SCOPE 49 - Methods to Assess Adverse Effects of Pesticides on Non-target Organisms

12.1 INTRODUCTION

The quantity of pesticides applied in world agriculture annually is estimated to be about 2.5 million tonnes at a cost of about $16.3 billion (US) (Helsel, 1987). Despite the use of this 5 million tonnes of pesticides and all other controls, pests in the world are probably destroying about 35 per cent of all potential crops before harvest (Pimentel and Pimentel, 1979). These losses to pests are primarily due to insects, plant pathogens, and weeds. After the crops are harvested, an additional 20 per cent of the crops are probably destroyed by insects, microorganisms, rodents, and birds. Thus, nearly half of all potential food in the world is being destroyed annually by pests, despite all efforts to control pests with pesticides and other types of controls.

Although large quantities are applied to crops, only a small percentage of the applied pesticides reach the target peststhis is estimated to be less than 0.1 per cent (Pimentel and Levitan, 1986). Thus, more than 99 per cent of applied pesticides go off into the environment and affect non-target sectors of the ecosystem, including soil, water, atmosphere, and the non-target biota. Pesticides, however, are generally profitable to use in crop production. On average, pesticide use on crops returns about $4 (US) per $1 invested for pest control (Pimentel et al. 1991). These are direct benefits, and do not include the social and environmental costs of using pesticides (Pimentel et al., 1980a).

Integrated pest management (IPM) is playing a role in slowing the use of pesticides in the world, but IPM practices that actually result in reducing pesticide use have been only gradually adopted (Pimentel, 1986). One important reason for the slowness is the problem of developing non-chemical controls to replace chemicals. Also, non-chemical controls are sophisticated, and this contributes to the difficulty of farmers adopting the new approaches for pest control. 

It is hoped that biotechnology and genetic engineering will help to develop new biological pest controls using microorganisms, and that these will be easy to use and effective (NRC, 1987; Pimentel, 1987). Although the new biotechniques for pest control will help reduce the use of pesticides, biotechnology and genetic engineering will probably create some serious environmental problems themselves (Pimentel, 1987; Pimentel et al., 1989). 

The objective of this paper is to investigate the ecological effects of pesticides and genetically engineered pest control organisms on non-target species in terrestrial ecosystems. Particular attention will be given to methodology for conducting sound ecological investigations of the effects of pesticides and genetically engineered biocontrol organisms on the environment. 

12.2 TROUBLESOME PESTICIDES AND BIOLOGICAL CONTROL AGENTS

There are nearly a thousand different types of pesticides in use (Metcalf et al., 1962; WHO, 1988). Clearly, it would be difficult to list all pesticides that are hazardous to the environment and the kinds of ecological effects each has on the environment. This is especially true when the environment includes 510 million species. The United Nations (WHO, 1988) completed a listing of the most hazardous pesticides to humans. This is a valuable list, but its limitation is that it is restricted to lethal dosages to humans based on rat and other laboratory animal studies. 

In general, the toxicity and the hazard to the environment have a relationship. Insecticides generally are the most hazardous to the environment, followed by fungicides and herbicides. This is a generalized statement, because certain herbicides are highly toxic and present a greater hazard to the environment than some insecticides. Thus, one has to be specific about which pesticide (including its dosages and methods of application) is being investigated in an ecological study.

DDT and related chlorinated pesticides are noted for their deleterious impact on non-target species. An important reason for the serious ecological effects of the chlorinated pesticides is that these pesticides have a high lipid/water coefficient, and thus have a tendency to accumulate in organisms (Pimentel and Edwards, 1982). Most organisms contain lipids and, hence, are likely to take up lipophilic pesticides (such as many of the organochlorine insecticides) from the soil and water environments. In certain environments, organisms may bioconcentrate these fat-soluble pesticides at 101000 times the level found in the ambient environment (Pimentel, 1971). Pesticides that are water soluble may also be a serious hazard because of their movement in water in the terrestrial ecosystem. For example, pesticides such as aldicarb and several herbicides have a tendency to be washed from the treated crop area to other parts of the ecosystem, including the aquatic system (Pimentel and Edwards, 1982). Another attribute that can make some pesticides hazardous is their relatively high activity at relatively low concentrations. Aldicarb and parathion are pesticides with these characteristics. This is why the UN (WHO, 1988) lists these materials as highly toxic and hazardous to use.

Persistence is another characteristic that makes pesticides hazardous to the environment (Pimentel and Edwards, 1982). Obviously, the longer a chemical remains in the environment, the greater the chance that it will have adverse effects, and the greater the likelihood that it will spread or be transported from the treated crop ecosystem to other ecosystems. Also, the pesticide may accumulate in the environment if the life of the chemical is greater than the frequency of application. Classic examples of pesticides that persist for years are many of the organochlorine insecticides like dieldrin (Pimentel, 1971) and the herbicide paraquat. Some pesticides that are readily adsorbed into organic matter or clay particles persist longer in the environment (Pimentel and Edwards, 1982). However, these polar compounds may be less biologically active than non-polar compounds. Conversely, non-polar compounds tend to persist for shorter periods, because they are not adsorbed and thus are readily available to living organisms.

Biotechnology has promised new biological controls to replace pesticides. Although these new biological pest controls will have some advantages over the use of pesticides, the new biotechnology and biological controls are not without risks themselves (Pimentel et al., 1984, 1989). For example, five vertebrate species that were intentionally introduced for biological controls have all become pests themselves in the United States (Pimentel et al., 1989). These biocontrol species include the Indian mongoose, the English sparrow, and the grass carp.

Such genetically engineered microbes as bacteria and viruses for biological control may reduce pesticide use but also may cause environmental problems (Pimentel et al., 1989). Several microbes have already been engineered for biological control. One of these is Pseudomonas syringae (Ps), which was modified for the biological control of frost-causing types of Ps (Lindow, 1982). Spraying enormous numbers of the altered bacterium, known as the `ice minus strain', on potatoes, tomatoes, strawberries, corn, and other crops allows the bacterium to outproduce and replace the wild form of Pseudomonas. This could extend the crops' growing seasons and increase yields.

Although preliminary tests suggest that this engineered organism may be relatively safe, Lindow reported that the naturally occurring Ps is an `important plant pathogen' in 17 crops and will also infect at least 100 species of plants in nature (Lindow et al., 1978). Clearly, under these circumstances, tests in contained environments were advisable to demonstrate that the modified strain was non-pathogenic in both crops and wild plants (Pimentel, 1987).

12.3 INJURIOUS EFFECTS ON ECOSYSTEMS

The injurious ecological effects of pesticides and biotechnology on terrestrial ecosystems are summarized based on currently available data.

12.3.1 STABILITY

Pesticides have been reported to influence populations of organisms and thus change the interactions and stability among species within ecosystems (Müller et al., 1981). The best documented cases of such ecological effects are from agroecosystems. When insecticides were first used on tropical cotton crops, for example, they controlled the two or three major pests of the crop and greatly increased yields. Within a few seasons, however, the chemicals reduced populations of parasites and predators, and a number of other arthropod species became serious pests (ICAITI, 1977).

Orchards are complex ecosystems that are easily perturbed by the extensive use of pesticides, and there are many instances of increased pest attacks in orchards after the use of pesticides (Pimentel, 1971). These include outbreaks of codling moth, leaf rollers, aphids, scales, and tetranychid mites because controlling parasite and predator populations had been drastically reduced (Brown, 1978; Tolstova and Atanov, 1982; Bostanian et al., 1984). Also, intensive use of herbicides, insecticides, and fungicides in orchards leads to reduced soil fauna and microflora and increased leaching of nutrients from the soil (Rusek and Kunc, 1983).

Pesticides can affect animal reproduction directly, as evidenced by the deleterious effect of the persistent organochlorine insecticides on reproduction in raptors and other birds. Eggshell thinning due to the uptake of organochlorine insecticides that affect calcium (Ca) metabolism has been observed in predacious birds (Keith et al., 1970; Newton et al., 1986; Wiemeyer et al., 1986; Opdam et al., 1987). Fish-eating birds are more severely affected than terrestrial predatory birds, because the fish-eating birds acquire more pesticides via their food chain than the other predators (Pimentel, 1971; Littrell, 1986).

Pesticides can also affect reproduction in the invertebrates; for example, sublethal doses of DDT, dieldrin, and parathion increased egg production by the Colorado potato beetle by 50, 33 and 65 per cent, respectively, after two weeks (Abdallah, 1968). The herbicide 2,4,5-T was found to reduce the reproduction of soil-inhabiting Collembola (Eijsackers, 1978). Populations of invertebrates with high rates of increase can recover stable populations much more rapidly than those of bird and mammal populations (Pimentel and Edwards, 1982).

In some cases, insect pests have altered their behaviour to avoid insecticides. For example, houseflies were found to avoid resting inside treated barns. Instead, the resistant flies rested most frequently on untreated vegetation outside the barns.

12.3.2 WASTE DISPOSAL AND NUTRIENT CYCLING

Most dead organic matter is broken down by the activities of the soil fauna and microflora, which together fragment and decompose the material (Pimentel and Warneke, 1989). These organisms assist in organic matter decomposition, incorporate the degraded material into the soil, and help to convert mineral elements into forms available for plant growth (Pimentel et al., 1980b). A large portion of the pesticides used ultimately reaches the soil, where it can affect soil organisms directly or indirectly (Fletcher, 1960; Edwards and Thompson, 1973; Broadbent and Tomlin, 1982; Prasse, 1985). Populations of earthworms, for example, which are among the most important decomposer organisms, are decreased drastically by some pesticides (particularly chlordane, endrin, parathion, phorate) and nematocides, fungicides, and carbamate pesticides (Edwards, 1980; Barker, 1982; Lofs-Holmin, 1982).

Soil microarthropod populations and rates of decomposition and mineralization have been reduced by insecticide treatments in agricultural and grassland ecosystems (Perfect et al., 1981; Pimentel and Warneke, 1989), forest areas (Weary and Merriam, 1978), and a desert ecosystem (Santos and Whitford, 1981). Such effects generally are transient, but they may result in a reduction in soil fertility and productivity (Perfect et al., 1979).

Most nutrients, especially carbon, nitrogen, phosphorus, potassium, and sulphur, are taken up by plants, which in turn may be eaten by animals. The amounts and forms of nutrients in soil and plants may be changed by pesticides affecting the dynamics of these elements in the ecosystem. For example, the herbicide simazine increased water and nitrate uptake in barley, rye, and oat seedlings, resulting in increased plant weight and total protein content of the plants (Ries and Wert, 1972).

12.3.3 DIVERSITY AND FOOD CHAINS

Pesticides are notorious for altering the interrelationships of species in ecosystems (Pimentel, 1971). Some species are sensitive to some or all pesticides and may be exterminated from an ecosystem so that species diversity (richness) is reduced (Pimentel and Edwards, 1982; Tolstova and Atanov, 1982; Prasse, 1985). For instance, the number of species of soil-inhabiting arthropods in a cereal crop were reduced considerably by a single recommended dose of DDT, and a greater reduction resulted from the use of aldrin (Edwards et al., 1967).

Parasites and predators frequently suffer greater mortality from insecticides than do their herbivorous hosts (Pimentel, 1971; Croft and Brown, 1975), either because of greater susceptibility to the chemical or because their food supply is drastically reduced after pesticide use. For example, treatment of cole (Brassica) crop plants with either DDT or parathion reduced the number of taxa of herbivores, but reduced the number of parasitic and predacious taxa even more (Pimentel, 1961a), with consequent effects on species diversity and the complex structure of this ecosystem (Figure 12.1). The selective application of some insecticides may, through the reduction of some predacious species, increase species diversity (Risch and Carroll, 1982).

In terrestrial ecosystems, herbicides can either increase or decrease plant diversity, depending on the initial floristics of the area and specificity of the chemicals used (Tomkins and Grant, 1977; Wakefield and Barrett, 1979). Where herbicides affect plant species composition, animal populations have been shown to respond in turn (Malone, 1969; Spencer and Barrett, 1980). For example, the large-scale application of herbicides in southern England suggests that the presence of these pesticides in cereals is leading to the elimination of insect-desirable plants. This has led to decreasing numbers of partridges, because there are fewer insects on which the young partridge chicks can feed (Potts, 1977). 

Figure 12.1. Relationships between the cole crop plant (Brassica oleracea), insect pests (), and parasitic (----) and predacious (^{. . . .}) enemies of the pests (Pimentel, 1961a)

12.3.4 ENERGY FLOW

Energy flow through ecosystems can be influenced by pesticides that alter primary production. The use of pesticides in agroecosystems usually increases primary production, and there is good evidence that primary production in grassland and forests is also increased (Shure, 1971). However, frequent use of pesticides does not always concomitantly increase crop and forest yields. For example, regular use of some insecticides on cotton and in orchards has created new insect and mite pests, thus sometimes reducing productivity (Edwards, 1973; ICAITI, 1977).

In orchards, the use of the fungicides benomyl or methyl thiophanate to control scab has sometimes led to increased outbreaks of the disease and reduced productivity (Edwards and Lofty, 1977; Lofs-Holmin, 1982). These chemicals are toxic to earthworms, which help to control apple scab by removing most of the infected apple leaves from the soil surface.

Organisms in ecosystems exist in complex interdependent associations, and the transfer of food energy dominates most interspecies relationships in ecosystems (Elton, 1927). In ecosystems, energy flows from producers through the lower trophic levels to higher levels in food chains. Clearly, if pesticides kill organisms lower in the trophic levels, less energy will be available for those species at the higher levels (Müller et al., 1981; Pimentel and Edwards, 1982). In the first years of herbicide application in orchards, an increased food supply (dead organic matter) and more favourable soil moisture led to a higher population density of Enchytraeidae and higher soil respiration activity in the soil (Rusek and Kunc, 1983).

12.3.5 POLLINATION 

Cross-pollination is essential to reproduction in many plants. A total of 90 US crops, valued at more than $4 billion (US), are dependent upon insect pollination; and nine additional crops, valued at more than $4.5 billion (US) are significantly benefited by insect pollination (USDA et al., 1969). In addition to the cultured plants, large numbers of wild plants require cross-pollination. 

An estimated 20 000 species of US native flowering plants are dependent upon biotic cross-pollination mechanisms (Pimentel et al., 1980c). Honey bees and wild bees carry out most of this pollination, and many of these species are highly susceptible to pesticides (Pimentel, 1971; Pimentel et al., 1980a). In some circumstances, farmers have had to rent honey bees or by other means encourage the presence of bees on their farms.

12.3.6 RECOMBINANT-DNA BIOTECHNOLOGY 

12.3.6.1 Ecological effects

The planned release of genetically engineered organisms (GEO) into terrestrial and aquatic environments raises concerns about potential short- and long-term effects that GEOs could have on processes and populations associated with many diverse ecosystems (Rissler, 1984; Milewski, 1985; Pimentel, 1987; Pimentel et al., 1989). A list of research issues includes:

1. Determination of the effects of GEOs on various components of ecosystem structure and function

2. Delineation of geochemical and biological processes most useful for describing ecosystem structure and function

3. Microcosm and/or field studies (i.e., verification of laboratory-derived methodologies and results) that estimate ecosystem responses in potentially impacted areas or under GEO-use conditions 

4. Verification of biological indicators developed in the laboratory on population, community, and ecosystem responses to GEO exposure 

5. Development of model systems to predict resiliency (impact and recovery) of populations, communities, and ecosystems exposed to GEOs 

6. Assessment of risks associated with release of GEOs by integration and interpretation of biological, chemical, and physical data on aquatic and terrestrial ecosystems (Crane and Moore, 1984; Lance and Gerba, 1984; Corapciolu and Haridas, 1985).

12.3.6.2 Adverse ecological effects

Although we are fortunate that no environmental disaster has resulted to date from genetic engineering, some environmental problems will probably result from the release of genetically engineered organisms for several reasons. First, none of the test protocols will allow us to predict with 100 per cent accuracy the impact of genetically engineered organisms on the environment (Pimentel et al., 1989). There is no way in which sufficient tests can be run in the laboratory and in controlled microcosms to eliminate all possibilities of a disaster (Hagedorn and Lacy, 1988).

Historically, we have been unable to distinguish accurately all beneficial organisms from potential pests (Pimentel et al., 1989). More than 128 species of crops and animals, for example, that were intentionally introduced as beneficial have become some of the most serious pests known in the United States. Farmers spend several billion dollars annually for their control. Johnsongrass, for instance, is the most serious weed pest in the southern United States (Williams and Hayes, 1984). Other potential adverse effects of biotechnology (Pimentel et al., 1989) include the following.

1. Developing crop resistance to herbicides will encourage the use of a wider array of herbicides on a variety of crops and probably increase the use of herbicides, thus intensifying the ecological problems associated with these pesticides. 

2. Developing microbial agents with recombinant-DNA (R-DNA) technology may help reduce pesticide use. The same agents, however, may harm or displace beneficial organisms.

3. Although genetically engineered microbes released to attack specific chemical pollutants have the potential to improve the environment, these microbes must be compound-specific and produce harmless by-products; otherwise, they could pose environmental hazards.

4. When unique genetic characters are released into the environment via microbes and other organisms, a few of these novel DNA sequences may be transferred to other microbes and other organisms.

5. Because most pest species are of native origin, using native organisms to genetically engineer new organisms is probably not any safer than selectively introducing foreign organisms.

6. Few ecological niches in the ecosystem are completely filled; therefore, natural communities are unlikely to resist invasion by foreign organisms, including genetically engineered organisms.

7. Although R-DNA technology has the potential to increase genetic diversity in agriculture and forestry, it is expected to reduce further genetic diversity in these systems.

8. Accuracy in predicting the adverse ecological effects of releasing a genetically engineered organism depends on the specific organism, the type of genetic information introduced, the particular environment into which it is released, and the availability of detailed ecological information.

12.3.6.3 Research approach

A lack of scientific understanding and a lack of appropriate methodologies exist for examining the effects of GEOs on ecosystem structure and function. Studies dealing with the effects of adding a GEO to a natural ecosystem will usually require more highly detailed microcosm simulations. Three principal research approaches that appear to be useful include:

1. Expanded use of microcosms as representations of various ecosystems including appropriate design features, characterization of the most critical parameters, calibration to the field, and standardization of the protocols; 

2. Establishment of ecosystem baseline data for any particular habitat to permit confident analysis of ecological effects (and the degree of change required to detect a significant effect); 

3. The generation of positive controls to ensure that test systems are appropriate for assessing ecosystem effects (Brezjnev et al., 1974; Hunter et al., 1986). However, Hagedorn and Lacy (1988) emphasize that, although these tests including microcosms are helpful, they are relatively poor in predicting ecological effects in nature.

With the wide diversity in ecosystem populations (especially microbial) and processes, it will be a major undertaking to attribute changes in any one ecosystem parameter to the presence or activities of a GEO. By using an appropriate modelling program, conducting experiments in a reasonable microcosm, and focusing on the most important processes, it may be possible to determine the potential that a GEO may have for causing environmental perturbations (Corapciolu and Haridas, 1984; Kook et al., 1987).

Research trends in examining ecosystem effects must include some element of experimental procedure that incorporates:

1. Designing a microcosm to simulate some important aspects of the proposed release area;

2. Evaluating the GEO by monitoring a set of parameters that are deemed `most critical' for the particular microcosm;

3. Developing a descriptive model of the microcosm that can account for process rates, perturbations, and analyses (Minogue and Fry, 1983; Hicks and Newell, 1984; Thingstad and Pengerud, 1985; Pimentel et al., 1989). Lastly, numerical analysis of data assumes a uniquely important aspect in ecosystem studies because of the large variation inherent in the components of the system and the difficulty of establishing cause and effect relationships (Rouse, 1985). It will be necessary to attempt to define normal ranges of variability and function for specific ecological populations and/or processes and naturally occurring levels of change (Gladden and Ginzburg, 1982). Only then can perturbations be detected and the probability be determined that the alteration was associated with the presence and/or activities of a GEO.

12.4 METHODS FOR DETECTION AND QUANTIFICATION

The methods employed for detection of the adverse effects of pesticides and biotechnology agents on terrestrial ecosystems depend on whether the system is a crop, grassland, forest, or soil terrestrial ecosystem (Table 12.1). Another important dimension of the experimental assessment is which species or groups of species will be assessed in the quantified investigation. In addition, the methods employed will depend on whether stability, species diversity, energy flows, waste decomposition (biogeochemical cycles), pollination, or a combination of these ecological factors will be assessed in the study (Table 12.1).

Table 12.1. Measuring the adverse ecological effects of pesticides and biotechnology agents on terrestrial ecosystems. Various aspects of potential investigations are listed herein

Since it is impossible to describe all experimental methods that might be employed for the detection and quantification of the adverse effects of pesticides and biotechniques on all the categories listed in Table 12.1, measuring the impact of pesticides on cole crops will be used as an example. In this model assessment, attention will be focused on the crop plants, invertebrates, and microorganisms both above ground and in the soil system. All aspects of the ecosystem dynamics are measured related to these species, except for pollination.

Often, the fauna and flora of crop ecosystems have fewer species than a natural forest and grassland; however, when all the species included in the soil ecosystem are taken into account, species richness is enormous (Odum, 1971). 

For the proposed model study, Brassica oleracea or cole plants (broccoli, Brussels sprouts, cabbage, cauliflower, collards, and kale) are proposed to investigate the ecological effects of pesticides on the ecosystem for the following reasons:

1. Cole plants grow easily and rapidly;
2. Cole plants are attacked by an abundance of insect pests and plant pathogens (Walker and Larson, 1958; Pimentel, 1961a; Root, 1973; Kareiva, 1985);
3. Cole plants grow rapidly under favourable conditions and therefore can tolerate relatively heavy pest attack without the need of pesticides (control untreated) for survival (without pesticides, however, the crop is usually unmarketable);
4. A wide array of different insect pests (aphids, caterpillars, beetles, and plant bugs) and plant pathogens (fungi, bacteria, and viruses) offer potential for possible extrapolation of the results to other crops; 
5. Cole crops are an important food crop in the world;
6. Cole plant ecosystems have a rich fauna and flora that make them similar to natural ecosystems.

Collards will be used as the representative crop plant species because of their morphological structureopen flat leaves that make insect and plant pathogen sampling relatively easy compared with other genotypes, like cabbage, of this species.

12.4.1 ABOVE-GROUND INSECT NUMBERS

The insect community associated with collards (Brassica oleracea) is ideal for study because the community is rich in numbers of both species and individuals (Pimentel, 1961b; Root, 1973). In addition, all animals occur on both natural and cultivated cruciferae in the northeastern US region; however, the arthropods are attracted to and prefer the cultivated B. oleracea species. Also important is the fact that the plant is a biennial and thus can be maintained in the field for an added season. Bimonthly, starting the first week in June, 15 plants per plot are measured and insect populations sampled for potential ecological effects of pesticides. To avoid bias in the choice of individual plants to be sampled, every fourth plant is selected for sampling. The starting plant is selected at random.

An estimate of the total surface area of the plant is made by placing a plastic sheet, which has been etched in centimetre squares, against the leaves and stem. Only one side of the leaf is included in the measurements. In making the visual counts of arthropods, only a single stage in the life history of each is used. The stages selected are those that give the best estimate of the density of the species and also its parasitism.

The bimonthly arthropod population samples detect a 1050 per cent difference in abundance at the P=0.05 level, depending on the variability in the species population (Southwood, 1978). Simultaneously, the bimonthly population samples (throughout the summer) often will suggest important trends, even if these samples are not statistically different (Steel and Torrie, 1980). These consistent trends are extremely important in the analysis and interpretation of the data (Risch et al., 1986).

12.4.2 SOIL AND LITTER INVERTEBRATE POPULATIONS

Litter and soil samples are taken during the summer at about one-month intervals during July and August. Sampling consists of taking 10 soil units (12 cm diameter x 8 cm deep). Before a soil sample is removed, any litter on the surface of the sample will be removed and placed in a ''/a in mesh screen cylinder (12 cm diameter x 8 cm) lined with tissue paper. The screen cylinder is used to keep the soil sample intact during handling and extraction. Both the cylinder and soil samples are then placed in a plastic bag and labelled. 

A total of 10 litter and soil samples are selected at random locations within each test plot. Tullgren funnels are used to separate the invertebrates from the litter (Southwood, 1978). These funnels are 20 cm in diameter at the top. Hardware cloth (¼ in mesh) covers the funnel about 5 cm from the top, where the diameter is 16 cm. About 60 cm above the funnels, reflector floodlights of 75 W drive the invertebrates from the litter.

The litter samples are first sprinkled with water and then covered with a piece of wet cheesecloth. Glass jars containing 75 per cent ethyl alcohol placed below the funnels are used to collect the invertebrates. After 96 h, the alcohol is strengthened and the jars labelled and closed for counting the organisms later.

To estimate the numbers of surface-dwelling invertebrates (e.g., Coleoptera, Diplopoda), pitfall-traps are used (Dunger, 1983).

12.4.3 BIRD AND MAMMAL POPULATIONS

If pesticide effects on bird and mammal populations are to be measured, then extremely large ecosystems have to be treated. To locate dead birds and small mammals after death is in itself most difficult because of the rapid disappearance of carcasses. For example, Balcomb (1986) reported that about 80 per cent of the bird carcasses disappeared within 24 h after death. In addition, if the birds die in a grassland or forest, it becomes almost impossible to detect even one out of 100 deaths. Attaching tracking devices to birds and mammals will help to locate them, but the cost of making more than a few would be enormous.

12.4.4 PLANT PATHOGEN INFECTIONS

The incidence of plant pathogen infections in the experimental treatments is measured by two procedures. First, starting the first week in June, each plant in each plot is visually searched bimonthly for signs of disease that include wilting, yellowing, black spots, brown spots, white spots, and other suspicious symptoms of potential disease. The diseased plants are removed, roots and all, and placed into a plastic bag with an appropriate label. At the laboratory, the plants are carefully examined and a diagnosis made of the disease. Then the plants are measured for leaf-surface area using the methodology described earlier for insect measurements. The intensity of the infection is assessed by measuring the extent of area of infection. In addition, in the last two weeks of June and in early September, 15 plants are selected at random and totally removed from each plot in each treatment. These plants are also placed in plastic bags, labelled, and taken to the laboratory for examination. This procedure helps us to detect any potential infections that are not sufficiently severe to cause noticeable wilting and yellowing conditions in the plants in the experimental treatments.

The common diseases of cole plants include clubroot, yellows, black rot, black leg, rhizoctonia, bacterial soft rot, white blight, black leaf spot, ring spot, powdery mildew, downey mildew, white rust, mosaic, and damping off (Walker and Larsen, 1958).

12.4.5 PLANT PRODUCTIVITY

Net primary production is one of the most important parameters of plant quality and measure of stress effects. Plant production supplies the food to the insects and plant pathogens and, therefore, plays a major role in the dynamics of the system. Some of the pesticide treatments are expected to reduce plant productivity.

To measure plant productivity, 15 plants selected at random in each plot are measured bimonthly for total above-ground surface area. This is accomplished by placing a plastic measuring device (25 x 25 cm etched in centimetre squares) against each leaf and stem and estimating plant surface area. At the close of each experimental season, 10 plants in each plot are dug up and the roots washed clean of soil. These plants are then oven-dried and weighed to determine dry weight.

12.4.6 NUTRIENT AVAILABILITY TO THE PLANTS

Some pesticide treatments should influence leaching and availability of soil nutrients to the plants in the different treatments. The availability of these nutrients to the vegetation is assessed to associate net primary production and possible insect, herbivore, parasite, predator, and saprophyte and plant pathogen abundance with these nutrient changes. The nutrients selected for investigation in the experimental ecosystems are Al, As, B, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, N, Na, Ni, P, Pb, S, Se, and Zn. These nutrients were selected because of their role in the biotic function of the experimental systems. Total nitrogen and KCl-extractable ammonium and nitrate in the soil receive particular attention because nitrogen is frequently a limiting nutrient, and the cycling dynamics of this element are largely determined by biologically mediated processes (e.g., ammonium mineralization, nitrification).

Ten 3 cm² soil cores (15 cm deep) are collected during May and September from each plot and these combined into one bulk sample for each plot. These are thoroughly mixed on a clean plastic surface. While in the field, 6 g subsamples of soil (approximate weights) are added to preweighed 120 ml polyethylene bottles containing 200 ml of 2 N KCl+PMA (phenyl mercuric acetate: 0.5 p.p.m.). Precise sample weights are determined by reweighing the KCl bottles upon return to the laboratory. Moisture correction factors, ash content, and total nitrogen determinations are made on the remaining bulk samples. In addition, bulk samples collected prior to treatment and at the end of the growing season are analysed for cation exchange capacity (CEC), total phosphorus, Ca, K, Mg, and S.

Nutrient movement and availability measurements are made in the plots of each treatment to determine nitrogen and calcium leaching from the soil. These measurements include soil sampling at depths of 20 and 50 cm to determine movement and concentration of plant nutrients. Soil samples are taken weekly in June and then once per month in July, August, and September. For these purposes, soil sampling is a superior method to lysimeters, although the method is more laborious (personal communications from E. R. Watson, 1986, and T. W. Scott, 1987, Cornell University, Ithaca, New York).

During May and September, one leaf at the top and one at mid-height of 10 plants in each plot is sampled. These are combined on a 1:1 ratio, based on weight, into one sample for each plot. The leaves are analysed for the nutrients listed.

12.4.7 DECOMPOSITION

Decomposition of organic matter in the experimental systems is essential to normal nutrient cycling in these systems. Because pesticides may influence the rate of organic matter decomposition, we plan to assess this component (Edwards, 1980).

Standard litterbag methods are used to determine the effects of pesticide applications on litter decomposition (Edwards and Heath, 1963; Hagvar and Kjondal, 1981). Collards are used as standard organic matter. About 4 g (oven dry weight) of collards are placed in nylon mesh litterbags (approximately 15 x 15 cm of 1 mm mesh). The litterbags are placed in the field in May. Each bag is placed 5 cm below the soil surface. A total of 10 replicate bags are placed within each plot and these are collected in early October. Weight loss determinations for litterbag materials returned to the laboratory are made after drying at 60 °C. These samples are ground to pass through a # 20-mesh screen, and sent to a soil-testing laboratory for an assessment of total nitrogen and nitrate.

12.5 SUMMARY AND CONCLUSIONS

About 2.5 million tonnes of pesticides are applied annually in the world at an estimated cost of $16.3 billion (US). Generally, these pesticides provide a significant benefit to agriculture and public health. At the same time, pesticide use is causing major problems:

1. Approximately 20 000 fatalities occur worldwide annually 
2. Large amounts of food products are contaminated 
3. Beneficial natural biota are destroyed 
4. The injurious effects to terrestrial environments include reduced (a) stability, (b) energy flow, (c) species diversity, (d) pollination, and (e) waste decomposition and cycling.

Methods for the detection and quantification of the ecological effects of pesticides on terrestrial ecosystems are complex and costly, depending on what aspects and what combinations of pesticides are the target of the investigation. There is a clear need for a single accepted procedure for assessing the ecological effects of pesticides on non-target species in terrestrial ecosystems.

12.6 REFERENCES

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