TECHNICAL
Pesticides have provided distinct benefits and, until now, the pesticide manufacturers have
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managed, broadly speaking, to keep at least one step ahead of the pests
the false pink bollworm. The application rate rose to 8-10 times per year. This reduced the problem of the aphid and the false pink bollworm, but led to the emergence of five further secondary pests. By the 1960s, the original two-pest species had become eight. There were, on average, twenty-eight applications of insecticide per year (Flint and van den Bosch, 1981). On a broader scale, changes in the overall pattern of weed infestation can be seen as an example of the outbreak of secondary pests. The herbicides in use until the 1960s, when they were selective at all, tended to be most active against dicotyledonous weeds. The result has been an upsurge in the importance of grass weeds (monocots), and the 1970s therefore saw the beginnings of a new drive towards the production of herbicides selective against grasses (Lockhart et al., 1990).
Evolved resistance
The final problem is, in many ways, the most serious one of all. Even before the advent of the organics, occasional examples of resistance to an insecticide had been found. For instance, A. L. Melander in 1914 showed that scale insects demonstrated resistance to lime-sulphur sprays. Between 1914 and 1946, eleven additional cases were recorded. The development of organic insecticides, such as DDT, gave hope that insecticide resistance was a dead issue. However, by 1947, 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
130 PC February/March 2019
Figure 3. This graph presents the chronological increase in unique cases of herbicide resistant weeds. A unique case is a SPECIES x SITE of ACTION. So, if a Conyza canadensisbecomes resistant to atrazine (Group C1), it is listed as one unique case, if another population of Conyza canadensisbecomes 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.
http://www.weedscience.org/
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). 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.g. fungicide resistance within sports turf, our concerns are dwarfed by those of our close relative, agriculture, where there is a greater understanding of these problems, because there has been a greater reliance on utilising pesticides to
provide cheap food for growing populations. 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.
The virtues of chemical pesticides
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. In spite of the steadily rising costs of pesticides - the result of increasing complexity (rising development and production costs) and of oil price rises, the cost/benefit ratio for the individual facility has remained in favour of pesticide use. 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 1947; or, to take one specific example, more than one billion
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