|Scientific Name||Lissorhoptrus oryzophilus Kuschel|
|Common Names||English: Rice water weevil; Spanish: Gorgojo acuático del arroz; Italian: Curculionide acquatico del riso|
|Description||The adult weevils are typical snout beetles, about 2.5 to 3.5 mm (0.09-0.14 inch) in length and gray, with a dark brown mark on their backs. The larvae are white grubs without legs: the last instar can reach 8 mm (0.31 inch) in length.|
The adults feed on the upper epidermis of the leaves, leaving typical long, narrow scars lying between the leaf veins in a lengthwise orientation.
However, the damage caused by the larvae is much more serious. The young larva briefly feeds inside the leaf sheath, then it moves down into the mud and starts attacking the roots, firstly by boring into them, but later, when it is too large, from the outside. The larvae can move over distances of up to 0.5 metres (19.6 inch) to reach new food sources. This pruning of the root systems causes seedlings to become stunted and chlorotic. The damage is most serious in young plants, but mature plants suffer delayed growth and produce less yield.
High larval populations can destroy the root system to such an extent that plants can be dislodged by the slightest physical strain and then float on the water surface.
Infestations tend to be most severe along field borders.
In spring, the adult weevils migrate into rice fields and start to feed on host plants. Oviposition and hatching of larvae can only occur where there is open water (the beetles are able to swim). When paddies are flooded, the female crawls down rice stems and uses its ovipositor to insert a total of ca. 50-100 eggs (over a period of about two months) singly into leaf sheaths below the water line.
The larvae hatch after 5-10 days and pass through four instars within about a month. For respiration, they penetrate the root tissue with a set of paired dorsal hooks that are connected to their spiracles. The final larval instar forms a watertight, mud-coated cocoon that is attached to the roots, within which it spends a pre-pupal stage of 1-2 days before the actual pupation. The adult emerges about a week later. After feeding on rice or weeds for a short time, the adults migrate, usually in late summer or autumn, either to hibernation sites or to fields with young rice plants in which they can produce another generation. During migration, the adults are strong fliers - at other times, the flight muscles are reduced.
Overwintering takes place in grass, weeds, plant litter or soil cracks. In diapause, the insects can tolerate hard frost.
There are usually two generations each year, corresponding with the usual number of rice harvests, but in some areas, either one or three generations can occur.
Besides rice, aquatic grasses and sedges can also be used as hosts by L. oryzophilus.
L. oryzophilus is facultatively parthenogenetic, and some populations in Asia and California consist entirely of females.
The rice water weevil originates from America. It is most common in the rice-growing areas of the USA but it is also found in parts of South and Central America. Since its accidental introduction into Japan in 1976, it has become widespread in Korea, India, Taiwan and eastern China as well. The insect has recently appeared in Europe, where it has been reported from the Lombardia region of Italy.
As one of the most destructive insect pests of rice, it causes considerable yield losses (up to ca. 30% in the USA, and as much as 60% in Asia).
Integrated Crop Management
Delayed flooding of paddy fields has been shown to reduce the severity of infestations. Older plants are more tolerant of attack by this insect, so rice crops planted early will generally show less yield loss.
This pest can be transported over long distances with infested plant parts, and secondary spread can be very rapid, so quarantine regulations must be strictly observed.
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