Acanthocelides obtectus (Say)
Common name: Bean weevil.
Geographical distribution: Cosmopolitan. Distribution Maps of Plant Pests # 625, 2002.
Host plants: Legumes, such as beans, vetch, alfalfa, soybean and lentils.
Morphology: The adult body is 2-5 mm in length, light to dark brown to black in color. The elytra, which do not completely cover the abdomen, are with longitudinal spots, a golden pubescence, and a red posterior border. The head is bent forward. The larvae are white with a brown head, setae and three pairs of reduced legs.
Life history: This pest is a warm-climate species, with optimal temperatures around 22-27°C. A female lays about 200 eggs, sticking them on bean seeds inside the drying pods in the field or on beans in storage. The emergent larva invades the seed and feeds therein, pupates and then gnaws a roundish emergence hole; several larvae may develop in the same seed. The adults do not feed, are strong fliers, live for about 3 weeks and usually complete 6 annual generations.
Economic importance: A significant pest of legumes, especially beans, in some parts of the world, attacking crops in the field and in warehouses. It may reduce yields by 60%. Due to feeding inside seeds, it may not be noticed until much of the crop is infested. Partially damaged seeds lose their germinating ability and taste quality.
Monitoring: As the feeding larvae cannot be seen inside the seeds, monitoring is conducted by maintaining potentially-infested seeds in a closed container and counting the emergence holes.
Horticultural methods: Strict hygiene in stores, which should be well-closed, and the removal of infested residues from last season’s harvest. Frequent aeration of the storage container may reduce infestation. Susceptible crops are to be planted at some distance from stores and legume fields.
Mechanical methods: The pest may be killed by exposing it to subzero temperatures, or to modified atmospheres. Maintaining seeds in storage within hermetic packaging will reduce infestations.
Plant resistance: Wild bean varieties and accessions are partially resistant to the pest due to small seed size, pods with a stronger wall and indigestible seed components. Some common Bulgarian bean varieties are also partially resistant to the pest.
Chemicalcontrol: Organophosphates and pyrethroids were formerly applied against the pest and in some cases are still being applied, but the emergence of pesticide resistance has curtailed their use. Nowadays essential oils and plant extracts are in common use. The extracts and vapors of several plants, including Lavandula hybrids, Rosmarinus officinalis Linnaeus, Urtica dioica Linnaeus and Taraxacum officinale Linnaeus have strong insecticidal activities, killing the pest and/or reducing its fecundity, egg hatchability and offspring emergence.
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