Insects crop losses

Injury to plants and vegetation is caused by a variety of factors, of which air pollution is only one. Drought, too much water, heat and cold, hail, insects, animals, disease, and poor soil conditions are some of the other causes of plant injury and possible plant damage (3). Estimates suggest that less than 5% of total crop losses are related to air pollution. Air pollution has a much greater impact on some geographic areas and crops than others. Crop failure can be caused by fumigation from a local air pollution source or by more widespread and more frequent exposure to adverse levels of pollution. 

Furthermore, significant amounts of food are lost to pests. Worldwide preharvest crop losses to pests (insects, diseases, and weeds) are estimated to be about 35% each year (10). These major pest losses are occurring despite the application of about 2.5 million metric tons of pesticide, at an annual cost of 18 billion (11). Even after crops are harvested, an additional 20% of the food is lost to pests (12). Overall about 48% of all potential world food supply for humans is lost to pests despite all efforts to protect it. 

The agricultural use of pesticides such as insecticides, herbicides, and fungicides has clearly reduced crop losses due to insects, weeds, and plant diseases in the US and throughout the world. The benefits from agricultural pesticides include improved crop yields, greater availability of fruits, vegetables, and grains, and lower consumer costs (Ecobichon, 1996). 

Food production in the last few decades has been improved greatly by applying chemical fertilizers, irrigation methods, rotation of crops, and plant-breeding practices. Despite these new technologies for increased food production, the crop losses caused by insect pests, pathogens,... 

Insects, weeds, disease, and rodents destroy 30% of the world s food supply. In developing countries, the crop losses may be even higher. The World Health Organization estimates that about 12,000 people starve daily, that is 4.4 million per year. Not only is the waste of food inexcusable, but it represents a waste of the energy used for production. 

In the United States crop losses due to pests are about 30% or 20 billion annually, despite the use of pesticides and other current control methods (3). Ware (3) addresses the question of "what would the losses be without the use of insecticides " Studies conducted in 1976-1978 compared the yields from test plots, where insecticides were used to control insects, to adjacent plots in which the insects were allowed to feed and multiply uncontrolled. Summary data from this study are shown in Table IV. These data suggested that, on the average, for the crops evaluated, half of the crops are lost to insects. Even with insecticides, 10% of the crop is lost. The issue, however, is what rate of return can a farmer expect from the use of insecticides on these crops The data suggested an average increased yield of 36%. Pimentel and Levitan (4) concluded that in economic terms, for the 3 billion invested in the United States in controlling pests through the use of pesticides, about 12 billion are returned in increased return on investment. 

Crop losses caused by insects went up from 7 to 13% in the period from 1945 to 1989, despite the use of ten times as much insecticide in 1989.76 The reasons probably involve the shift to intensive monoculture and the loss of natural predators. As mentioned earlier, the lack of specificity and the toxicity of insecticides are problems that need to be solved in a system of sustainable agriculture. 

The best-known recent aphid damage is that of Russian Wheat aphid, Diuraphis noxia which was first discovered in Texas in 1986 but has now spread to 16 million of the total of 59 milhon wheat and small grain acres in the Great Plains states causing 37,000,000 annual crop loss (5). Due to the widespread use of synthetic pyrethroids killing beneficial insect predators, aphids are becoming increasingly important on cotton. 

Baby leaves and herbs are attractive to a wide range of pests especially aphids, leaf miners, caterpillars and slugs. The contamination of the crop with insects is also a commercial issue. Seedling diseases can affect the crops such as Pythium and Rhizoctonia but the main disease is downy mildew which can lead to complete crop loss if untreated. Resistant lines are available for some of the larger volume crops like spinach but downy mildew evolves new strains rapidly and plant breeders are always playing catch up . 

Careful estimates suggest that the total losses to agricultural crops from insect attacks in the United States average about 10% of production and amount to more than 14 biUion annually (1). Worldwide agricultural losses from insect attacks have been estimated as about 14% of production (2). 

Pests and Insecticides. The most destmctive pests of the cotton plant are the boU weevil and the boUworm/budworm complex. They are serious threats to the cotton industry in countries around the world. The boU weevil migrated from Mexico around 1892 and spread over the entire cotton belt within 30 years. The domestic cotton crop lost to the weevil is worth 200 million a year. In addition, about 75 million a year is spent for pesticides to control this destmctive pest (8). Unfortunately, some insecticides used to control the weevil kill many beneficial insects. Among the undesired casualties are insects that help to control the boUworm and the tobacco budworm, pests that cause another 200 million loss in cotton. 

Furthermore, the share cf crops lost to Insects has nearly doubled during the last 40 years (Table 1), despite a more than 10-fold increase in both the amount and toxicity of synthetic insecticide used (17-19). Up to the present time the increased insect losses, in terms of yields per hectare, have been offset by increased crop yields obtained through the use of higher yielding varieties and greater use of fertilizers and other energy-based inputs (20, 21). 

Synthetic pyrethroids now account for at least 30% of the world insecticide market and are rapidly replacing other agricultural chemicals for control of insect pests. Fenvalerate is one of the more widely used synthetic pyrethroid insecticides. It is derived from a combination of a-cyano-3-phenoxybenzyl alcohol and a-isopropyl phenylacetate ester. Technical fenvalerate is a mixture of four optical isomers, each occurring in equal amounts but with different efficacies against insect pests. Fenvalerate does not usually persist in the environment for >10 weeks, and it does not accumulate readily in the biosphere. Time for 50% loss (Tb 1/2) in fenvalerate-exposed amphibians, birds, and mammals was 6 to 14 h for reptiles, terrestrial insects, aquatic snails, and fish it was >14 h to <2 days and for various species of crop plants, it was 2 to 28 days. Fenvalerate degradation in water is due primarily to photoactivity, and in soils to microbial activity. Half-time persistence in nonbiological materials is variable, but may range up to 6 days in freshwater, 34 days in seawater, 6 weeks in estuarine sediments, and 9 weeks in soils. 

Fenvalerate persists for <10 weeks in the environment and does not accumulate readily in the biosphere. Time for 50% loss (Tb 1/2) in fenvalerate-exposed amphibians, birds, and mammals is 6 to 14 h. For reptiles, terrestrial insects, aquatic snails, and fish, it is usually >14 h to <2 days and for crop plants, it is 2 to 28 days. In nonbiological compartments, Tb 1/2 is up to 6 days in freshwater, 34 days in seawater, 6 weeks in estuarine sediments, and 9 weeks in soils. 

As alluded to above, even a crop that sustains losses from an insect pest may actually be capable of inhibiting insect growth. A number of aspects of the insect dietetics may be interacting to produce such a situation. The feeding insect must ingest food "that not only meets its nutritional requirements, but is also capable of being assimilated and converted into the energy and structural substances required for normal activity and development" (2,. ... 

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