Insecticide Resistance Management Strategies

Project and Seminar Material for Agricultural Science Students

Abstract

The use of chemical products to control arthropod pests has occurred for thousands of years, beginning with the use of inorganic products in ancient cultures, and progressing to the development of synthetic insecticides beginning in the early 20th century. As these chemicals have imposed selection pressure on insects, the insects have adapted to this pressure, leading to the development of insecticide resistance. Since it was first reported in 1914, insecticide resistance has grown to be a major concern facing agricultural production, as each insecticide chemistry introduced is impacted by the evolution of resistance. This report was therefore carried out on insecticide resistance management strategies. In the latter half of the 20th century, renewed focus was placed on non-chemical pest management practices and their use in Integrated Pest Management programs to help improve the sustainability of insect pest management and slow the development of insecticide resistance. New insect management products such as plants modified to express Plant Incorporated Protectants (PIPs) provided new options to producers who were interested in diversifying their management strategies, but they brought with them resistance management challenges of their own. To maintain the utility of insecticides for insect pest management, it is necessary to take steps to utilize effective Insecticide Resistance Management, through measures like rotating insecticide modes of action, utilizing resistant plants, planning planting and harvests to minimize pest pressure, and many others. These tactics will continue to be necessary in the future but increasing plant varieties will further help to reduce the pressure placed on insecticides in pest control. Research into the basis of resistance in insects will help inform decisions regarding resistance management approaches that may be effective. New technologies for PIPs such as RNAi can help to diversify the products available to producers and potentially help to increase the efficacy of tactics already in use. The future of Insecticide Resistance Management will require integrated, research-based solutions to maintain the sustainability of insect management.

Chapter One

1.0 Introduction

1.1 Background of the Study

Insecticide resistance is an ever-growing concern in crop production. New insect species develop resistance to various chemistries every year, and some pest species have become very difficult to control due to their resistance to a wide variety of insecticides. This is not a new concern, however. Insecticide resistance has been observed in some species since shortly after the first synthetic pesticides were marketed, and it is likely these concerns will continue. To maintain the efficacy of our management practices and products, it will be important to continue to explore and discover new ways to manage insects effectively.
Humans have been utilizing insecticides in one form or another to protect crops from various insect pests for thousands of years. Ancient Sumerians and other cultures four thousand years ago utilized inorganic sulfur dusts to protect their crops (Oberemok et al., 2015). Other records from the same time mention using poisonous plants to protect against insect pests. Within the last several hundred years, several plant extracts with pesticidal properties have been identified and widely utilized for pest control. Dried dusts, known as “Persian dust,” produced from Chrysanthemum roseum and other closely related flowers which can be dried to produce an insecticidal dust known as pyrethrum (Casida, 1980), were utilized in Europe some 200 years ago for control of a variety of insect pests (Davies et al., 2007). Extracts of elderberry flowers, tobacco, and wormwood have all been shown to have insecticidal properties and have been used effectively against various insects (Oberemok et al., 2015).

In the middle of the 19th century, several new chemicals began to be used to protect plants from insects. The inorganic compound Paris green (copper-acetoarsenite) was first successfully applied in 1871 to aid in control of Colorado potato beetle, Leptinotarsa decemlineata (Say), as it spread across the United States (Alyokhin, 2007). DDT was also first synthesized at around this time, though its insecticidal properties were not discovered until 1939 (Jarman & Ballschmiter, 2012). DDT was widely used during World War II for control of insect-born disease in humans, though it later was also used for control of agriculturally relevant insects. It became widely used for control of codling moth (Laspeyresia pomonella), pink bollworm (Pectinophora gossypiella), and many other pests (Madsen & Hoyt, 1958; Tsao & Lowry, 1963). A number of other organochlorine insecticides, such as aldrin and dieldrin, were first utilized at around this same time. Within the next few decades, many of the synthetic insecticide families and chemistries that are widely used today were developed and released to the public. In the 1940s and 1950s, the first organophosphate insecticides were introduced, and the carbamates were introduced in the 1960s (Casida & Durkin, 2013).

At around this same time, the use of pyrethrins, the insecticidal esters providing the insecticidal properties of pyrethrum, reemerged in pest control, and research of their chemical structures led to the development of the first synthetic pyrethroid chemicals in the 1950s (Davies et al., 2007). Over the following decades, new, more stable pyrethroids were developed, and they entered widespread use in the 1970s (Matsuo, 2019). The relatively low mammalian toxicity of the pyrethroids greatly promoted the use of these chemicals over some of the other popular chemicals at the time.

Few areas of applied entomology have advanced as rapidly or received such widespread attention in recent years as that of insecticide resistance. This reflects both the increasingly severe impact of resistance on pest and disease management programmes, and, as is evident from several other chapters in this book, the exciting contributions that resistance is making to fundamental knowledge of insect genetics, biochemistry and physiology. These insights should, in turn, offer greater prospects for developing or fine-tuning strategies aimed at circumventing the impact of resistance on pest management, or even preventing its appearance in the first place.

Opportunities for exploiting knowledge of mechanisms to combat resistance were outlined very effectively by George Georghiou in his landmark paper entitled “Management of resistance in arthropods” (Georghiou 1983). As well as presenting a novel classification and a comprehensive review of operational tactics for combating resistance to insecticides, this paper also highlighted several areas of research needed to refine such tactics and adapt them to specific resistance problems. These included detailed work on the toxicological and biochemical nature of resistance mechanisms, in order to improve the detection of resistance genes and provide means of mitigating mechanisms through the use of synergists and/or novel „resistance-defeating‟ toxicants.

1.2 Problem Statement

While insecticides are a very useful tool in the pest management toolbox, they also carry with them risks, and improper management has led to widespread development of insecticide resistance. Similar views have been expressed by a number of authors including Scott (1990) and McKenzie (1996). Holloway and Forrester (1998) also emphasized how an improved understanding of resistance mechanisms may contribute to effective recommendations for resistance management strategies. Similarly, McCaffery (1998) considered an understanding of the mechanisms underlying resistance to be central to optimizing the use of existing insecticide chemistry in cases which resistance already has evolved, and to understanding cross-resistance patterns within and between chemical groups. Other attributes of resistance mechanisms such as their effective dominance and phenotypic expression under field conditions also play a critical role in the development and management of resistance, but have been reviewed comprehensively elsewhere (Roush and Tabashnik 1990; Denholm and Rowland 1992; McKenzie 1996).

This seminar report explores the extent to which research on resistance mechanisms is contributing or might contribute to managing resistance in practice.

1.3 Purpose of the Study

The main purpose of this report is to assess the insecticide resistance management strategies. Specifically, the report seeks to achieve the following;

To review the resistance impact of insecticides
To describe major types of resistance mechanisms and how cross resistance develops
To identify strategies to properly manage and control insecticide resistance using proper chemical class and m monitoring techniques

1.4 Research Questions

What is insecticide resistance and its impact on crops?
What are the major types of resistance mechanisms and how does it develop?
What are the strategies to properly manage and control insecticide resistance?

1.5 Significance of Study

This study will explore the insecticide use trends of the 21st century, and how they led to the rapid development of insecticide resistance throughout the world, as well as some of the early attempts to slow this development and change the way insecticides were used. Also, the report will explore more recent developments and technological advancements related to insecticide resistance and its management, as well as discussing the extent of insecticide resistance and some of the most concerning cases of resistance. Finally, the study will explore potential methods for the future to support insecticide resistance management, including alternative insect management methods and how new technologies can aid in understanding and managing insecticide resistance.

Chapter Four

4.0 Summary, Conclusion and Recommendation

4.1 Summary

This report focused on the insecticide resistance, factors influencing its effectiveness, and strategies to improve insecticide resistance management. From the forgoing, the theoretical framework of population genetics predicts that resistance is far easier to circumvent when the underlying resistance genes are still rare and/or very localized in their distribution (Roush and Daly 1990). Unfortunately, the development of resistance often goes unnoticed at the critical early stage, only becoming apparent once the proportion of resistant individuals is sufficiently high to visibly impair the efficacy of pesticide applications. By then, the options for modifying control regimes to combat the threat are very limited and far less likely to be effective. In addition, scope for switching to new chemical groups once resistance becomes established is diminishing as novel chemicals become increasingly difficult to discover and register, and cross-resistance conferred by broad-spectrum detoxification systems and insensitive target proteins becomes increasingly pervasive (Denholm et al. 1998a).

Monitoring programmes to detect resistance genotypes and/or phenotypes, as early as possible, and to document their distribution, should therefore be a key component of any resistance management strategy. Whole-organism bioassays, involving topical application or exposure to pesticide residues on surfaces or in food, have long been the cornerstone of such programmes, but are limited in their application and power of resolution (Roush and Miller 1986). Comparisons of LD50 or LD90 values of samples from populations – the most widely adopted approach – may be useful for detecting a high frequency of resistant insects but are far too insensitive for detection incipient resistance. Use of a „discriminating‟ dose or concentration corresponding to the LD99 or higher of baseline susceptible populations, although a better use of resources is still subject to important statistical constraints. Firstly, the estimation of these doses is challenging because the fitting of probit or logit models is usually imprecise at the extreme ends of dose-response relationships. Secondly, unless doses are perfectly diagnostic (i.e. killing 100% of susceptible ones but no resistance ones, which is rarely the case), sample sizes required for the reliable detection of even 1% resistance may be prohibitively large.

Although of relatively recent appearance, insecticide resistance is now very widespread and is being increasingly used as a model system for understanding how organisms adapt to human activity and environmental stress. In recent years, biochemists and molecular biologists have established a role at the forefront of such research. Their contribution should be evaluated first and foremost for its fundamental significance to evolutionary biology rather than its short-term practical benefits.

4.2 Conclusion

Although this work does have important implications for pest management, particularly for the design of new toxophores less vulnerable to known routes of resistance, its direct relevance to managing resistance is constrained on at least two counts. Firstly, time lags between discovering and characterizing resistance mean that data on the underlying mechanisms may not available when they are most needed, i.e. at the early stages of resistance development. Secondly, even if such data were available, attempts to combat specific mechanisms are continuously threatened by the genetic plasticity of insect pests, and hence the risk of selecting for alternative mechanisms. Despite our rapidly increasing knowledge of the biochemical and molecular nature of resistance, it seems certain that management strategies will continue to rely largely on „broad-brush‟ tactics equally applicable to whatever mechanism may be present.

4.3 Recommendations

The following recommendations were made;

In many cases, a specific Mode of Action insecticide can be used across a range of crops to control multiple pests that have the ability to move from crop to crop. There is interaction between intensive horticulture and broad acre farming, as with Diamondback Moth (DBM) in Brassica vegetables and resistance strategies that could be compromised by widespread use of insecticides for DBM control in canola.
Also, the pest complex for a specific crop will vary within production regions. For this reason, CROP by PEST strategies can be flawed and further Insecticide Resistance Management (IRM) advice for specific pests should always be sought on a local basis.
An alternative to the CROP by PEST strategy is that of “Regional strategies” such as those for Cotton, Brassicas and the Southern NSW and Northern Victorian IRM strategy for grain and annual horticultural crops”.
The overall Resistance Management Strategy of avoiding overuse of individual Modes of Action insecticides should be followed, not just on a specific crop and pest but on a broad perspective of crops and pest complex.