Pesticide Options

For all of their negative connotations, pesticides remain an indispensable part of pest management. Pesticide modes of action have continued to expand despite perennial concerns expressed as early as the 1970s about the finite number of active ingredients remaining to be discovered and the tremendous cost of development of each new insecticide.66 Nonetheless, whole new insecticide classes have been discovered and commercialized since that time, as insecticides in general have become safer and more effective.101 The resulting diversification of insecticides has provided the knowledgeable pest manager with many more treatment options than were formerly available with broad-spectrum modes of action. It was not a matter that too few products were available in the early decades of synthetic organic insecticides, but that the modes of action were limited to just two or three target sites in insects. This limitation must have resulted in greater selection pressure being exerted compared with the much larger selection of modes of action that are available in today's pest management. Indeed resistance to insecticides quickly rose to become a major problem in pest management and remains a serious concern as always. With greater awareness of the problem, however, and a much wider selection of products representing many times more modes of action than in the past, the outlook for managing resistance is much brighter.

Pesticides are powerful agents of destruction that are intended for specific pest species but often have much broader impact on non-pest organisms as well. This is a principal reason why they need to be used with the greatest care and restraint, to avoid as much as possible destroying non-target organisms that contribute to a stable equilibrium among pest and non-pest species. In addition to the obvious beneficial insects that include predators and parasitoids of the pest species of concern, non-target also designates all those other arthropods in a crop that generally do not merit concern from a plant injury standpoint, but whose presence may contribute to food web complexity and encourage greater biological control. Insecticides that are physiologically more selective in activity towards a target pest are ones that express higher relative toxicity to the pest species than to non-target species. This is possible because molecular receptors at the target sites where the insecticide molecules interact vary in receptivity according to species level differences or at higher taxonomic categories. Thus, activity spectra among insecticides vary tremendously with those affecting a broader range of vertebrate and invertebrate animal species being considered ''broad-spectrum'' insecticides. From an environmental and public health perspective, narrow spectrum or higher selectivity insecticides are those that are most toxic to insects only and are relatively non-toxic to vertebrate animals including humans. However, insecticide selectivity extends beyond the vertebrate/invertebrate divide to where particular taxonomic categories of insects are either more or less susceptible to an insecticide. From an IPM perspective, selective insecticides that are more toxic to pest species than to non-target species are ones that are good candidates for integrating into biologically intensive IPM. The overall selectivity of an insecticide treatment can be improved further by timing an application to when a target pest is more exposed and vulnerable, or possibly by reducing the dose that is still effective against the pest without decimating non-target organisms. These latter approaches fall into the category of ecological selectivity and should always be considered prior to an insecticide application so as to minimize exposure to non-target organisms.

Using insecticides that have systemic mobility in plants can further enhance ecological selectivity. While a number of organophosphate and carbamate insecticides have systemic properties, they are more commonly used in spray formulations with the exception of aldicarb, a highly toxic compound that is applied as granules to the soil for plant uptake. Imidacloprid is a much more widely used systemic insecticide that was commercially introduced in the early 1990s. It was the first member of the neonicotinoid group of insecticides102 that has since expanded to include additional compounds that also can be soil-applied for systemic uptake. The advantage of soil application from an IPM standpoint is that there is no direct contact by foraging beneficial insects with an insecticide applied as a spray to the crop or as a residue after application. Imidacloprid is often applied to the soil ahead of the seed drop at time of planting, especially in situations where there is chronic pest pressure and no uncertainty about whether the pests will invade the crop. Herbivorous insects that feed on treated plants should be the only mechanism by which exposure occurs. In reality, however, the translocation of imidacloprid from the roots through xylem vessels to all parts of the plant results in contamination of nectar and pollen. Honeybees are thus vulnerable along with other nectar and pollen feeders. It is now also known that certain weather and soil moisture conditions that are conducive to guttation occurring in a plant can result in residues of imidacloprid being deposited on leaf surfaces. Despite these drawbacks, soil-applied imidacloprid is much more compatible with biocontrol agents than most foliar insecticides simply because the exposure levels are much lower.

There are still newer groups of insecticides than the neonicotinoids that also exhibit systemicity within plants. One of these is the Insecticide Resistance Action Committee (IRAC)102 group-28 diamide insecticides that have good activity against lepidopteran pests that also helps fill an important gap in the pest spectrum the neonicotinoids do not cover. A second new insecticide group is the tetronic and tetramic acid derivatives (group 23) from which the compound spirotetramat is both xylem and phloem mobile within plants (see Figure 9.5). This compound is unique in that it is applied to foliage rather than to the soil, yet is translaminar in the leaf where it moves into phloem tissue, translocated to the roots and then back up the plant via the xylem. Foliar treatments of spirotetramat defeat the limited exposure concept of soil-applied systemic compounds, but nevertheless have little contact toxicity due to their mode of action that inhibits lipid synthesis and growth regulation.

In addition to conserving natural enemies, a second principal reason for the restrained use of pesticides is to conserve active ingredients by protecting them from resistance. The capacity to diversify insecticide treatments has increased dramatically over the past two decades. For example, at the time of the B. tabaci outbreak in the Imperial Valley in 1991 (see Figure 9.3A), only three modes of action were available that included pyrethroids, cyclodiene organo-chlorines, and the organophosphates and carbamates that share the same target site. The number of modes of action registered for use on crops against

Spirotetramat

Chlorantraniliprole

Figure 9.5 Structures of spirotetramat (Insecticide Resistance Action Committee's group 23: tetronic and tetramic acid derivatives) and chlorantraniliprole (IRAC group 28: diamides).

Spirotetramat

Chlorantraniliprole

Figure 9.5 Structures of spirotetramat (Insecticide Resistance Action Committee's group 23: tetronic and tetramic acid derivatives) and chlorantraniliprole (IRAC group 28: diamides).

B. tabaci in the USA is now ten, more than triple the number from two decades ago. These newer products have been indispensable to restoring control of B. tabaci in regions where population outbreaks were exacerbated by resistance. Not only are the new products more effective control agents at reduced rates compared to conventional insecticides, they are much less destructive to natural enemies. Many of the diverse modes of action have highly specific activity against certain stages of development and therefore must be used in the proper context according to circumstances in the field. This requires a high level of understanding on the part of pest managers to be able to make correct treatment decisions. Good stewardship of these valuable products requires that they be used knowledgeably and with restraint to ensure food security and sustainability.

9.5.4 Biotechnology Options

The development of biotech crops represents a revolutionary change in pest management, but one that has been restrained by societal concerns from becoming an even larger part of the pest management repertoire. Nevertheless, the trend towards increasing acceptance of biotech crops appears to be accelerating with their adoption having occurred on all continents with the exception of Antarctica. As of 2008, biotech crops were planted in 25 countries on a total of 125 million ha, a 9.4% increase over the previous year.103 More impressively, when the area planted to biotech crops is considered in terms of acres, 2008 marked the first time that biotech crops exceeded 2 billion acres. The time required to attain the first billion acres was 10 years, from 1996 till 2005, but then only 3 years (2006-2008) were required to accumulate the second billion acres.103 Herbicide-tolerant biotech crops accounted for 63% (79 million ha) of the total area in 2008 compared to only 15% (19.1 million ha) of insect-resistant crops. However, another 22% (26.9 million ha) consisted of stacked double or triple traits that included at least one insect-resistant trait.103 There are currently several new biotech crop products under development that feature multiple traits including insect-resistant, herbicide-tolerant and other agronomic traits that are expected to become dominate in the near future.103 All insect-resistant biotech crops commercialized thus far have drawn from the family of cry genes isolated from the soil bacterium Bacillus thuringiensis, commonly known as Bt in crop protection (see Chapter 8). The first insect-resistant biotech crops were transformed by splicing into the crop plant genome just a single cry gene, e.g. cry1Ac in cotton. The translated Cry proteins from their respective cry genes vary in toxicity against several lepidopteran and coleopteran pest species. By pyramiding two or more cry genes into a plant, it has been possible to expand the pest spectrum to which a particular biotech cultivar expresses resistance. For example, the original Bollgard® cotton expresses Cry1Ac protein that is highly active against lepidopterans that feed on cotton bolls, such as Heliothis virescens and Pectinophora gossypiella, and reasonably effective against Helicoverpa armigera and H. zea.104 By pyramiding the cry2Ab gene along with cry1Ac, Bollgard II® was created that offers much improved protection against H. armigera and H. zea, while retaining outstanding activity against H. virescens and P. gossypiella. Another example of pyramided genes includes the cry1F and cry1Ac genes in cotton, which adds protection against Spodoptera spp. while retaining efficacy against bollworms. In addition to increasing the breadth of protection against pest species, pyramided cultivars putatively decrease the likelihood of resistance by raising the toxicity to a higher level for a resistance mechanism to overcome. This approach ties into the high dose/refuge strategy of resistance management that posits an inverse relationship between the expression of a toxin and the frequency of a genotype able to survive. The theory behind the refuge part of the strategy is that if, or when, a resistant genotype appears in a population, a surplus of susceptible genotypes maintained in an untreated refuge will greatly outnumber resistant genotypes. The probability of a cross occurring between the rare resistant and abundant susceptible genotypes to produce a heterozygote at the resistance locus will be increased in the presence of a susceptible refuge. Assuming that the inheritance of the resistant allele is recessive, the heterozygote will not survive a high dose with any more likelihood than a fully susceptible genotype. After 15 years of deployment of insect-resistant biotech crops, there have been very few signs of resistance development in insect populations, either in countries with mandated resistance management programs or ones without. Recently, however, populations of H. zea in the southeastern USA have shown increased tolerance to Cry1Ac and Cry2Ab in Bt cotton in laboratory bioassays, but without any apparent loss of field efficacy.105

The accumulated data since the first deployment of insect-resistant biotech crops present a very positive picture regarding pest management, agronomic and environmental benefits. Significant global declines in insecticide use have been recorded in insect-resistant maize (-35.3%) and cotton (-21.9%) during the period 1996-20 08.106 Other evaluations suggest an even larger reduction in insecticide use in individual countries, e.g. 50 and 65% reductions in India and China, respectively.107 In China and perhaps elsewhere, the impact of insect-resistant crops has been even greater as decreased insecticide use has been measured in other crops (corn, peanuts, soybeans, vegetables) across regions where Bt cotton is grown and H. armígera is a primary pest.108 This is an example where Bt cotton has served as a trap crop109 for all other crops in the region that also faced infestation by the polyphagous H. armigera, effectively becoming a dead-end for any mated female that deposited eggs in the Bt cotton. In China and elsewhere, farmers have benefitted not only from reduced insecticide costs, but also by gains in yield and gross profit margins. This is believed to be a major reason why the rate of adoption of biotech insect-resistant crops has been so rapid, especially in developing countries.107

There are a number of metrics that have been developed to estimate the environmental impact that has occurred through adoption of biotech crops. Consider for a minute the amount of fossil fuel required and the emissions generated to fly crop-duster airplanes over fields or to drive tractors up and down rows to spray pesticides. Then there is the release of volatile organic compounds into the atmosphere, the runoff into aquatic systems and contamination of ground water, the toxicity to birds and fish and mammals, health effects on humans, etc, all from the millions of tons of pesticides applied to the earth each year. Researchers at Cornell University developed an environmental impact quotient (EIQ) that incorporates three principal components that include farm worker, consumer and ecological components, each one consisting of multiple rating factors.110 An EIQ equation is then used to distill all of the factors for each pesticide into a single EIQ value. In a counterfactual analysis using data obtained from conventional crops, Brookes and Barfoote106 estimated the pesticide applications that were not made as a result of cultivating all biotech crops, herbicide tolerant and insect resistant. They determined that a 16.3% change in reduced environmental impact has occurred globally from 1996-2008 as a result of biotech crops. The two biggest changes occurred in insect-resistant maize (-29.4%) and insect-resistant cotton (-24.8%). There have also been enormous savings in fuel consumption and carbon dioxide (CO2) emissions from the cultivation of every one of the biotech crops.106 For example, the fuel savings in insect-resistant cotton (assuming four tractor passes per ha, 1.045 l ha 1 fuel consumed per pass) over the 1996-2008 period amounts to 124.99 million liters, with 343.75 million kilograms of CO2 emissions also saved.106

The most apparent negative effect from growing insect-resistant biotech cotton has been an increase in various hemipteran species, most probably due to a decline in insecticide use. With the bollworms and other foliage feeding Lepidoptera kept in check by the Bt proteins expressed in the insect-resistant biotech cotton plants, plant bugs and stinkbugs have become more of a focus of pest management based on a slight increase in insecticides targeting these pests.111 In northern China, mirid bugs have increased progressively in conjunction with Bt cotton adoption. Cotton under conventional management acts as a sink for mirid bugs, but the decrease in insecticide sprays in Bt cotton has now made it a source.112

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