BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.8. Human Impact on Ecosystems

10.8.4. Pesticides and the Environment

Pesticides are chemical substances used to control harmful organisms. They encompass several groups of agents: herbicides, which destroy weeds; insecticides, which kill pest insects; fungicides, which eradicate pathogenic Fungi, and so on. While most pesticides are poisons designed to kill target organisms, the category also includes sterilants (substances causing Infertility) and growth inhibitors.

In Britain, such agents are primarily used in agriculture, although they are also employed to protect food supplies, timber, and other natural products. In many countries, chemical control is used against forest pests, as well as vectors of human and domestic animal diseases (such as malaria mosquitoes; Chapter 15).

Ecologically Important Properties of Pesticides

The ecologically significant properties of pesticides include their toxicity, persistence, and Specificity.

TOXICITY. The toxicity of a substance to a particular species is usually measured by its median lethal dose (LD₅₀). This is The amount of pesticide (per individual) that kills half of the treated experimental population. In field conditions, where organisms are subjected to additional stresses and higher doses of pesticides are applied, mortality among target species can be higher, but by definition, some individuals survive. Total eradication is rarely the goal; rather, the objective is to reduce damage to an acceptable level, which is determined largely by economic considerations. Unfortunately, this survival provides a basis for Selection towards resistance to the agent, and species with short life cycles, such as insects, rapidly develop populations against which previously effective pesticides become useless.

PERSISTENCE. This parameter is determined by the length of time a substance remains in an ecosystem—including its biotic component—before breaking down and losing its pesticide properties. An example of an extremely persistent chemical pesticide is the organochlorine agent DDT, which was widely used from the 1940s to the 1960s.

Generally, high pesticide persistence is undesirable (especially on food crops), but in certain situations—such as controlling animal ectoparasites or soil pathogens—it is practically and economically important. However, the long-term presence of a chemical pesticide in the environment can lead to unpredictable and potentially dangerous consequences. For example, in the mid-1960s, DDT was discovered in the livers of penguins in Antarctica, very far from where the chemical had been applied.

Toxicity and persistence are practically interrelated: a potent but short-lived agent is often less harmful to nature and humans in the long run than a weak poison that retains its properties for years. The latter has a higher chance of entering food chains, where it may be metabolized into an even more dangerous form or (typically) become concentrated in the bodies of top-level trophic predators (see Question 10.21).

Pesticide poisoning has a devastating effect on many carnivores, especially birds. For instance, the peregrine falcon disappeared entirely from the eastern United States As a result of DDT use there. Birds are particularly sensitive to this chemical because it induces hormonal changes affecting calcium METABOLISM, which leads to the thinning of eggshells. Consequently, large numbers of eggs break even under normal incubation.

DDT is now banned in most developed countries, including Britain and the USA. However, it is relatively cheap and is still considered an effective remedy in certain situations, such as controlling malaria mosquitoes. When deciding whether to use a particular pesticide, one often has to choose the lesser of two evils. For example, DDT has helped completely eradicate malaria in many countries.

SPECIFICITY. Pesticides vary in their specificity, i.e., the range of organisms they affect. DDT, for instance, has a broad spectrum of activity, killing many species of animals. Pirimicarb has a much narrower spectrum, affecting aphids and dipterans while sparing beetles and many other insects. Similarly, dalapon kills monocotyledonous plants while sparing dicots, whereas phenoxyacetic acid herbicides exhibit the exact opposite effect.

The Use of broad-spectrum pesticides carries the risk of pest "resurgence," i.e., their reappearance after Treatment in greater numbers than before. This occurs because the preparation kills not only the pests but also the predators that kept them in check.

A good example of this is the use of DDT to control caterpillars of the small white butterfly (Pieris rapae), a pest on Brussels sprouts. Initially, DDT treatments yielded a noticeable effect, but gradually the Abundance of pests became even higher than in control (untreated) plots (Fig. 10.31). The difference was even more pronounced with repeated Applications of DDT to "suppress" new pest outbreaks. Analysis of the agroecosystem showed that the pesticide concentration in the leaves eaten by caterpillars declines rapidly due to the overall growth of the plant's green parts. However, the level of the chemical in the soil remains high, especially if post-harvest plant residues are plowed in. As a result, caterpillars hatching from eggs laid on leaves after treatment suffer little, whereas the numbers of their primary enemies—ground beetles (Harpalus rufipes) and harvestmen (Phalangium opilio)—decline. Facing less pressure from predators, the pests significantly increase their survival chances, a benefit that outweighs even The impact of the chemical. Further pesticide applications only worsen the situation (Fig. 10.31). Predators often suffer more from pesticides than herbivorous pests because predator population densities are initially lower, making them more vulnerable and slower to recover after a population crash.

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Fig. 10.31. Differential effects of DDT on animals living on plants and in the soil. Small white butterfly caterpillars feed on green leaves. DDT spraying effectively destroys these pests only for a brief period in the first year (A). Because pesticide residues accumulating in the soil kill the predators that eat the caterpillars, the pest population increases significantly with repeated treatments (B). (J. D. Dempster (1968) The control of Pieris rapae with DDT, J. Appl. Ecol. 5, 451–62.)

10.21. Fig. 10.32 shows DDT concentrations at various trophic levels of a food chain (data obtained in the USA).

a) If the concentration of DDT in the Water surrounding Algae is 0.02 ppm^-1, What is the concentration factor of this substance when it passes into:

1) producers;

2) small fish;

3) large fish;

4) apex predators?

b) What Conclusions can be drawn from the Answers to question a)?

c) At which trophic level:

1) might DDT have the strongest effect;

2) is DDT easiest to detect;

3) are crop pest insects located (a typical target for DDT)?

d) How can DDT find its way into Antarctic penguins?

e) Clear Lake is a large recreational lake in California, widely used by sport fishermen. The disruption of this natural ecosystem by eutrophication (nutrient enrichment; Section 10.8.2) led to a population surge in the 1940s of Blood-sucking dipterans, namely gnats. To control them, spraying with DDD (a pesticide closely related to DDT) was carried out in 1949, 1954, and 1957. The first two treatments killed about 99% of the gnats, but their population quickly recovered, and the third treatment proved virtually ineffective.

Analysis of small fish from the lake showed that the DDD content in their Muscle tissue (consumed by humans) was 1–200 ppm^-1, and in their adipose tissue, 40–2500 ppm^-1. The breeding population of western grebes there died out, and DDD was detected in the adipose tissue of these fish-eating birds at a concentration of 1600 ppm^-1.

1) Why did DDD fail to eradicate the gnats, and why did their population rapidly recover after the third treatment?

2) Observations indicate that many animals die from DDT poisoning during periods of food scarcity. Based on the data above, explain why this happens.

f) In Great Britain, the winters of 1946–1947 and 1962–1963 were particularly severe. Bird mortality was high in both cases, but much higher In the second than in the first. Given the information about DDT provided above, explain this difference.

Fig. 10.32. Biomass pyramid and DDT content (ppm^-1) at different trophic levels of a food chain.

The long-term effects of pesticides, especially at low doses, and their potential synergism with other environmental pollutants and disease vectors remain poorly understood due to the relative novelty of most synthetic chemicals. There are growing concerns that "harmless" traces of their metabolites persisting in food—while not exerting toxic or lethal effects—may nevertheless lower Disease resistance and gradually accumulate in the body to dangerous levels. Many scientists link the presence of pesticide residues in the North Sea to the rapid spread of viral diseases in the harbor seal population in the summer of 1988.

The overall effect of pesticide use is a reduction in species diversity. Pesticides also typically increase productivity at lower trophic levels while decreasing it at higher ones. Their impact on decomposers is poorly understood, and the consequences of all these changes for nutrient cycling and soil fertility also require further study. Fig. 10.33 summarizes the Main Pathways of pesticide impact on ecosystems. Consider what changes occur in ecosystems as a result of pesticide application.

Fig. 10.33. Main pathways of pesticide impact on ecosystems. C — competitor species; P — food species (prey); M — habitat species; X — predator species. (Adapted from: N. W. Moore (1967) A synopsis of the pesticide problem, Advances in Ecological Research, J. B. Cragg (ed.) pp. 75—126, Blackwell.)

The problems described above—particularly The Development of resistance in target species, the "resurgence" of pest populations, and the hazards of agricultural chemicals to human health—have prompted an intensified search for alternative pest management strategies. Below we briefly characterize two of them: biological control and integrated pest management. The latter involves more targeted pesticide applications combined with biological Methods, quarantine measures, etc.

Biological Pest Control

Biological pest control is traditionally understood as The regulation of pest numbers by their natural enemies—predators, parasites, and pathogens. This is a form of population management that prevents their uncontrolled exponential growth (Section 10.7.3 — growth curves), i.e., population explosions. Some scientists also include genetic manipulation among biological control methods. Cultural control, sometimes considered a subset of biological control, encompasses plant protection practices such as specialized crop rotations, specific tillage operations, intercropping, removal of crop residues from the field, and shifting sowing or harvesting dates to periods unfavorable for phytophages and/or favorable for predator population growth.

Classical biological control has been most successfully applied to introduced species whose populations in a new Location are not limited by the physical or biotic factors acting in their native range. The first scientifically grounded and successful experiment of this kind involved the management of the cottony cushion scale (Icerya purchasi), which was damaging new citrus plantations in California in the late 19th century. This insect had been imported along with citrus nursery stock from Australia. Field studies in the pest's native range revealed two of its natural enemies: the parasitic fly Cryptochetum iceryae and a predator, the vedalia beetle Rodolia cardinalis. They were brought to California and, after thorough study in the new environment, released into the orchards. The parasite and predator spread rapidly and, within a matter of months, achieved effective and stable suppression of the scale population.

Success is not always achieved so quickly. Sustainable Introduction of a beneficial species requires suitable climatic conditions and adequate interactions with the local biota as a whole. For example, in an attempt to control the walnut aphid (Chromaphis juglandicola) in California, its parasite, the wasp Trioxys pallidus, was introduced from near Cannes, France. This yielded some positive results in the coastal zone, but in the hot interior of the state, where the main walnut groves are located, all the wasps died out within a single season. Only 10 years later, following a thorough survey of regions with a similarly hot and dry climate, was it possible to source another strain of Trioxys from Iran. These wasps successfully overwintered and cleared more than 130,000 km² of territory from the pest within a year, with the parasitism rate exceeding 90%.

Biological control is widely used in the enclosed environment of commercial greenhouses. Soil sterilization in winter kills all beneficial predators. New plants introduced in spring are usually already infested with pests. These pests lack natural enemies, and their population grows very rapidly; therefore, it is either necessary to use pesticides or to quickly restore the predator population. The latter are introduced into the greenhouse when the pest population has become large enough to sustain the predators with food, but before it causes noticeable damage to the crops. This method is highly effective and yields chemical-free produce.

Any species can be used for biological control only after a thorough analysis of the ecosystem into which it is intended to be introduced. In particular, all potential prey of the introduced predator must be identified. Otherwise, unpleasant surprises cannot be ruled out. For example, the mongoose, introduced to Jamaica to control the black rat, preferred to attack its local natural enemies instead. Furthermore, it significantly reduced populations of several bird species and completely eradicated several reptile species on the island.

Integrated Pest Management

This involves a thoughtful integration of biological and chemical methods for pest population management. Pesticides are used only as a supplementary measure when absolutely necessary, and applied in the gentlest way possible. The primary objective is to maintain pest populations at an economically acceptable level (or prevent their proliferation altogether) while minimizing any disruption to the agricultural or natural ecosystem, and most importantly, preserving the beneficial predators and parasites resident within it.

To achieve this, highly specific (selective) pesticides are employed, such as pirimicarb, which selectively targets aphids. This compound enabled the development of an integrated pest management program for the peach potato aphid (Myzus persicae) and spider mites (Tetranychus sp.) on greenhouse chrysanthemums in Britain. Previously, both of these pests were controlled using organophosphorus insecticides, which lack specificity and have led to the development of resistance in aphids.

Due to economic constraints—the research and development costs for selective chemicals are quite high relative to their limited market demand—an alternative approach is also applied: more targeted applications of broad-spectrum pesticides. In this case, their effective specificity is enhanced through localized application or meticulously timed usage. Spatial targeting of treatments is greatly facilitated by pheromones (Ch. 17). For instance, sex attractants are used to lure pest insects into traps containing a toxicant or sterilizing agent. Pheromones can also be deployed to concentrate target pests in a restricted area, followed by a blanket treatment of that specific zone with a pesticide. This drastically reduces the overall demand for chemicals, causing minimal harm to harmless species and the ecosystem as a whole. Another promising application of pheromones is the disruption of pest behavioral responses, such as mating inhibition caused by saturating the air with a sex attractant (which requires very low concentrations of the substance). The animals become habituated to the constant stimulus, their search for a mate is suppressed, and consequently, population growth is curbed.

The frequency of pesticide applications can be minimized by precisely timing treatments to coincide with the period of maximum vulnerability for the pest (such as the mating season), while ensuring impact on non-target species is kept to a minimum. This requires a detailed Study of the life cycles and ecological interactions of all community members potentially affected by the chemicals.