17 to 34 °C. The three species support relative humidity of 45 to 100%.

They are considered a primary pest because the adult is capable of damaging healthy grains and the larvae feed inside them. Upon emerging, the adult leaves typical holes in the grains. In flour and milling products it is considered of secondary importance since it is not capable of multiplying.

Sitophilus granarius (wheat weevil)

It is considered the oldest known species among those that attack stored grains; It is believed to originate from the Mediterranean and is widespread in all temperate regions of the world.

It primarily attacks cereal grains such as wheat, rice, corn, barley, oats, sorghum and others; sometimes chickpeas, noodles and peanuts.

The adult of this weevil is cylindrical in appearance, dark brown, brown to shiny black in color. Its length does not exceed 4.5 mm. The head is provided with a long trunk; The pronotum is covered with scores that are mostly oblong. The geniculate antennae end in a club. The elytra are provided with finely dotted parallel striae and are welded. This last characteristic, together with the fact that the membranous wings are atrophied, prevents it from flying. There are no spots on the elytra.

The females make holes in the grains, with their mouthparts, where they deposit their eggs. Each female lays between 50 and 250 eggs, which take 4 to 14 days to incubate, depending on the temperature and relative humidity of the environment.

Curculioniform apoda larvae hatch, which devour the entire interior of the grain; then, after 21 to 28 days, they pupate. The pupa is typically exarata and remains in the pupal state for about a week to 10 days, completing one generation every 30 to 45 days. The adult lives 7 to 8 months.

The attacks are located in any part of the grain mass. It is considered a primary pest because the adults pierce the grains and the larvae develop inside.

Sitophilus zeamais (corn weevil)

It is considered a primary pest because the adults are capable of piercing grains just like the rice weevil; It attacks all cereals, being extraordinarily destructive in the case of corn.

Morphological in appearance and color very similar to the rice weevil, Sitophilus oryzae, its elytra also have four reddish-yellowish spots. For a long time it was considered to be the same species as the rice weevil, although slightly larger in size. It is now recognized as a different species. Its color is slightly darker than the rice weevil, although this is not a characteristic that allows them to be differentiated; For this it is necessary to dissect its genitalia to corroborate the species.

It is an insect with great flight capacity that infests cereals from the field. It is found mainly in warm humid, tropical and subtropical areas. It has gradually displaced the rice weevil in some countries with tropical climates, which was originally predominant.

Its biological cycle is very similar to that of Sitophilus oryzae. In practice it is very difficult to find notable differences.

Sitophilus oryzae (rice weevil)

It seems to be native to India and, although it has spread throughout the world, it prefers warm regions.

It mainly attacks cereals, both in the field and in storage. The adult and larvae feed voraciously on grains such as wheat, corn, rice, sorghum, barley, oats and rye. It has been found attacking other cereal products with a hard consistency, such as noodles and cookies. It has occasionally been found in ground products, although it hardly multiplies in this medium.

The adult is very similar in appearance to the wheat weevil. Sitophilus granarius. The head is projected in the shape of a proboscis; The antennae are angled and end in a club. The prothorax is densely covered with irregularly arranged circular depressions. It has wings and flies with great ease. The elytra have two yellowish or reddish brown spots on each one. The adult measures 2.5 to 3.5 mm and varies in color from brown, reddish brown to black.

In the stored grains, the female deposits an egg in each perforation, which is subsequently covered with a secretion, so its presence goes unnoticed. Each female lays between 300 and 400 eggs that take between 4 and 6 weeks to transform into adults. The larva, lacking legs, feeds, transforms into a pupa and finally into an adult inside the grain. The adult lives 4 to 5 months. The female reaches her maximum oviposition activity after 3 weeks of emerging. Upon emerging, the adult leaves typical holes in the grains.