Rhyzopertha dominica (Fabricius)
Common name: Lesser grain borer, rice weevil.
Geographical distribution: Cosmopolitan. CABI Distribution Maps of Pests, Series A, Agricultural Map #318, 1973.
Host plants: Polyphagous, especially cereal seeds.
Morphology: Adults 2-3 mm long, body reddish-brown and shiny, cylindrical, head not visible from above. Pronotum tuberculate and elytra with punctate, parallel-sided striations. Larvae whitish with a brown head, thoractic segments broad, posteriorly narrow, elderly larvae with head and thorax recurved ventrally, about 5 mm long.
Life history: The females lay 200-500 eggs on grains. The larvae initially feed on frass and then on cereal seeds and on foods and animal feed, and can bore into intact kernels. At 34°C complete development requires about 3 weeks and humidity content of above 9%. The pest cannot develop below 20°C or above 38-40°C. The adults, which are good flyers, can live for several months. The males produce a pheromone that attracts both genders.
Economic importance: This borer is a major pest of stored products throughout the tropics, subtropics, Australia and the USA. Due to the trade in products and its ability for prolonged flights it also occurs in temperate countries. The pest feeds mostly on stored cereal seeds as well as on beans, ginger, cassava chips, biscuits and many others. The feeding of larvae and adults reduce grain weight, causing losses of up to 40%, and can turn rice to dust. Damaging the seed germ reduces germination rates and may be followed by secondary pests and fungi. However, R. dominica often occurs along with other stored food pests, thus it is difficult to evaluate its individual damage and the accompanying control costs.
Monitoring: Trapping adults in pheromone-baited flight traps. Sieving grain samples for adult presence. Thermometers with remote electronic readouts placed in storage structures can supply rapid indications of grain temperature, provided they were placed near “hot spots”. Holes in grain and other commodities indicate insect presence, but not their specific identity.
Horticultural methods: Cleaning grain stores and keeping them airtight, maintaining low temperatures (below 20°C) and hermetically sealed packaging can conserving seed viability.
Controlled atmospheres By increasing the levels of C02 and reducing that of O2 in grain stores the numbers of R. dominica were greatly reduced.
Plant resistance: Rice varieties with high phenolic content and hardness show some resistance to the pest, as do some wheat varieties.
Control by heat treatment: A thermal method for the disinfection of post-harvest pests consists of high temperature treatments. When applied against R. dominica infesting rice it killed 80% of the pests but seed quality decreased at temperatures of 80 and 85°C.
Chemical control: In food stores the beetle was often controlled with various gases, but it had developed resistance to the oft used compound, phosphine and to many pesticides. Certain plant oils are also used for their fumigant activity against R. dominica in large-scale storages. The pest is susceptible to spinosad. The combination of Insect growth regulators (IGRs) and entomopathogenic fungi was also effective, although some methoprene-resistant strains have been noted. The combination of an IGR and a chitin synthetase inhibitor totally suppressed the production of pest progeny.
Biological control: Several predators of R. dominica were reported from different regions. They include Histerids, Tenebrionidae, Anthocoridae and various mites. The parasitoids Choetospila elegans Westwood and Anisopteromalus calandrae (Howard) (both Pteromalidae) were very effective in reducing populations of R. dominica. The pest is susceptible to preparations of Bacillus thuringiensis (BT) and to various entomopathogenic fungi, like Beauveria bassiana Bals.-Criv.) Vuill. The application of entomopathogenic nematodes had only a partial effect on pest numbers.
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