However, by , just one year later, housefly resistance to DDT had evolved in Sweden. The evolution of pesticide resistance is simply natural selection occurring more rapidly than usual and on a particular obvious character. Within a large population subjected to a pesticide, one or a few individuals may be unusually resistant perhaps because they posses an enzyme that can detoxify the pesticide.
If such individuals exist at the outset, resistance can begin to spread in the population immediately; if they arise subsequently by mutation, then there will be a lag in the evolutionary response before this chance event occurs.
In either case, the resistant individuals have an improved chance of surviving and breeding and, if the pesticide is applied repeatedly, each successive generation will contain a larger proportion of resistant individuals figure 3.
Figure 3. This graph presents the chronological increase in unique cases of herbicide resistant weeds. So, if a Conyza canadensis becomes resistant to atrazine Group C1 , it is listed as one unique case, if another population of Conyza canadensis becomes resistant to ALS inhibitors Group B , then it is counted as a separate "unique" case, but if a third population is found with multiple resistance to ALS and Triazine herbicides it does not count, as the other two already cover the sites of action.
One answer to the problem of pesticide resistance is to develop strategies of 'resistance management'. This consists of two approaches; reduce the frequency with which a particular pesticide is used, thus depriving the pest of a series of generations over which resistance may evolve. This may be done by using a range of pesticides in a repeated sequence, especially when they have different target sites or modes of action.
The second strategy is to ensure that pesticides are applied at a concentration high enough to kill individuals heterozygous for the resistance gene, since this is where all the resistance genes are likely to reside when resistance is rare.
Thus, together, the problems of resistance, target pest resurgence and secondary pest outbreaks have frequently met with a predictable but, in many ways, regrettable response: the application of more and more pesticides, leading to further resistance, further resurgence and further secondary pests, and so to more pesticide, more problems and more expense; what has become known as a pesticide treadmill which managers can find difficult to get off. Whilst these problems exist within the amenity sector, e.
Talk to any cereal grower in the south east about Blackgrass Alopecurus myosuroides , and they will be all too aware of the strategies that are continually changing in order to maintain reasonable yields figure 4.
Much of the discussion around pesticides throughout this article has been focused on agriculture, with good reason: it is as a result of changes or innovation within this sector that utterly dictates the agronomic approach to pesticides within the amenity sector. However, turf managers currently have just one insecticide available to them - Acelepren - that will hopefully be provided Emergency Authorisation once again this year.
This product is also administered via stewardship, clearly indicating an increased level of control is expected to maintain the availability of this product for the foreseeable future. Figure 4. The records for blackgrass Alopecurus myosuroides within Britain and Ireland since Pesticides have provided distinct benefits and, until now, the pesticide manufacturers have managed, broadly speaking, to keep at least one step ahead of the pests.
Pesticides themselves are being used with increasing care. Many are now used as an integral part of a more varied armoury. Pesticides have also worked, in the past at least, as disease control agents. For instance, the chlorinated hydrocarbons, despite all their attendant problems, have saved at least seven million lives since ; or, to take one specific example, more than one billion people have been freed from the risk of malaria Miller, , although, since , malaria has made a remarkable come-back, owing in large part, to the insecticide-resistant strains of mosquitos, leading to an increased emphasis on possible biological control measures.
For instance, customers in richer countries have become conditioned to demand that their foodstuffs are unblemished. This means that pests have to be eradicated rather than simply reduced to a level where nutritional, rather than aesthetic or cosmetic harm is negligible.
In summary then, the case in favour of chemical pesticides is that they have worked in the past, as judged by objective measures such as 'lives saved', 'total food produced' and 'economic efficiency of food production' and that they are continuing to do so as a result in advances in the types of pesticide produced and the manner in which they are used. We have also seen though, that many of the disadvantages of chemical pesticides - widespread toxicity, secondary pests, resistance, escalating costs - are undeniable, such that the case for chemical control can only ever be one in which the advantages are shown to outweigh the regrettable disadvantages.
Since , most acres planted with major crops have been treated with herbicides, including over 90 percent of corn, cotton, and soybean acres, leaving limited potential for increased herbicide use.
As a result, the application of improved active ingredients with new modes of action and lower per-acre application rates resulted in a slight downward trend in pesticide use since the peak, to million pounds in In , corn, soybeans, cotton, wheat, and potatoes accounted for about 80 percent of the quantity of pesticide measured in pounds of a.
Soybean production had the next largest share in at 22 percent, near its all-time high share of 25 percent in Cotton accounted for just over 7 percent in , down significantly from its 40 percent share in the early s. The quantity of pesticide applied to cotton has trended downwards since due to the replacement of DDT and other older insecticides with more effective products requiring the use of smaller quantities , the eradication of the boll weevil, and the adoption of insect resistant Bt cotton that contain genes from the soil bacterium Bacillus thuringiensis that produce a protein toxic to specific insects.
The pesticide types applied by U. Insecticides accounted for 58 percent of the quantity of pesticides applied in , but only 6 percent in Herbicide applications increased from 18 percent in to 76 percent in The intensive use of registered products and a lack of alternatives has resulted in another negative side effect: the widespread emergence of chemical resistance.
Resistance emerges when genetic changes in target agripest populations results in decreased susceptibility to a previously effective pesticide. New pesticides have been developed via increased knowledge of botanical insecticides, e. However, these botanical extracts are still very toxic insecticides, and several neonicotinoids a class of insecticides have attracted concerns over their impact on non-target species such as bees. Meanwhile, despite most farmers using herbicides within regulatory limits, no-till farming has resulted in Australia having one of the biggest problems worldwide with weed species resistant to herbicides, while farmers are required to keep livestock away from sites once used for dipping sheep due to potential arsenic poisoning having leeched into the soil.
Scientists continue to raise concerns regarding wide-spread pesticide use on farm worker health, the environment and resistance evolution. These concerns often reflect the fact that regulatory systems were designed to mitigate acute risks, but do not address the impacts that wide-spread and cumulative use of multiple chemicals have in our farming systems today.
While integrated approaches have not been universally adopted, there are some successful examples. The Australian cotton industry today has moved from a catastrophic situation in the s of insecticide applications per season to control moth pests. Today cotton farmers see the benefits of a well-used integrated pest management system combined with an effective resistance management strategy to support the long-term use of genetically modified cotton plants that express a naturally occurring soil bacterium, Bacillus thuringiensis Bt.
The reality today is that alternatives to pesticides are often more costly, harder to implement, and riskier for farmers.
There is also little incentive for farmers to manage resistance evolution and adopt stewardship practices, for which they are almost solely responsible. These challenges are compounded by a general unwillingness from industry and consumers to absorb costs associated with growers adopting more sustainable practices.
Who will pay for alternatives that cannot be packaged and sold from a shop front? Food shortages caused by agripest outbreaks are rare in the modern world, but with the ongoing evolution of resistance and withdrawal of chemicals from the market, the need for new tools for crop protection will continue.
Secoy, J. ISBN , A Short History of Fungicides, V. Morton and T. Staub, APSnet, March Advanced Search. To Footprint Database. Home History of Pesticide Use.
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