Acanthoscelides obtectus (Say, 1831)

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POLYPHAGA Emery, 1886

CHRYSOMELOIDEA Latreille, 1802

CHRYSOMELIDAE Latreille, 1802

BRUCHINAE Latreille, 1802

Acanthoscelides Schilsky, 1905

Native to Central America this species was repeatedly transported with grain cargoes to Europe towards the end of the 19th century where it became established under artificial conditions, the following decades saw a gradual spread through Europe and Asia and during the early 20th century it spread worldwide and is now among the most serious of stored bean pests. It was first recorded from the UK in the 1920s and since that time has occurred regularly among imported beans, it can breed continuously and survive year round under artificial conditions and adults are occasionally recorded from the wild but they cannot survive through the winter. They thrive at temperatures between 24 and 27°C and at high humidities and so control measures usually include keeping temperatures below zero and removing moisture from stored products but fumigation is also used and insecticides are often applied to field crops. Adults feed on foliage and only the larvae damage seeds, typically reducing yield by more than 50% and destroying the taste or germinating ability of surviving seeds. In warmer southern parts of the Palaearctic region they have become established and breed outside on a range of legumes, usually producing two generations each year and so providing a continual source of infestation for commercial operations. Winter is spent as early stages among crops or in the adult stage among litter in sheltered sites remote from the crops. Adults become active when the temperature reaches 16oC, they emerge from winter quarters and migrate to growing crops where they feed on foliage of both wild and cultivated legumes before mating and moving into bean fields to begin ovipositing. Females bite holes into developing seed pods and insert between 5 and 20 eggs, they move between pods and eventually produce up to 200 eggs. Larvae emerge after a month or so, when the pods have grown to a decent size, and penetrate into the seeds, completely consuming them from the inside and become fully-grown within 3 to 3.5 weeks, they pupate among the remains of the seeds and adults emerge two to four weeks later. In the wild the cycle from egg to adult takes between 100 to 110 days. Under artificial conditions development may be faster and the usual cycle of responding to increasing spring temperature and migrating to crops to feed before mating is suppressed; large populations build up and mating occurs continually and without maturation feeding as new adults emerge, and among unlimited host material the female behaviour is also modified as they do not bite into the seeds to oviposit but use

Acanthoscelides obtectus

Acanthoscelides obtectus

secretions to stick eggs directly onto the surface of pods or grains. Populations developing under artificial conditions tend to have a strong ability to disperse by flight into the wild when conditions become favourable, and in warmer latitudes there is a continual movement of adults between natural and artificial environments. The most serious damage is recorded from haricot beans, maize, chickpea, lima bean and cowpea but a very wide range of natural and cultivated legumes are attacked, both in the wild and under artificial conditions. They rarely occur outside in the UK but sweeping suitable vegetation in the vicinity of commercial agricultural operations may produce adults, and sometimes they simply occur spuriously as we found when we collected one from local wasteland quite remote from any agricultural activities.

2.5-5.0mm. Broadly-oval and densely scaled, dorsal surface light to dark brown or grey with numerous contrasting elongate spots that form more or less distinct transverse bands on the elytra. Abdomen, legs, except for dark ventral margins to the middle and hind femora, and often the elytral apices orange, antennae pale with segments 6-10 dark. Easily identified among our fauna by the combination of a strongly narrowed pronotum which has smooth lateral margins, and the form of the ventral margin of the hind femur which has a large tooth and next to a series of two or three smaller teeth.

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