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A Thorny Problem

Strategies for managing pesticide resistance are imperative if we are to keep horticultural pests in line.

Max Suckling and Graeme Bourdôt

Food production and human living standards have risen dramatically in many parts of the world during this century, especially in the industrialised countries. Chemical pesticides, pharmaceuticals and other xenobiotics have emerged as powerful and highly cost-effective tools in the fight against the ravages of pests and diseases. Antibiotics and drugs provide excellent protection against diseases in humans and domesticated animals, while herbicides, fungicides, insecticides and miticides contribute greatly to the higher yields and improved quality of food and fibre crops that are characteristic of modern agricultural systems.

However, the high efficacy of these xenobiotics has proven to be a powerful agent for evolution in the targeted pest populations. It is perhaps not surprising that their widespread and repeated use has resulted in several significant problems, including ecological disturbances of various kinds. However, certain agricultural and horticultural sectors remain reliant on synthetic pesticides while more environmentally benign alternatives are developed.

Pesticide resistance now threatens the long-term viability of plant protection chemicals worldwide. For example, more than 500 species of pest insects are resistant to one or more chemical groups. While the numbers of plant pathogens and weeds showing resistance to pesticides are lower, the problem is showing no sign of being any less severe. New Zealand has not escaped this looming problem, despite ongoing attempts to make our agriculture and horticulture less dependent on pesticides. Resistance management is now an imperative we cannot afford to ignore.

The Force of Evolution

Pesticide resistance is the development of genetic change in a population, following exposure to selection. It is pre-adaptive, in the sense that the genes for resisting a chemical agent exist in the population before the agent is introduced. Such resistant genes usually occur at a very low frequency, at or close to the mutation rate (generally estimated at between 1 per 100,000 to 1,000,000).

Individuals with such traits normally have associated lower fitness in other attributes, which prevents these genes from becoming fixed in the population (the organisms die before passing on their traits to another generation). However, strong directional selection for a certain trait, combined with limited gene flow, and backcrossing with the wild type can lead to the emergence of high frequencies of resistant genotypes.

In insects, a range of resistance mechanisms have been identified, including reduced rates of penetration, sequestration, or biochemical detoxification with the battery of enzymes developed for dealing with plant defense compounds. Many of these enzymes have enabled insects to readily adapt to new compounds. In experiments conducted at Michigan State University, insects have even developed resistance to household cleaners!

In plants, herbicide resistance may either be inherited quantitatively (being controlled by multiple loci or by multiple alleles at a single locus), or in a Mendelian fashion through a single recessive, semidominant or dominant gene. It is generally assumed that repeated use of herbicides favours selection of existing resistance genes, and that herbicide use does not increase the mutation rate.

Rising Resistance

There has been a rapid escalation of herbicide resistance worldwide during the 1980s, particularly involving the new and widely-used acetolactate synthase inhibitors and the new acetyl coenzyme A carboxylase inhibitors. Resistant plants may utilise many different mechanisms to overcome herbicides, including target site modifications that prevent herbicide action, and metabolic breakdown of the herbicide.

There is now evidence that plants are accumulating resistance mechanisms, resulting in multiple herbicide resistance in individual weed species. While the complexity of the interactions between weed species, herbicides and environment makes resistance mechanisms and their associated mutations difficult to predict, one certainty is that novel and multiple mechanisms will continue to develop.

Despite many similarities in the factors affecting resistance evolution, some fundamental differences in the biology of organisms in the different kingdoms have a major part in the development of resistance. For example, most weeds have a substantial seed bank, which acts to buffer the rate of resistance development. However, once this reservoir of susceptibility is polluted with resistance genes, the seed bank will then act to substantially increase the lag time before resistance disappears, and allow rapid resurgence of resistance if the herbicide is used in future years.

In insects, similar reservoirs of susceptibility exist ("refugia"), particularly with species possessing a wide host range other than the crop to which the pesticide is applied. Insect movement, and consequently gene flow, is a major factor delaying resistance, but only up to the point at which the immigrants into the crop remain fully susceptible. The rate of emergence of resistance accelerates once the immigrant population begins to reinforce the locally-selected resistant trait, and the benefits of dilution by susceptibles disappears.

Another difference between kingdoms is in the mode of inheritance of resistance traits. With many organisms, the offspring of a cross between purely resistant and susceptible parents may be recessive (like the susceptible parent), dominant (like the resistant parent) or, very commonly, intermediate between the two parents. However, some exceptions exist which are unique to insects. For example, coffee berry borer from New Caledonia, which is 1,000-fold resistant to endosulfan (an organochlorine relative), is intermediate in dominance in females, and either dominant or recessive in males depending on the mother's genotype.

Problems in New Zealand

Cases of pesticide resistance were first documented in New Zealand during the 1960s, when DDD resistance was found in leafrollers from Nelson apple orchards and DDT resistance in grassgrub. Repeated episodes of miticide resistance in apples in the 1960s and 1970s led to the development of successful biological control of mite pests, once organophosphate-resistant predatory mites became widespread. This situation permitted the use of these insecticides against insect pests, without disturbing biological control. However, increasing numbers of insect species have exhibited organophosphate resistance, due to the intensive reliance on this group by growers involved in export horticulture.

There has been increasing interest in resistance management since the 1980s. Somewhat surprisingly, New Zealand has yet to see codling moth resistance, although this phenomenon is widespread in Europe, the US, Australia, and South Africa. The problem of insecticide resistance is now mostly confined to the sectors such as vegetables or pipfruit, where the industry reliance on broad spectrum insecticides is greatest.

Research teams at HortResearch, Crop & Food Research and AgResearch are engineering resistant plants, which will have some protection against caterpillars incorporated into the plants, using the gene from a bacterial insect pathogen (Bacillus thuringiensis). One key part of this research is to plan the strategy for delaying the emergence of insect strains capable of surviving on the new plants, whether they are clover, brassicas or apples.

The elements of resistance management are essentially no different for transgenic plants, compared with other xenobiotic delivery systems. An integrated approach is needed, with reliance on a range of tactics simultaneously (i.e. Integrated Pest Management). Tactics such as the combination of transgenic plants and mating disruption using sex pheromones may offer promise in the future deployment of these plants, but unique solutions will be needed for each cropping system.

Herbicide resistance in weeds in New Zealand was discovered in the early 1980s. New cases have continued to emerge both on cropping and pasture land with the number of species involved currently standing at six. Many years of reliance on MCPA-based herbicides for pasture weed control throughout New Zealand has resulted in the widespread occurrence of giant buttercup (Ranunculus acris) and nodding thistle (Carduus nutans) populations resistant to these chemicals in dairy and sheep pastures respectively. Although there is some indication that a new sulfonyl urea herbicide may be effective against MCPA-resistant giant buttercup, the outlook for nodding thistle control with herbicides is less optimistic. Biological controls with insects, fungi and grazing animals are possible solutions here.

Management Methods

Resistance management aims to slow or prevent the development of resistance in the target populations, usually by reducing selection pressure. However, pesticide use is driven by economic necessity -- for example, exporting growers need to meet international requirements, while non-pesticidal controls are currently inadequate or uneconomic. Farmers use herbicides to prevent yield losses and reductions in the marketable value of produce. This leads to the dilemma of how to reduce selection pressure, while maintaining pest, disease or weed control at acceptable levels. There has been extensive international debate about possible strategies for resistance management in the last 10 years, but most attempt to reduce selection pressure.

Theory suggests that lower pesticide use rates, which permit more survivors, may lead to the development of polygenenic resistance -- the accumulation of several low level resistance mechanisms in the same individuals. Alternatively, use of high doses may achieve complete kill of the target population, ideally including resistant individuals. However, this strategy can lead to rapid resistance development if this high level of kill is not achieved. This occurs most rapidly when the dose affecting the population is sufficient to cause the resistance to be functionally dominant.

Multiple or cross resistance is an increasing risk with many pesticides, including fungicides, when resistance mechanisms are capable of permitting survival of different types of pesticide chemistry. Prior exposure to one chemical group can result in an increased frequency of several resistant types in the population, so that the effective life of new pesticides may be seriously shortened. Reliance on new chemistry from industry for solving resistance problems is unlikely to be sustainable in the long term.

Prevention of herbicide resistance must be the aim of any weed management programme. There is a compelling need to educate growers to rotate herbicides with different modes of action and thus reduce the selection pressures for particular resistance mechanisms. Prophylactic or "insurance" spraying must be replaced by a truly integrated approach with herbicides being used less often, and only in combination with other methods.

Herbicide resistance may be permanent once dominant in a weed population. Ecological studies with giant buttercup in New Zealand have shown that the frequency of resistant phenotypes declines only slowly when herbicide selection pressure is removed. Experimental data suggest that 28 years of discontinued herbicide use will be needed for reversion to the resistance frequency found in populations never exposed to herbicide. Such a prognosis demonstrates the need for farmers to implement preventative herbicide resistance management measures.

There has been a certain degree of interest in resistance, and resistance management since the 1980s in New Zealand. The New Zealand Committee on Pesticide Resistance was formed, as a joint government and industry initiative, to develop and promote pesticide resistance management strategies. Two task groups were established to deal with insecticide and fungicide resistance problems. The result was the development and ratification by industry of resistance management strategies for spider mites, leafroller caterpillars, benzimidazole fungicides and demethylation inhibitor fungicides.

In the early 1990s, some new resistance problems began to emerge in New Zealand, including an increasing number of cases of resistance in insects and fungi, as well as previously unknown cases of herbicide resistance in pasture and cropping land weeds (which resulted in the formation of a herbicide task group).

Before developing management strategies, it is essential to understand that pesticide resistance is an evolutionary phenomenon governed by two key biological processes, genetic variability and selection intensity.

The process over which we have some control is selection. Reducing the intensity of selection for the resistant mutants -- which population biology theory demands will be present in most pest populations -- forms the basis of Pesticide Resistance Management across a wide range of organisms.

Strategies for fungicide, insecticide, miticide and herbicide resistance problems all focus on tactics by which the grower can minimise the selection of pesticide-resistant mutants in pest populations, while maintaining economic production.

A unifying theme across all pests is the need to combine pesticide usage with other methods of control, instead of relying solely on the pesticide. While such integrated pest management (IPM) is the only practical approach to reducing the rate of resistance evolution, it is rarely practiced. This is often because growers are forced into short-term decision-making by the need to confront immediate problems, frequently under severe financial constraints. Commercial considerations force pesticide manufacturers to use pesticide marketing strategies that maximise pesticide usage and hence resistance evolution -- even if their research departments acknowledge the necessity of resistance management.

The challenge, as Steve Powles from the Cooperative Research Centre on Weed Management in South Australia points out, is "to achieve grower adoption of IPM practices which accommodate commercial stringencies and yet recognise the biological reality that resistance will result if IPM is not conducted". It is imperative that this challenge be met if agriculture is to retain the powerful technology that chemical pesticides represent.

New pesticides, offering more environmentally benign pest management opportunities for the future, are as much at risk as existing older products, so grower adoption of management strategies is likely to be of enduring benefit. Whether these strategies are effective or not will depend on whether growers adopt them. Adoption will only happen if growers can recognise the economic advantages in resistance management.

The costs of failing to implement resistance management in New Zealand could be very significant, especially in high-value export industries, where market access can be affected. At the same time that we recognise the need for resistance management, we need to urgently develop more integrated and inherently sustainable insect, pathogen and weed management strategies to avoid these problems in future.

The authors have recently edited a book entitled Pesticide Resistance: Prevention and Management, on behalf of the New Zealand Plant Protection Society.

Graeme Bourdôt works for AgResearch based in Lincoln, Canterbury.
Max Suckling works for HortResearch at Lincoln, Canterbury.