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The economic importance of rodent pests
There are three major reasons why rats and mice are considered pests:
After harvest the crop attains its highest value, taking into account all the costs of producing it, processing, (packaging), storage and distribution prior to consumption. The actual value of the losses caused by rats vary by crop, variety, year, geographical location, pest species involved, length and method of storage and climate (Gratz 1990). The exact post-harvest losses are difficult to assess. A review can be found in Jackson (1977) and Meehan (1984). Some examples based on surveys are given below which indicate the huge financial losses that have been found and can generally be expected.
Surveys conducted in small warehouses in the Philippines indicated losses of 40 to 210 kg of grain in each (Rubio 1971, Agnon 1981 as cited in Benigno and Sanchez 1984); at the time this was equivalent to about US $ 80 for each unit. Interviewing farmers in Bangladesh on rodent damage inside houses provided an estimated loss equivalent to US $ 29.50 for a six month period (Bruggers 1983). This figure is supported by Mian et al (1984) who found that, on average, households were each infested by about 8 mice and 2 rats. At 10.5 million households the annual losses are estimated at US $ 620 million for the entire country in houses only. Higher estimates were found by Krishnamurthy et al (1967) in a similar study in India. In large grain stores the situation may be even worse. For example, Frantz (1977) estimated that each godown in Calcutta had, on average, a population of about 200 Bandicoot rats. At an estimated 50 gm one rat can destroy in one night appreciable losses will accumulate.
To these food losses, costs for cleaning produce, the losses due to damaged packaging (Meehan 1984) as well as structural damage have to be added. It is impossible to put an exact estimate on these losses, but it is obvious that the damage caused by rodents is enormous.
Diseases transmitted by rats to man pose a serious public health problem in tropical countries. Apart from causing human suffering, they are responsible for financial losses incurred through work-days lost and additional medical bills.
While this topic is not directly related to post-harvest problems, it bears relevance to postharvest rodent control. As rodent pests in stores and households are controlled the rate of disease transmission will be reduced. Gratz (1988), Fiedler (1988) and Meehan (1984) have reviewed this aspect in detail.
Just as it is difficult to put exact figures on losses caused by rodent pests, it is not easy to estimate the exact benefits of rodent control. However it is apparent that rodent control is mostly if not always cost effective.
In the U.S.A. the annual loss to rodents is estimated at US $ 900 million (Clinton 1969 as quoted in Meehan 1984), while the annual cost of control is estimated at US $ 100 million (Brooks 1973 as quoted in Meehan 1984). According to Sumangil (1990) losses of rice in the Philippines were reduced from US $ 36 million to US $ 3.5 million with the advent of organised rat control programmes.
In Bangladesh two national strategic multi-media rodent control campaigns were organised and analysed in detail. Net profits were calculated at US $ 800,000 for each campaign, based on a single crop and season (Adhikarya and Posamentier 1987). Calculated benefits would be a multiple of this figure, if all crops could have been surveyed and the reduction in structural damage and human suffering could be quantified. Further field studies in the same country have shown clearly that losses can be reduced by 40-60% at farm level also (Posamentier 1989).
Dubock (1984) and Richards (1988) describe some examples of rodent control in urban and rural situations, including warehouses, in various Asian and Central American countries. The cost-benefit ratios ranged from 1:2 to 1:30. Hernandez and Drummond (1984) found that in Cuban warehouses the loss of 1% of the amount available to human consumption could be readily preventable by standard control techniques.