The bean weevil (Acanthoscelides obtectus) is an insect belonging to the order Coleoptera, family Bruchidae. It is one of the most fearsome parasites for bean cropsespecially during storage. This kind of weevil or bruchide it is originally from Latin America, but today it is considered cosmopolitan, that is, present practically all over the world.

To avoid the serious damage of this beetle it is necessary to know how to recognize the presence of the parasite and its biological cycle, in order to be able to implement adequate prevention and biological defense strategies. Let’s see how.

Description of the bean weevil

bean weevil
L’Acanthoscelides obtectus when adult it is a small beetle with a dark gray background color and numerous longitudinal greenish-yellow streaks. In the upper part of the body there is also a thick greyish hair with blackened parts.
The shape is ovoid, slightly convex. The elytra (wings) do not cover the final part of the abdomen. The legs have a toothed end and a reddish-yellow color. The antennae are club-shaped, with the first segments of a reddish-brown color. Male specimens of the bean weevil are smaller than females. The final dimensions of the beetle range from 2.5 to 4 mm in length.

Eggs and larvae

The egg of the bean weevil is elliptical in shape, milky in color and with no visible trace of the micropyle.
The first stage larvae have 3 pairs of long legs, with a yellow head and a whitish body. At the second stage they are arched, apodic and with a brown head.

Damage of weevils to beans

Harm of bean weevil
This small beetle lives mainly on the seeds of the bean. The damage is caused by the larvae that penetrate inside the seeds and develop there. From the seeds, then, the adults come out, which open their way by making a net round hole. Each bean seed (depending on size) can house up to 8-10 weevil larvae. Beans infested with weevils lose their germination capacity and, of course, also their food and commercial value.

Damage in the field and in the warehouse

bean weevil damage in warehouse
The infestations of the bean weevil already begin in the field, but it is in the warehouses where the foodstuffs are stored that the most serious damage occurs, which can even reach the complete destruction of all the beans (seeds).

Varietal susceptibility to bean weevil

Not all bean varieties are equally susceptible to weevil attacks. For example, varieties that keep their pods quite green when ripe are less affected. More susceptible, on the other hand, are the varieties with mottled or yellow pods, which dries in advance of the seeds contained within. Late ripening varieties are also less affected.

Other legumes attacked by bean weevils

L’Acanthoscelides obtectus it is a weevil that in addition to beans can infest the seeds of others common legumesthat is to say: Fava beans, lentils, chickpeas And peasmore rarely the corn.

The life cycle of Acanthoscelides obtectus

Bean weevil
Bean weevils spend the winter in the adult stage and inside warehouses. Adults fly to bean crops in July-August, when the pods contain well-formed seeds or in the final stage of ripeness.
The fecund females pierce the pod with the mandibles, near the point of insertion of the seed on the carpellar suture line, and lay up to 40 eggs in this hole. Embryonic development can last over a month (temperatures below 15 ° C) or only a few days (temperatures around 25 ° C with average humidity). Once hatched, the larvae penetrate the seeds and begin to feed by digging tunnels. They reach maturity in about thirty days, usually when the beans have been collected and taken to the warehouse, pupating just under the seed coat. New adults flicker after 8-10 days.

Other generations and temperature sensitivity

This first generation is followed by others, which are developed completely in the warehouse. Usually in the coldest warehouses of the Northern regions another couple of generations take place. In the hottest warehouses it goes up to 4-5. This is because the activity of the bean weevil is greatly reduced with temperatures below 10 ° C and stops completely at 2-5 ° C. With temperatures of -1 ° C the weevil does not survive more than a month, while temperatures of -10 ° C are lethal even if they occur for a few hours.

How to prevent and eliminate bean weevil

To avoid bean weevil infestations there are gods natural enemies that parasitize the eggs of the insect, but we recommend that you practice the right agronomic precautions instead. In the time of sowing it is advisable to choose late ripening varieties, therefore less susceptible. Or, if you do not want to change the variety, you have to carry out a late sowing, calculating the times so that the beans ripen in late August or September, thus avoiding the attacks of the weevil.
Another precaution is at the time of collection. Once the ripe pods have been picked, they must be shelled as soon as possible, in this way we will prevent many newborn larvae from moving from the oviposition hole to the seed.
Between permitted products in organic farming that can be used in the field, near the harvest and against adult weevils, we point out the natural pyrethrum (that found here) and theazadirachtin (which instead found here). It is advisable to carry out two interventions 10-15 days apart.

Defense in the warehouse

First of all, it is necessary to proceed with a thorough cleaning of the premises, a precaution, this, in general, essential to defend against parasites of foodstuffs, such as: mealworm, anobio of bread, grain awl, banded moth, weevils etc. In the case of the bean weevil, the room must be dry and with temperatures below 10 ° C which greatly limit the activity of the parasite. If the bean harvest is abundant, to prevent infestations from spreading from a few infested seeds, it is recommended to use thick canvas bags for storage. If the attack is more limited, glass demijohns with a wide mouth protected by a very fine mesh net can be used. Well dried beans can be protected from weevil and its proliferation by subjecting them to low temperatures (-10 ° C) for a few hours.

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Philip Owell

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