Acanthoscelides obtectus
Acanthoscelides obtectus
The bean weevil (Acanthoscelides obtectus Say, 1831) is a beetle belonging to the Chrysomelidae family.
Systematics –
From a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Subkingdom Eumetazoa,
Bilateria branch,
Phylum Arthropoda,
Subphylum Tracheata,
Superclass Hexapoda,
Class Insecta,
Subclass Pterygota,
Endopterygota cohort,
Superorder Oligoneoptera,
Coleopteroid section,
Order Coleoptera,
Suborder Polyphaga,
Infraorder Cucujiformia,
Superfamily Chrysomeloidea,
Chrysomelidae family,
Subfamily Bruchinae,
Bruchini tribe,
Subtribe Acanthoscelidina,
Genus Acanthoscelides,
Species A. obtectus.
It is basionym:
– Bruchus obtectus Say, 1831.
They are synonyms:
– Acanthoscelides obsoletus;
– Bruchidius obtectus (Say);
– Laria obtectus (Say);
– Mylabris mimosae Olivier, 1811;
Geographic Distribution and Habitat –
Acanthoscelides obtectus is a beetle native to Central America which was introduced in Europe towards the end of the 19th century, quickly colonizing the territory. It had already reached all of France in the 1920s, by which time it also reached the Crimea.
Currently the insect is widespread worldwide being documented in all European countries, in the entire American continent, in Africa, Asia, Australia and also some islands such as Hawaii.
The breeding habitat of this insect is both in the open field and in the warehouse. In the open field it lives on beans and seeds of other legumes, alfalfa, vetch, lupine, cicerchia and, secondarily, on soybeans or lentils and, if necessary, also on cereals.
Morphology –
The Acanthoscelides obtectus is a small beetle, long, when adult, from 2 to 5 mm.
It has an ovoid-shaped body, slightly convex, with a light or dark brown color and covered with greenish-yellow down with longitudinal spots above, and light gray below.
The elytra are dark reddish brown or greenish gray, and do not cover the end of the abdomen; the pronotum is the same color which is wider than long.
The legs are reddish-yellow, and the posterior femur has a toothed tip.
The prothorax is conical; the antennae are clavate and with the first 4-5 segments of a reddish-brown colour.
The larva is arched and whitish in color and has three pairs of long legs and a yellow head in the first instar, while they are apodous, arched and with a brownish head from the second instar. The pupa is also white and about 4 mm long.
The eggs are milky, 0.6 x 0.25 mm.
Attitude and biological cycle –
The Acanthoscelides obtectus is an insect that reproduces and develops on various species of legumes or cereals and in stock.
The females lay the eggs (up to two hundred, in groups of five to twenty) on the pods of the host plant, near the sutures or in an opening they dig with the mandibles near the mature seeds.
The eggs hatch after 30-45 days and the larvae enter the seeds (even more than one per seed) digging tunnels as they feed on them.
These complete their entire life cycle, of about three weeks, inside the seed; once mature they dig an exit tunnel and then pupate.
Adult individuals emerge after 9-29 days.
This beetle, if necessary, can overwinter, always inside the seeds, in all stages; the adults emerge when the external temperature rises above 11-12 °C, but remain inactive until it reaches 16 °C; above 21 °C, in dry and sunny weather, they also move by flying.
The species can have multiple generations per year; in optimal conditions, such as inside the warehouses where the legumes are stored once harvested, it can even arrive two or three.
Ecological role –
The Acanthoscelides obtectus is a beetle that lives, in the larva stage, inside various seeds, especially the bean.
The damage is therefore due to the tunnels it digs inside the seeds. The seeds, which are emptied and pitted, become inedible and lose their germination capacity.
The containment of this beetle is based on a series of measures.
Meanwhile, the rooms intended for storing the seeds must be perfectly impenetrable by insects. Doors and windows must have devices that allow them to be hermetically sealed. The same building must also be isolated in the foundations, to allow for any disinfestation fumigation, even under pressure; the windows must be fitted with metal or fine-meshed nylon nets to prevent the entry of adult insects
In warehouses and processing rooms, they find effective applications: food traps, electric discharge light traps, sex traps.
Different results can be obtained with these traps, i.e. both mass trapping and monitoring trapping, where the latter allows identifying the most propitious moment to intervene with disinfestant products and only when the size of the population is such as to cause real economic damage.
The intervention threshold ranges from 1 to 2 insects per trap.
The disinfestation is carried out with fumigants or residual action insecticides; it can be done both with infestations in progress and with empty premises, for preventive purposes.
Furthermore, the doses and periods of exposure must be strictly respected to prevent the stored product from taking on odors which are then also transmitted to the bread and other derivatives.
However, in addition to these techniques, adopted up to now, given the residual ecological effects, the conservation of foodstuffs is moving towards the use of two new technologies: controlled atmosphere and refrigeration.
To adopt these new techniques, specially constructed and naturally watertight environments are required.
The controlled atmosphere technique is carried out with the use of nitrogen or carbon dioxide instead of oxygen.
The best results are obtained with Carbon Dioxide which requires shorter application times, compared to Nitrogen, even in the presence of a certain % of Oxygen.
Insects die from suffocation and from the toxic effect of CO2 at the cellular level.
The refrigeration technique makes it possible to effectively preserve the products for long periods as the metabolism of the insects is blocked.
The refrigeration times vary according to the chosen drop in temperature, which depends on the species of insects present and their stage of development.
Conservation could also be integrated, i.e. using both techniques: low temperatures associated with a controlled atmosphere.
In the open field and in organic farming, on the other hand, natural pyrethrum and azadirachtin can be used near the harvest and against adult weevils; in this case it is advisable to carry out two operations 10-15 days apart.
Guido Bissanti
Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Global Biodiversity Information Facility.
– Russo G., 1976. Agricultural entomology. Special Part. Liguori Publisher, Naples.
– Pollini A., 2002. Handbook of applied entomology. Edagricole, Bologna.
– Tremblay E., 1997. Applied entomology. Liguori Publisher, Naples.
Photo source:
– https://inaturalist-open-data.s3.amazonaws.com/photos/270076928/original.jpeg