Pollution is essentially the wrong substance, in the wrong place, in the wrong concentration, at the wrong time. More formally, pollution can be defined as the introduction of human-made substances (or natural substances released by humans) and forms of energy into the environment that are likely to damage ecosystems or their constituents, amenities, or structures.
Insects, like other living organisms, are affected by pollution. However, insects are also used to assess the effects of pollution as surrogates or representatives of the larger assemblages of organisms in communities and ecosystems. We refer to insects in this latter role as biomonitoring agents.
Pollution can be caused by a variety of substances and activities: sewage and other organic enrichment, fertilizers (e.g., nutrients such as phosphorus), siltation (e.g., from erosion), pesticides (e.g., herbicides, fungicides, insecticides), metals (e.g., cadmium, mercury, selenium), organic compounds (polychlorinated biphenyls, polycyclic aromatic hydrocarbons, industrial atmospheric emissions (e.g., sulfur dioxide and NOx, which are precursors to acid rain; greenhouse gases such as carbon dioxide and methane), radiation (as in the Chernobyl disaster), heat (e.g., thermal pollution from power plants), and habitat destruction (e.g., clear-cutting, stream channelization, reservoir creation). Disturbances that have consequences similar to those of human activities can also be caused by natural events such as volcanic eruptions or forest fires, but the products of these disturbances do not fit our definition because they are not deliberately introduced by humans.
POLLUTION EFFECTS ON INSECTS
The effects of pollution on insects occur at a variety of spatial and temporal scales. For example, effects can occur at the molecular level in fractions of seconds (i.e., biochemical effects), at the ecosystem level over several decades, and at various scales in between these extremes (Fig. 1). The most easily detected responses are at the level of the individual (as in bioassays), in which evaluation is often based on whether an insect lives or dies when exposed to a contaminant. However, population and community levels are generally used when effects are examined in nature (Fig. 1). Within a population, we can see shifts occurring in the frequency of organisms of different sizes.
FIGURE 1 Relationship between ecological relevance, spatiotem-poral scale, mechanistic understanding, and specificity across levels of biological organization. MFO, mixed-function oxidase; AChE, ace-tylcholinesterase; DNA, deoxyribonucleic acid.
For example, early instars in a population seem to be more susceptible than later instars because early instars have higher surface-to-volume ratios, they are more active, and they have thinner cuticles. Eggs, pupae, and diapausing insects are usually more resistant stages.
Common community-level changes observed with the onset of pollution involve decreases in species richness (i.e., the total number of species), decreases in species evenness (i.e., the distribution of numbers of individuals among species), and alterations in species composition. Severe sewage pollution in fresh waters presents a good example of the kinds of changes that can occur. With sewage input, the total number of species decreases immediately downstream of a sewage source and species evenness decreases because the remaining tolerant organisms proliferate. Also, pollution-tolerant organisms such as dronefly maggots (Eristalis tenax), some species of midge (Chironomidae) larvae, and aquatic earthworms replace less tolerant ones, and these tolerant organisms can reach extremely high densities.
Alteration of feeding-group structure is another response to pollution. For example, the removal of a riparian zone surrounding a stream can lead to increased production of algae because of more open conditions and greater sunlight reaching the stream substrate, a decline in the leaves falling into the stream, and an increase in grazing insects (feeding on algae), which replace the insects that would normally feed on leaves.
More subtle community-level effects are also possible. For example, the suppression of parasites by air pollutants may enhance outbreaks of forest insect pests whose populations are normally held in check by the parasites. These indirect effects (Fig. 1) are often more difficult to detect than the direct effects discussed above.
Natural variability is a problem in evaluating the effects of pollution on insects. Only the accrual of long-term baseline data on insect species present and their abundances, and the use of an experimental approach to establish causation, can lead to understanding of the effects of pollutants on insects.
USE OF INSECTS IN BIOMONITORING
The responses of insects to pollution can be used in biomonitor-ing of air, land, and water quality. The most extensive biomonitor-ing has been developed in aquatic habitats, perhaps because aquatic insects are directly exposed to water pollution and their responses are easy to measure.
Insects present a number of advantages for biomonitoring: (1) they are ubiquitous, so they are exposed to pollution in many different habitats; (2) the large number of species offers a range of responses; (3) the sedentary nature of many insects allows spatial analysis of pollutant effects; and (4) their long life cycles allow temporal analysis of pollutant effects. Unlike relying on instantaneous measurement of physical and chemical variables, the use of living organisms, like insects, provides a temporal integration of pollutant effects over their life span.
Biomonitoring can be done at a variety of spatial and temporal scales (Fig. 1). The smallest scale is biochemical; for example, exposure of stonefly species to the pesticide fenitrothion can be measured by depression of acetylcholinesterase activity in the stonefly head. The largest scale is the ecosystem, in which measures of processes such as productivity and decomposition can indicate pollutant stress. However, biomonitoring is most commonly done at individual, population (or species assemblage), and community levels. Common examples of biomonitoring at the individual level include the use of insects as sentinel organisms and for measuring morphological deformities. The use of morphological deformities to measure pollutant effects is more common in freshwater than in terrestrial habitats. The head capsules of midge larvae are frequently used for this purpose. Bioassays often involve exposure of individuals to potential pollutants to detect sublethal responses (e.g., diminished growth or fecundity) or lethal responses (e.g., mortality).
Biotic indices are popular ways to summarize information and assess pollution at the population or species assemblage levels. For example, indices of the trophic status of lakes have been developed using midges, and an index of acid stress is available that uses a group of macroinvertebrate organisms, each of which has a different sensitivity to acidification.
However, biomonitoring is most frequently done at the community level, and measures vary from simple taxa richness to biotic indices that use the whole community (e.g., Family Biotic Index) to complex multivariate statistics that use the “reference condition” approach.
Bioassays are widely used in determining toxicity of industrial byproducts before and after they are released into the environment and can be a valuable adjunct to field biomonitoring. They can be used to answer questions such as: (1) At what concentration does a pollutant become toxic to a certain insect species? (2) What are the physical and chemical conditions under which the pollutant is most harmful?
(3) Which stage of the life cycle is most susceptible to the pollutant?
(4) What are the effects of acute vs. chronic exposure? (5) Which species are most susceptible to the pollutant? Bioassays can be as simple or as complex as needed (e.g., single vs. multiple species, static or flowthrough setup, laboratory beaker vs. field mesocosm).
Biomonitoring using insects can involve volunteers as well as professionals. Work with insects is usually labor intensive, and volunteers can provide much of this labor. In fact, volunteer biomoni-toring programs involving school classes and local community groups are currently very popular throughout the world.
In conclusion, although pollutants have affected living organisms, habitat destruction may be more important in determining whether organisms can recover. For example, it may be possible to clean air and water of their pollutants but recovery of ecosystems is impossible if the habitat remains impaired.
