Section 7 - Biology and control of other storage pests

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Rodents in storage and their control
Rodent control research and future needs in the philippine grain storage system
Birds as pests of grain stores
Post-harvest microbial infection of cereal grain

Rodents in storage and their control

Melinda M. Boque

I. Economic Importance

With the sole exception of man, the most successful and abundant mammals on earth today are rats and mice. They would not have enjoyed this without man's inadvertent help. Rats and mice are considered commensal for the fact that these animals live at man's expenses, invading his home, eating his food and destroying his commodities. They are also capable of transmitting diseases to man, who thus derives no benefits from the relationship.

Stored foods are particularly prone to rodent attack, with the items concerned varying in different regions. The most common and therefore most vulnerable, are maize, rice, sorghum, millet, barley, oats wheat and cereal products. Much food loss occurs as the result of contamination, supplies being rendered unfit for human consumption by rodents hairs faecal droppings and urine, which are shed liberally as the animals forage nightly for their food.

While rodent attack on stored foods is widespread, estimates of damage are poorly documented. The best information available has been summarized by Holf and co-workers (1976) and is presented in Table I. The amount of food lost through direct consumption by rodents is considerable. An average-sized Norway rat eats about 259 of food a day, or the equivalent of 9kg in a year. It was found that small colonies of Norway rats (10 to 26 animals), each with access to sack wheat for 12 to 18 weeks, contaminated 70% of the grain and caused a 4.4% loss in weight. The main monetary loss however, resulted from damage to the sacks. Total losses amounted to 18.2% of the value of the wheat and the sacks.

Three species of worldwide distributions are the most important commensals: the Norway or brown rat (Rattus norvegicus) the roof rat (Rattus rattrs) sometimes called the black rat or shiprat and the house mouse (Mus musculus). Their characteristics are considered briefly in here. This paper therefore will discuss the reproduction, ecology and control of commensal rodents which are essentially pests in storage.

II. Taxonomy and Distribution

The identifying characteristic of commensal rats and mice are summarized in Table II.

The Norway rat (R. norvgicus) is essentially a temperate climate spectes. It is more abundant and widely distributed across central Asia, Europe and Northern America. The range of this species continues to expand due to changes in urban environments favorable to its habits and to occassional introductions resulting from the traffic. This species frequently live in and around residences, in cellars stores, warehouses, slaughter houses, docks and sewers. On farms they infest silos, granaries, piggeries, poultry houses, stables, warehoused and dockside structures in port areas.

The roof rat (R. rattus) is at home indoor or out, depending on the climate. It is a semi arboreal species, climbing shrubs, vines and trees in habitats ranging from river banks to tropical rain forest. This species inhabits a wide range of buildings in temperate areas, including houses, shops and large foodstore, warehouses, poultry houses, barns, market, restaurants and grain elevators. It also lives in close association with man in many cities and villages in the tropics. This species is more extensively distributed worldwide than is B. norvegious. The ancestral home of the roof rat is the southern Asian mainland, Southern China, parts of India, Indonesia and the Philippines but it is also distributed in several countries of the Southern and Northern hemispheres.

The house mouse (M. musculus) is distributed in temperate, tropical and semi-desert regions.

TABLE 1. ESTIMATED DAMAGE AND LOSSES OF STORED CROPS AND OTHER FOODSTUFFS DUE TO COMMENSAL RODENTS IN TROPICAL AND SUBTROPICAL AREAS (Hopf et al 1976)

TABLE 2. FIELD CHARACTERS AND MEASUREMENTS OF COMMENSAL RODENTS

It habitually infest food storage and other premises in both urban and rural surroundings and it is also found occupying such varied habitats as cold stores, rice, sugarcane and cereal grain fields, garbage dumps, salt marshes, and coal mines.

Other rodents develop into commensals where their habits bring them into close contact with man. Most notable of these species are the lesser bandicoot rat, Bandicota bengalensis, and multimammate rat, Mastomys natalensis.

The bandicoot rat is distributed in South and Southeast Asia. While it is mainly a pest of agriculture and the predominant field rat in many parts of India, Bangledesh, Burma and Thailand, it has invaded cities, towns and villages and become the main urban commensal in Bombay, Calcutta, Madras, Dhaka, Rangoon and Bangkok. This species feeds extensively on field crops but also infests food storeswhere it feeds on paddy or wheat, both in warehouses and farmers' houses.

The multimammate rat (M. natalensis) is regarded as a peridomestic rat in most of Africa where it is found in close association with man. It nests in underground burrows or in dark protected areas when living in human habitations. It is abundant in farm houses as it is in the fields.

III. Biology

Reproductive activity in commensal rats and mice is characterized by early sexual maturity, short gestation period, post-partum oestrus, breeding throughout much of the year and large litter size. These traits give commensal rats and mice the potential for very rapid population growth and for quick recovery when their numbers are reduced by poisons, traps or other means.

The results of reproductive studies of female commensal rodents are summarized in Table 3 and 4. Under conditions of optimum climate, surplus food and abundant shelter, commensal rodents population tend to breed throughout the year. These conditions most commonly occur indoors in food stores, warehouses, farm buildings and on ships.

Male animals generally remain in breeding condition throughout the year although the testes may be retracted into the abdominal cavity during periods of cold weather when they might appear to be infertile. Norway rats construct a nest of grass, waste paper, twine or other suitable materials in a separate chamber within a burrow system in the natural spaces within buildings. Likewise, house mice build nest in walls and roof cavities, in stacks of food or cabinets or drawers. Roof rats living outdoors often build in shrubs or trees, constructing them from twigs, leaves, grass and other plant materials. The young for all three species need constant maternal care for at least three weeks after birth.

IV. Ecology

The important physical elements necessary to sustain commensal rodent populations are food, water and shelter. Their abundance and distribution has a direct bearing upon how many rodents can be supported in a given environment. Commensal rodent populations thrive when all three resources are abundant and close together.

The main food sources for commensal rodents are stored food and garbage in urban areas, and in rural areas, field crops, natural vegetation and seeds. Stored food are available in mills, warehouses and godowns, port facilities, food processing plants. Feed bins and corn cribs, waste food and spilled grain are other important food sources. Warehouses containing food stored in bags or in bulk are particularly vulnerable to rodent attack unless they are protected. Improperly stored and handled garbage increase rat problems in the urban environment and it is a major cause of the persistence of rat populations in many cities and towns.

Water , generally available to rodents in urban and rural localities but its supply can be a problem for Norway rats, particularly those living in well designed warehouses or in areas with dry seasons. The Norway rat needs food with a high moisture content or a supply of free water, the lack of which restricts its destribution and spread into new areas. House mice, which utilize metabolic water more effectively, tolerate dry habitats without difficulty and the roof rat can also withstand water deprivation better than the Norway rat.

A rodent population that has been reduced in size recovers slowly at first and then at an increasingly faster rate. As it approaches the capacity of the environment, the primary limitations being food, water and shelter, growth slows down and the population tends to level off. In an uncharged environment, the population would be expected to remain at essentially the same level but in practice it tends to fluctuate in size, around the carrying capacity of the environment.

Evidently, a confined colony of rodents competes more fiercely for food, shelter and living space as the population density increases. At very high densities, rats and mice spend abnormal time and energy in aggressive attacks and in the defense of territories. Reproductive success of females can also be seriously lowered as the result of aggressive behavior.

IV Methods of Control

The control of commensal rodents is important to safeguard human health and to prevent economic and other losses. Most control work is directed towards preventing rats and mice from living in and around buildings in both urban and rural areas or eradicating populations that have already become established in them. Thus, varied control methods have been devised to come up with an appropriate strategy for certain types of condition. These include environmental sanitation, physical, chemical, and biological control and other methods.

A. Environmental Sanitaion

Environmental sanitation concerns the orderly management of the stored product. Basically it means good housekeeping, the proper storage and handling of food stuffs and organic waste and elimination of harborage for rodents and other pests. Commensal rodents are opportunistic and readily take advantage of man's misuse of the environment. Poor sanitation generally results in food stuffs and harborage being abundant and easily available to rodents and it enables their rapid establishment. Almost any pile of debris indoors or out, can provide rodents with suitable cover for nesting and breeding. In general, the maintenance of a cleared area, as extensive as possible, can do much to deter rats and mice infesting food stores.

B. Physical Control Methods

These have been appropriate measures since they do not basically contaminate the stored products. Some of the approaches are ratproofing buildings where rodents are denied shelter and food. It is the most effective and long-term remedy against rats. Preventing entry of rats and mice is important for successful rodent control. This includes keeping openings to buildings and bins tightly closed when not in use. Heavy wire screening or sheet metal barriers on lower portions of openings will help keep rodents out. However, this is a difficult task since commensal redents like mice can pass through 12 mm apertures and very young rats can enter 14 mm openings.

The method is only ideal where storage activity is less. Trapping is the preferred method of killing or capturing rodents in situations where the use of rodenticides is considered undesirable. This technique has to be properly done. Because of the cautious behavior of rats, they tend to be wary of traps and abundance of alternative foods, makes them difficult to capture or control by this approach. In campings against them, traps should be placed near runs and at other locations where there are clear sings of rat activity. Trapping success can be imporved by leaving the traps baited but unset for a few days and using baits of proven acceptability to rodents which include bacon, peanut butter, fresh, smoked of ried fish, and ground meats or bread for Norway rats. Baits that dry out or spoil should be replaced immediately by fresh ones. Traps should be examined daily to remove dead rats, which should be buried or incinerated, and to reset those traps that have sprung.

For economic or other reasons, traps are of little value in contriling large infestations of rats and mice but these are useful in helping to catch the survivors of any poison treatment.

The effectiveness of snap-traps against rodents especially on house mice is largely determined by the sensitivity of the traps, their placement and number of traps used. Traps should be set close to walls and in other areas where active runs are evident.

Electric fences which have been used on occasion, both to exclude and enclose rats in the field, can be effective in protecting stored product but obvious caution should be emphasized for possible shocks or electrocution.

The use of very high frequency sound, ultra sound, has been proposed as a means of preventing rats or mice from freely moving into a building or from one area to another. Field and laboratory studies have generally failed to support these contentions. Reports on the repellency of four commercially available ultrasound generating machines tested against a population of wild Norway rats in a large outdoor enclosure using an uninterrupted beam of sound 0.5 m from where they were accustomed to feed shows initial and partial repellency for a day or so but thereafter the feeding behavior of the rats was unaffected.

C. Chemical Control

Most measures to control commensal rodents depend on the application of poison incorporated in either bait, dust or water formulation. Rodenticides are usually classified as either chronic (multiple dose, slowacting) ar acute (single dose; quick acting) commpounds. Of most widespread use and particular importance are the anti-coagulant poison, since these slow-acting compounds are now regarded as first choice rodenticides againts commensal rodents in most control operations. Acute rodenticides still have a part to play, but they are principally and most effectively employed in situations demanding a rapid reduction of high-density population.

1. Anticoagulant rodenticides have a physiological action in that they discrupt the mechanism that controls blood-clotting and cause fatal internal hermorrhage to develop. Examples of this rodenticides are warfarin, coumachlor, coumatetralyl, diphacinone, and chorophacinone. The new anticoagulants which are so called "second-generations" are difenacoum, brodifrocoum bromadiolone and flocoumafen.

2. Acute rodenticides. The physiological action of this type is either on the stomach or nervous system. This is categorized based on hazardinuse such as:

a. Compounds that are highly toxic and extremely hazardous to man and animals, e.g. arsenic trioxide, Flouroacetamide, sodium furoacetate (1080)

b. compounds that are both moderately toxic and hazardous to man and animals, requiring considerable care in use, e.g. ANTU (alphanaphthyl-thiourea), calciferol-zinc phoaphide.

c. compounds or relatively lower toxicity that are least hazardous to man and animals. Some of these compounds are norbormide and red squill.

Attention has been given to the possible use of reproductive inhibitors or chemosterilants for the control of rodents. Field trials in England show some indication of reduction of rat populations due to ceased breeding. However, 6-14 months after treatment, a resumption in breeding was observed. This was attributed to a return to normal fertility of long surviving resident rats ang to immigration from nearby populations.

The future of suitable chemosterilants in the field of rodent control is uncertain, though this has and advantage over rodenticides in that they pose a less immediate hazard to human beings, pers or domestic animals. The major disadvantage is that rodent populations treated in this manner decline rater slowly. Meanwhile, damage, contamination and disease problems have to be accepted.

Fumigants are also used to kill rodents and their ectoparasites living in accessible areas in buildings, ships or in burrows in the soil.

Fumigants most commonly used against rodents are calcuim cyanide, methyl bromide, chloropicrin and hydrogen phosphide. These are used mainly indoors for insect control and can also be used against rodents in storage. Experience and skill are required therefore in their application.

Repellants are chemicals that are distasteful to rodents; their use in preventive measures is based on the extreme sensitivity of rodents to certain compounds and odors. A number of compounds have been found to possess repellents. The majority are also odorous or toxic to man however and, besides being difficult to work with, few of them have been found to have a long lasting effect.

D. Biological Control

This includes predation, disease and parasitism nad genetic manipulation. There are dangers in the introduction of predators which may become pests themselves. The use of diseases and parasites has temporary effects on the population and rats can easily adopt and become resistant. Most of the potential infections agents like Salmonella also cause disease disease in man and are therefore unsafe for use.

Genetic mutations through in radiation that induces mutations have been tried on R.r. mindanesis and R. argentiventer (Medina et al, 1973). More rsearc on inducing lethal genes and sterile male strains need to be done before this approach can become practical.

E. Other Methods

1. Rats as food

These was a time when people were encouraged to eat rats ("STAR meat") to help control infestation and to augment people's protein requirements. Rats are now socially acceptable as food in many rural areas of the Philippines.

2. Bounty System

Paying cash reward for dead rats as evidenced by tails or head as one control/ method in the field may be useful in rat control in storage areas.

Based on experience, the use of any combinat ion of different methods provide a better result than just making use of one control method

References:

Brooks, J.E. and F.P. Rowe 1987*. Commensal Rodents Control. In Vector Control Series: Rodent Training & Information Guide. WHO, 107

Brooks, J.E. 1973. A review of commensal rodents and their control. CRC Critical Review of Environmental control 3:403

Hopf, H.S., G.E.J. Morely, J.R.O. Humphries. 1976. Rodents damage to growing crops and to farm and village storage in tropical and subtropical regions, London, Centre for overseas Pest Research, Tropical Products Institute, 115 p. University of California. 8.

Sanches, F.F and E.A. benigno, 1985. Rodents Biology and Control. University of the Philippines at Los Banos. 152.

* Most of the contents of this paper were taken from this reference.

Rodent control research and future needs in the Philippine grain storage system

by ENWIN A. BENIGNO

INTRODUCTION

Rodents are serious pests in both agricultural field and storage. Although the same problem species may be involved in both cases, their presence is less tolerated in storage than in the field. This is understandable since food in storage is nearer to human consumption and we can ill afford to lose at this stage. In the field, for instance, early crop damage by rodents can still be partly recovered by the plants' ability to produce more than normal to compensate for the injury or damage. On the other hand, whatever rodents consume in storage is total loss; only the spilled or contaminated grains can be partly recovered but at an extra cost of re-processing.

In the Philippines, Rubio (1972) placed annual rodent damage at 0.80 to 4.12 cavans (40-206 kg) per rice mill-warehouse in Laguna. Aganon (1982) estimated annual grain loss per warehouse in Nueva Ecija due to rodent consumption, contamination and spillage at 1.92 to 2.93 cavans (96-14 Kg). Sayaboc et al. (1984) observed an average loss of 3.6 kg/day in commercial grain storage.

Added to these, there is permanent damage to the storage structure itself by the rodent's gnawing habits hence, food losses are potential. This is also one main reason why these pests must be controlled.

Rodent Research

The need for rodent research was deeply felt with the rodent outbreaks that devastated the farmlands in the island of Mindanao in the 1950's. With assistance from US-AID, the Philippine Government established a Rodent Research Center (RRC) on the campus of the University of the Philippines at Los Banos in 1968. Among the concerns of the center was the training of local expertise at the University and the Denver Wildlife Research Center at Colorado, USA. Quite naturally, the first research focused on rodent control in ricedields where most of the rodent problems were encountered. Research later branched out to include other crops like corn, coconut and sugarcane; and also research was initiated on birds. The activiteis of the RRC were absorbed by the National Crop Protection Center in 1976.

Storage problems came as extension of the rice production system. Surveys of private mill warehouses were conducted as part of student theses (Rubio, 1972; Aganon, 1982). RRC also studied rodent problems in farm storage in upland multiple cropping. More in-depth research in storage is now the concern of the National Post Harvest institute for Research and Extension (NAPHIRE).

Research Studies

Population dynamics mainly focused on reproductive potential and movements inside and outside the warehouse. One of the significant products of these studies was the inclusion of rodent lossed in a warehouse stock inventory system. Bird pests were also studied although to a lesser extent than rodents. Some of the specific findings are presented here to help us in our discussions.

Rodent Species

The major species affecting food in storage in the Philippines are Rattus norvegicus, the Norway rat, Rattus rattus mindanensis, the common ricefield rat, and Mus musculus, the house mouse.

These species differ in their habits, B. norvegicus is expected to be dominant over the other species because of its size. Conversely the mice will be confined to a smaller area in the warehouse.

Damage patterns in the warehouse. In a survey of 20 ricemill-warehouses in Nueva Ecija, Aganon (1982) discovered that the amount of rat damaged grain is a function of the number of sacks gnawed (r = 0.50). Further regression analysis based on the summarized annual data showed this relationship (disgregarding partial recovery of spilled or contaminated palay) to be:

y = 92.2 + .369X

where
X = number of gnawed sacks
Y=total grains loss in kg. (consumed + contaminated + spilled)

In warehouse adjacent to ricefields, damage is not related to rainfall and air temperature but to the crop stage. Damage is highest at land preparation and lowest during the rice reproductive stage. Ricefield rats also migrate to the warehouse during land preparation (Sayaboc, et al., 1984).

Larger capacity warehouses had lower incidence of gnawed sacks (r = 0.81) and warehouse near to garbage dumps, slum dwellings, and commercial establishment showed more damage. The different types of warehouses had the same amount of damage (Table 1 and 2).

Some previous studies showed that rat proofing is Types I and 11 warehouses did not significantly reduce infestation and subsequent physical losses, maybe because they were not properly maintained.

The stomach contents of trapped rodents from modern government warehouses (Type I and II) were observed to have a 99.5 percent grain component, while those collected from private warehouses (Type III and IV) contained 90 percent grains. This indicates that rats in government warehouses depend only on stored grains for food. Rats in private warehouses have alternate foods available such as feeds, grasses, fruits, etc.

It was also observed that while feeding, rodents spill 7.5 times as much as they consume indicating potentially more serious losses. Spilled grains were infected with Aspergillus flavus and A. ochraceous, the storage fungi associated with the production of carcinogenic compounds (mycotoxins) capable of causing liver and kidney damage in man. Samples were also infected with bacteria.

Poor sanitation, irregular baiting programs and improper warehouse design, i.e. unfit doors, drainage canals and gutters, are some of the factors that contribute to the rodent problem in both private and government warehouses (Sayaboc at al., 1984).

Rodent Control with Anticoagulants

Baiting was found effective and economical in controlling the rodent population in one warehouse. Prior to control, losses for 6 months were estimated at 976.65 kg paddy valued at P 1,659.09. During the first and second months of baiting, losses were reduced to 161.03 (94.72 kg) and 58.87 (34,63 kg) respectively.

No signs of rodent infestation such as damaged grains, gnawed sacks, feces, were observed during the third and fourth months of implementing the control program. The cost of control which includes: amount of poison, rice brewers, labor and cost of bait station was P 485.05 for six months. A benefit cost ratio of 1.36 was achieved using this control program on a sustained basis (Table 3).

Control Threshold

A control threshold (ETL) of 62 rats or 8.65 kg of spilled grains was computed based on consumption and baiting experiments. This figure still has to be validated.

Modeling

A rat population model was constructed using a leslie matrix. With consumption and control functions, rodent control corresponding losses can also be simulated. Results of our simulations are summarized in Table 4.

The model indicated that if an initial population were low (a pair of rats) rat problems occur starting at the 10th month of storage. Without any control measure the simulated loss is 5669.36.

The model also showed that it pays to have a high population control success (80-90%). For 90% control a two-year cost/benefit ratio of 1:14 is simulated.

P = Philippine peso
(14 P = 1 USD, 1986)

Future Needs

At present, there is no formal research project on rodent control in storage, although the importance of rodent pests in storage is still recognized and included in all training courses of NFA and NAPHIRE. Besides research, there is also a need for computerized data storage and retrieval to facilitate information exchanges among researchers.

The future needs of rodent research in storage will depend largely on the type of storage system in the future. The trend of storage system seems to be slanted towards bulk storage and storage with controlled atmospheres. It has been observed that insects bore into plastic covers enclosing CO2 treated stacks stored for 8-10 months. Going into the second year of storage, rodent gnawing of the sheets would be a greater potential problem, than insect boring. Likewise, rodents may damage structures in warehouses sealed for controlled atmosphere storage (CAST).

Presently we do not have information on how rodents would react physiologically and behaviorally to high concentrations of CO2. We have observed rodent footprints and droppings on flatbed dryers. Rodent population can possibly become established in grain silos as they have established in cold storage. These are the undnown areas for rodent research.

Table 1. Classification of warehouse site included in the field survey (Sayaboc et al., 1984)

Table 2. Average rodent population per warehouse according to type/design and daily consumption of paddy (Sayaboc et al., 1984)

1/Non-significant at 1% and 5% levels

Table 3. Cost-benefit analysis for six months (Sayacoc, et al., 1984)

Table 4. Simulated two-year rat control programs with varying effectiveness in a warehouse starting with a pair or rats.

a/ETL corresponding to percent control

LITERATURE CITED

Aganon, T.M. 1982. Studies on rat damage in Nueva Ecija ricemill-warehouses. CLSU Scientific Journals. 2(2):19-30.

Benigno, E.A.1985. Use of a population model for planning control strategies against Rattus rattus mindanensis. 1985 ASEAN Technical Seminar on Post-harvest Technology. Phil. 6-9 August, 1985.

Rubio, R.R. 1971. Survey of rat damage in Laguna in Laguna ricemill-warehouses Unpublished Undergraduate Thesis. UP at Los Banos.

Sayaboc, P. D. F. M. Caliboso, E.A. Benigno and J. M. Hilario. 1984. Rodent losses in commercial grain storage. Proc. 7th Asean Technical Seminar on grain post-harvest technology. Kuala Lumpur Malaysia 21-24 August 1984. 101-116.

I. Agencies Conducting

Rodent and Bird Research

• Rodent Research Center (1968-1976)
• National Crop Protection Center (1976 to present)
• National Post Harvest Institute for Research & Extension (NAPHIRE)
• National Food Authority
• Bureau of Plant Industry
• Ministry of Agriculture and Food
• International Rice Research Institute
• Philippine Coconut Authority
• Philippine Sugar Commission
• University of the Phil. at Los Baños
• Central Luzon State University

II. Rodent Research in Storage

• Surveys (species; damage)
• Feeding habits
• Population dynamics
• Control (Physical and chemical)
• Warehouse stock inventory
• Population modeling

III. Other Vertebrate Pests:

• Birds

1. Survey (species; damage)
2. feedig habits
3. control (nets traps and repell ants)

IV. Researchable areas:

• Intra-and interspecies competition
• Simplified warehouse stock inventory
• Model validation
• Longterm IPM program in existing storage system
• Control in other storage systems (e.g. controlled atmosphere, bulk storage)

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