The rodent species that infest stored foods in farm and village structures differ in the several regions of the world. The primary rodent pests are the cosmopolitan, commensal rodents, of which the roof rat (Rattus rattus), also known as the black or ship rat, is the major rat species found in food storage facilities world-wide, followed closely by the abundant and ubiquitous house mouse (Mus musculus). Since the mouse consumes far less food daily, it may be of lesser importance than the rat species in terms of overall amounts of food lost. The Norway rat (Rattus norvegicus), also known as the sewer, barn, or brown rat, is not as common and widely distributed as the roof rat, but is a formidable pest of food stores in many temperate areas of the world.
Table 7. Regional commensal and peri-domestic rodents of stored foods.
Area |
Scientific name |
Common name |
Remarks |
Latin America: |
Calomys laucha |
Vesper mouse |
Commensal |
|
C. musculinus |
" " |
" |
|
C. callosus |
" " |
" |
|
Akodon azarae |
Grass mouse |
|
|
Sigmodon hispidus |
Cotton rat |
Peri-domestic |
|
Oryzomys longicaudus |
Rice rat |
" |
Africa: |
Mastomys natalensis |
Multimammate rat |
Commensal |
|
Acomys caharinus |
Spiny mouse |
" |
|
Arvicanthis niloticus |
Unstriped grass rat |
Peri-domestic |
|
Tatera species |
Gerbils |
" |
Asia: |
Bandicota bengalensis |
Lesser bandicoot rat |
Commensal |
|
Rattus exulans |
Little house rat |
" |
There are several rodent species that are important regionally (Table 7). Some are true commensals and live in human structures and others are occasional invaders, living mainly in field situations but entering structures in certain seasons (peri-domestic).
It is important to know about how much stored foods the several rodent species may consume daily, and, sometimes, the amounts eaten in a year, so this can be related to the amounts of stored foods at risk. In large central food stores, the amounts of grain in storage may be 500 to 5,000 metric tons (500,000-5,000,000 kg). Obviously, the estimated 5 to 9 kg of grain eaten yearly by one adult Norway rat represents an immensely small percentage of this stored mass. But when 5 roof rats (3-4 kg/year/rat), 3 bandicoots (6-9 kg/year/rat), and 10 house mice (1 kg/year/mouse) eat into a farmer's stored 1,000 kg of rice, the aggregate amount consumed equals 43 to 57 kg/year, a 4-6 percentage loss. These daily and yearly values are given for each rodent species in Table 8.
Table 8. Amount of food consumed daily and yearly by rodents infesting stored foods.
Rodent Species |
Amount of food consumed daily (g) |
Amount of food eaten yearly (kg) |
Norway rat, Rattus norvegicus |
15-25 |
6-9 |
Roof rat, R. rattus |
8-12 |
3-4 |
Polynesian rat, R. exulans |
5-8 |
2-3 |
Bandicoot rat, Bandicota bengalensis |
15-25 |
6-9 |
Unstriped grass rat, Arvicanthis niloticus |
5-8 |
2-3 |
Multimammate rat, Mastomys natalensis |
5-8 |
2-3 |
Spiny mouse, Acomys cahirinus |
3-6 |
1-2 |
House mouse, Mus musculus |
2-3 |
0.7-1 |
Vesper mouse, Calomys laucha |
3-5 |
1-2 |
Grass mouse, Akodon azarae |
3-5 |
1-2 |
2.1.1. Biology and Behaviour
Mus musculus (House mouse)
Description: Head and body 75-100 mm, tail 65-90 mm. Body weight about 15-20 g. The dorsum is dark brownish-grey, the venter light grey to creamy white. The tail is uncoloured, dark, sparsely-haired. The body is small and slender, the eyes and ears are prominent (Figure 1).
Range and Habitats: House mice have now spread around the world, occurring from the sub-Antarctic to the near Arctic, from temperate, tropical, steppe, and semi-desert regions. They habitually infest food stores and other premises in both urban and rural surroundings. They are found occupying such diverse habitats as cold stores; rice, sugarcane, and cereal grain fields; garbage dumps, salt marshes, and coal mines. In Alaska they have been captured in open tundra, far distant from human dwellings.
Natural history: House mice are terrestrial/arboreal. They are nocturnal. They make burrows in field situations, but also frequent the upper parts of rural farm structures in the tropics, nesting in the roofs and attic lofts. They make and use runways in fields and structures. Their range of movement is limited; usually an area of no more than 1-3 m2 suffices. Their food requirements are 2-3 g of food daily. In feeding, they nibble on foods, sampling the several types in the living area. They quite often consume only the kernel of the grain, discarding the rest, thus ruining the grain far in extent of what they actually consume. They are capable of breeding year-round. The gestation period is 19-20 days; litter size averages 5.8 young, the young are weaned at 21 days, and sexual maturity is reached at 42 days.
Rattus rattus (Roof rat)
Description:
Head and body 140-220 mm, tail 160-250 mm uncoloured. The body is slender, the muzzle sharp, the ears prominent and mostly bare of hair (Figure 2). The body weight is 120-260 g. The dorsum is brownish-grey, the venter varies from light grey to creamy white to lemon yellow.
Range and Habitats:
Roof rats are found throughout the world due to use of manmade transport and colonising efforts. They inhabit a wide range of buildings in temperate areas, including houses, flats, shops, large food stores, warehouses, poultry houses, barns, markets, restaurants, and grain elevators. They also live in close association with humans in many cities, farms and villages in the tropics. They are still the most common rat found on sea vessels, from which they derive their name, "ship rat". They are found in sugarcane fields, and are orchard pests in many areas, causing damage to citrus fruits, macadamia nuts, cocoa, coconut, date palms, carob, and avocado fruits. In the Pacific Coast of North America, they occupy riparian habitat covered with the dense growths of the introduced blackberry, Rubus species.
Natural History:
Roof rats are arboreal and nocturnal. When living in structures, because of their agile climbing abilities, they prefer the upper parts; the attics, lofts, and open beams. They are the common rat in food stores, markets, grain elevators, poultry houses, and rural farm houses in the tropics and subtropics. They quite often nest outdoors in trees and tall shrubs. They feed on cereal grains, seeds, fruits, and nuts. They freely change their dietary needs, however, taking insects and herbivorous foods if necessary. They eat about 8-12 g of cereal grains daily. They live in close association with humans in many parts of Asia and even in interior parts of Africa, where they have invaded during the last half century. They are the common rat in food storage facilities throughout the tropic and subtropic world, from Latin America,. Africa, and Asia. Only in the temperate pats of their range are they replaced by Norway rats.
They have a definite social structure. A single dominant male was always present in a colony in Ghana. There were usually two or more females which were subordinate to the most dominant male but were themselves dominant over all other members of the group. A group territory around the feeding area was defended by the resident rats against strangers.
Roof rats have a gestation period of 20-22 days, average 6.2 young per litter, young are weaned at 28 days, and the young reach sexual maturity in 68-70 days. Breeding is usually bimodal, with peak production in the spring and fall months, but may be essentially continuous when climate, food, and shelter are optimum.
Rattus norvegicus (Norway rat)
Description: Head and body length 180-260 mm; tail 150-210 mm. The tail is bare, lighter coloured below than above. Body weight varies from 250-600 g depending on age. The ears are sparsely haired and set closer to the head than in the roof rat. The dorsum is brownish-grey, the venter is light to dark grey (Figure 3).
Range and Habitats: Norway rats are now found throughout the world. In the tropics they are essentially restricted to the seaports and along rivers. They infest food storage centres in coastal areas, but rarely are found inland. They occur indoors and outdoors. Inside buildings they prefer living in spaces between walls and floors, beneath piles of rubbish and waste foods. Outdoors they frequently are found near water, by drains, along ditches, streams, rivers, marshes, and distributed from northern South America southward to Argentina.
C. laucha occurs from southern Bolivia and southern Brazil to central Argentina and Uruguay, C. musculinus occurs in Argentina; and C. callosus ranges from Bolivia and southern Brazil to northern Argentina. Vesper mice occur in a variety of habitats, including montain grasslands, brushy areas, and forest fringes. They may find shelter in bunch grass, in holes in the ground, in rotting tree stumps, or among rocks. They are active mainly at night and possibly in the evening and early morning. They are found in open grasslands, on rocky hillsides, and in edges situations but occasionally are captured in the vicinity of marshes and swamps. They also frequent human dwellings and outbuildings.
Natural History: Vesper mice are terrestrial/arboreal and nocturnal. They do not make runways and rarely use those of other species. Nests have been found under boards and rocks, in crevices in the ground, and even high above ground in trees. They climb well and on the ground they often hop on the hind legs in the manner of Dipodomys. The diet is predominately vegetative but insects also are taken. They may eat about 3-5 g of food daily. The breeding season apparently extends from October until the following April and two litters may be produced. The gestation period is about 25 days in C. laucha. The data from a captive colony of C. musculinus showed the following: gestation, 24.5 days; litter size, 5.4 young; female sexual maturity, 72.5 days; male sexual maturity, 82 days. Calomys is a vector of the viral disease, Argentine haemorrhage fever.
Akodon azarae (Grass mice)
Description:
Head and body 90-140 mm, tail 55-100 mm. Akodon azarae range from 10-45 g body weight. The short tail and neck suggest the body form of voles (Figure 5). The dorsum is dark brown; the venter is greyish. Mice of the genus Akodon have been described as heavy-bodied, short-limbed, short-tailed, vole-like mice. The pelage is soft and full.
Range and Habitat:
Members of the genus range over all of South America; A. azarae occurs from extreme southern Bolivia, Paraguay, northern Argentina, southern Brazil, and Uruguay. South American grass mice occur in a variety of habitats, including relatively open country, grasslands, humid forests, and mountain meadows. They tolerate second-growth forests and man-made clearings. They range from near sea level to about 5,000 m. Some species are found frequently in human houses.
Natural History:
Although terrestrial, burrowing is not very important, but nests have been found 12-15 cm below ground. Various species of Akodon have been reported to be diurnal, nocturnal, and crepuscular, or active any time of day. The diet of A. azarae has been found to be herbivorous with a substantial amount of insects and other invertebrates. Considering the body weight, they probably eat about 3-5 g of food daily. The data on breeding are: gestation period, 23 days; litter size, 4.6 young; the young are weaned at 14-15 days; sexual maturity at about 60 days. The breeding season extends from August to May and there are probably two litters per year. Peak densities of 200/ha have been recorded from north-eastern Argentine pampas, decreasing to about 50/ha by late winter. At a semi-arid site in north-central Chile, the density of A. olivaceus was 30/ha in August and increased to 97/ha by November.
Rattus exulans (Polynesian rat)
Description:
This is a small, slender rat (Figure 6). The head and body measure about 120 mm, the tail is about the same length. Adults rarely weigh more than 110 g in the wild, usually much less (30-60 g) when living in houses. Their ears are relatively large and thin. The dorsum is brownish-grey, the venter a pale grey.
Range and Habitats:
Polynesian rats, sometimes also called little Burmese house rats, range from eastern Bangladesh, through Myanmar, Thailand, Malaysia, Cambodia, Laos, and into Vietnam. They also occur on islands in the South Pacific and the Hawaiian Islands. They were thought to have been carried to the islands by the Polynesians, hence their name. They are a common house rat in Indonesia and in south-eastern Asia, assuming the role of a house mouse in this environment. They live indoors in human dwellings, preferring the upper parts of the structures like roof rats. They also live outdoors in some island habitats, preferring jungle edge and tall grassy areas. They do not burrow or live in ground burrows or crevices.
Natural History:
Polynesian rats are arboreal and diurnal/nocturnal. They are herbivorous and, to a small extent, insectivorous. They feed on berries, seeds, coconuts, sugarcane, grass stalks, and insects. They consume 5-8 g of cereal grains daily. Home range is limited: 70-85 percent of recaptures were within 20-25 m of the original point of capture on Ponape, Guam, and Hawaii. The gestation period is 19-21 days, average litter size is 4.0-4.1 young, young are weaned by 20-28 days, and sexual maturity is attained at 90 days in both sexes. Polynesian rats coexist with other rat species; they were trapped from the same houses in Rangoon, Myanmar from which roof rats and lesser bandicoot rats were captured. In Indonesia they are reservoirs of plague and are a threat to humans when cohabiting in rural households.
Bandicota bengalensis (Lesser bandicoot rat)
Description: Lesser bandicoot rats are Asian versions of Norway rats. They measure 160-270 mm in head and body, the tail is 130-220 mm, dark in colour. Body weight is 250-600 g. The fur is coarse and rough-looking and the longer guard hairs are prominent (Figure 7). Animals range from brownish-grey to almost blackish-grey on the dorsum, the venter is light grey. Mammaries range from 12-20, averaging 16.
Range and Habitats:
Lesser bandicoot rats range from north-western Pakistan, India, and Nepal eastwards to the east coast of Vietnam, and southward through India, into Sri Lanka, Sumatra, and Java. They are primarily field rodents and pests of cultivated crops, but in many parts of India, Bangladesh, Myanmar, and Thailand, they have invaded cities, towns, and villages and become the main urban commensal rats in Bombay, Madras, Dhaka, Yangun (Rangoon), and Bangkok. In small towns and villages, they frequently occur inside and outside houses and food stores. In cultivated fields they occur in rice, wheat, and sorghum fields, and in waste areas between fields. They are peerless burrowing rodents, making extensive burrow mounds and easily-seen runways connecting burrows with feeding areas. Some burrow systems cover 10 m or more across and are up to 60 cm deep. They are good swimmers but weak climbers, since they are too heavy-bodied.
Natural History:
Lesser bandicoot rats breed throughout the year in Southeast Asia, but appear to have a bimodal cycle, with a peak in the dry winter months and a lesser peak during the monsoon months. The gestation period is 21-23 days, litter size averages 7-8 young, weaning takes 28 days, and the young become sexually mature in 60-75 days. They eat about 25 g of grain a day, but are notorious for hoarding 3-4 times more food a day than they eat. Their range of movement is limited to 30-50 m in diameter in fields and in urban environments they may move up to 150 m from shelter to food stores. They are gregarious, nesting in colonies under households in Myanmar towns and cities, yet are extremely aggressive when first approached by humans when in traps. They respond similar to Norway rats when exposed to baits and show almost the same tolerance to the several poisons so far tested for.
Mastomys natalensis (Multimammate rat)
Description:
Small rats, measuring 100-150 mm head and body length, with a tail about the same length (Figure 8). Body weight is 60-120 g. The dorsum is grey to brownish-grey, brown, or reddish-buff, the venter is lighter coloured. Females are distinguished by having 8-12 pairs of mammaries, continuously distributed from the pectoral to the inguinal regions.
Range and Habitats:
Multimammate rats occur as field and house rats over parts of Africa south of the Sahara Desert. They are regarded as peri-domestic rats in most parts of Africa where they occur, living in close association with humans. They occur in central food stores, in towns, markets, and in rural households. They are found in fields of maize, rice, groundnuts, millet, sorghum and also live in grasslands, and savannahs. They are typical unspecialised rats, showing a great ecological diversity in their extensive range.
Natural History: While omnivorous and having cannibalistic tendencies, they are mainly granivorous, living on seeds of wild grasses, millet, maize, and rice. They will also eat grass stems and rhizomes, and insects may comprise a large part of the diet. They are responsible for considerable damage to food stuffs in stores and houses, consuming 5-8 g of food daily. They are important from a public health standpoint because they are involved in the transmission of plague and Lassa fever. They can be prolific breeders: the gestation period is 23 days, litter size averages 10-12 but can be as much as 19 young, young are weaned at about 21 days, sexual maturity occurs after 90-100 days, and breeding appears to be strongly correlated with rainfall. Breeding increases most commonly a few weeks after the onset of the seasonal rains.
Acomys cahirinus (Spiny mouse)
Description:
Head and body measures 93-130 mm, tail measures 85-135 mm and is bicoloured. The dorsum ranges from brownish cinnamon to a uniform grey brown, the venter may be pure white to an overall slate grey. The hairs from behind the shoulder onto the rump are spiny; the sides are not (Figure 9). The ears are naked, greyish-black in colour. The weight varies from 20 to 64 g.
Range and Habitats:
Spiny mice range all across the drier parts of north Africa and into the Nile Valley, south as far as central Uganda and Kenya, the Middle East including the Arabian Peninsula, eastward across southern Iran and into western Sind, Pakistan. They are common in houses, stores, gardens, date groves, and rocky hills and cliffs bordering the Nile Delta and Valley. They are numerous in tombs and temples in Egypt. Desert specimens usually inhabit rocky hillsides, cliffs, and boulder-strewn canyons but can be found in settlements and native huts.
Natural History: Spiny mice are nocturnal and crepuscular; they are terrestrial but some have been trapped in trees. They are opportunistic feeders. Food consists of a variety of plants, grass seeds, leaves, dropped grain, nuts, flowers, and various animal matter; dates are a staple in some areas. They could eat 3-6 g of food daily. They are relatively generalised mice resembling the genus Mus. The gestation period is prolonged, 35-42 days; litter size averages 3; the young are semiprecocial and are weaned at 14 days; sexual maturity is reached at 2-3 months. They may breed over the greater part of the year.
Arvicanthis niloticus (Unstriped grass rats)
Description:
Head and body are 106-204 mm, tail 100-152 mm. They are heavily-built, shaggy-coated rats, weighing 115 to 150 g (Figure 10). The dorsal pelage is greyish-brown to dark brown, the ventral pelage is light-brown to medium-brown with white tips. The head is rounded with a blunt nasal region. The tail is covered with small hairs, dark above and light below. The mammae number 3 pairs.
Range and Habitats:
Unstriped grass rats range in a broad belt across the Sahel region of Africa, from Senegal to Ethiopia and extending north into the Nile valley through Egypt and south into the Rift valley as far as mid-Tanzania. The genus-species-complex may consist of as many as five species: A. niloticus found in the Nile Valley of Egypt and the south-western tip of the Arabian Peninsula and from Senegal to Ethiopia, Kenya, Tanzania, and Zambia; A. abyssinicus, from Ethiopia, a high altitude form endemic to Ethiopia; A. blicki in central Ethiopia, a form found in Afro-Alpine moorlands only in Ethiopia; A. nairobae, from east of the Rift Valley in Kenya and Tanzania; and A. somalicus, from east-central Ethiopia, Somalia, and Kenya. They frequent grassy savannahs, riverine habitats, into irrigated agricultural fields, and occur around human habitations. They are localised in their distribution because of their water requirements, preferring irrigated croplands and other rather moist habitats.
Natural History:
Unstriped grass rats feed mainly on the seeds, leaves, and shoots of grasses; they are polyphagous, however, eating insects, the bark of certain woody plants, agricultural crops (millet, sorghum, vegetables), and stored foods. They consume from 5-8 g of food daily. They are basically diurnal (active during daylight hours) but, in response to intense heat, may also be nocturnal. They are capable of breeding year-round but usually breed during the dry season. The gestation period is 21-23 days, the litter size averages 5-6 per litter but can run as many as 12 young. They are gregarious, living in colonies and sharing groups of burrows. Densities vary greatly and may increase greatly under favourable conditions, ranging from 12/ha in normal years to 100/ha during an outbreak in Senegal in February 1976 and from 65-250/ha in the Semien Mountains National Park of Ethiopia.
Tatera species (Gerbils)
Description:
Head and body length is about 90-200 mm, depending on species; tail is 120-240 mm, some with short hairs and others with long hairs in the tail, and some with either a white or dark tail tip. Body weight varies from 50-220 g. The pelage is soft to medium. Colour ranges from pale sandy grey to dark brown mixed with grey above; the underparts are white. The body form is heavy and rat-like (Figure. 11). The head is rounded; the eyes rather large.
Range and Habitat:
The genus is mainly African except for T. indica, which ranges from Syria to India and Sri Lanka. Other members of the genus are found throughout the drier areas of Africa: T. robusta from Senegal to Somalia and central Tanzania; T. leucogaster from Angola and south-western Tanzania to South Africa; T. valida in savannah zones from Senegal to western Ethiopia and south to Angola and Zambia. They inhabit sandy plains, grasslands, savannahs, woodlands, and cultivated areas.
Natural History:
Gerbils are nocturnal and terrestrial. They usually walk on all four limbs but when alarmed they flee by means of running bounds of up to 1.5 meters in height and more than 3 meters in length. They are able to do this because of their powerful, well-developed hindlimbs. They are burrowing animals, with deeper burrows with many chambers and tunnels, used for resting during the day, rearing the young, and for food storage. They are mainly granivorous but will eat fruits, some leaves and roots, and insects (especially in the dry season). Gestation varies from 22 to 30 days; overall litter size for the genus is 1-13, with averages of 4.5 in T. leucogaster, 5 in T. indica, and 6 in T. robusta. Young are born in an undeveloped state and remain in the nest for a month before accompanying the adults out to forage. Some species breed during the rainy season (T. leucogaster), all-year in T. indica, and during the dry season following the rains in T. robusta.
Birds, not prone to living inside the farm or village structures as rodents are, rarely have the opportunity to damage or consume stored foods. It is only in those outdoor situations where grains or seeds are exposed during drying or threshing that birds have the chance to eat them, or they may get into stored grains where they are stored in open cribs. Consequently, the losses of stored foods due to bird activities are small as compared to those caused by rodents. The several species of pest birds that are found in post-harvest situations in South and Southeast Asia are house and tree sparrows, Passer domesticus , P. montanus (Figure 12), common pigeons (Columba livia) (Figure 13), doves (Streptopelia species) (Figure 14), Asiatic house crows (Corvus splendens), and common mynas (Acridotheres tristis) (Figure 15). These are mainly pests in threshing and grain drying yards in farm household situations. The amounts of grain they consume are small as compared to that taken by rodents in farm stores. Birds rarely infest structures as rodents do; instead they rely on their mobility to quickly seek out places to feed.
Post-harvest losses of cereal grains due to birds are seen at threshing and drying stages. When grains are threshed in the field, or in clearings at the farm households, birds are attracted by the abundant grain. When grains are spread on the ground for drying in the tropic sun, birds are similarly attracted. Unless birds feed in large flocks, the amounts of grain they consume are negligible as compared to rodents. Garg et al. (1966) found that sparrows, mynas, crows, and pigeons visited and fed on wheat at threshing yards and estimated the losses caused mainly by the birds (a few yards had small numbers of rats) at 244 g/day/yard or about 7.3 kg/yard for the 30-day threshing season at Harpur, India. In Pakistan, mynas, crows, and pigeons were seen frequently at grain storage centres and at farm yards but they mainly fed on waste or spilled grain at these sites.
Libay et al. (1983) reported on losses of feed for ducks on farms in the Philippines due to European tree sparrows, Passer montanus. Losses at 4 farms where bird counts averaged 149-177 over the 14-month study were estimated at 4.5-5.0 kg/day of rice. This was equivalent to US $0.40-0.45/day for the 4 farms or $146-164/year. At the largest farm the loss represented 4 percentage of the total amount of duck feed used.
The common pest birds in food storage situations in Africa are sparrows (genus Passer) and weavers (genus Ploceus). Maize and sorghum are sometimes stored in open crib-type structures and in these situations these birds may cause damage.
The easiest method of control is to keep birds from access to stored foods. This is done by using wire mesh as screening, or using local-made netting, where foods are to be stored in open crib-type structures. The inside of the structure is lined with the wire mesh or netting before the grains are placed inside. Where birds are pests at threshing and drying yards, the use of bird-scaring devices or human bird scarers will help to keep birds from feeding.
The nests of house sparrows in the eaves or crevices around farm structures may be destroyed by pulling the nest contents out with a hooked stick or a wire bent to form a hook.