6.10.4. Mutagens and toxins

Stressful conditions induced by toxic or mutagenic chemicals may affect insect growth and form to varying degrees, ranging from death at one extreme to slight phenotypic modifications at the other end of the spectrum. Some life-history stages may be more sensitive to mutagens or toxins than others, and sometimes the phenotypic effects may not be easily measured by crude estimates of stress, such as percentage survival. One sensitive and efficient measure of the amount of genetic or environmental stress experienced by insects during development is the incidence of fluctuating asymmetry, or the quantitative differences between the left and right sides of each individual in a sample of the population. Insects are usually bilaterally symmetrical if grown under ideal conditions, so the left and right halves of their bodies are mirror images (except for obvious differences in structures such as the genitalia of some male insects). If grown under stressful conditions, however, the degree of asymmetry tends to increase.

The measurement of fluctuating asymmetry has many potential uses in theoretical and economic entomology and in assessment of environmental quality. For example, it can be used as an indicator of developmental stability to determine the effect on non-target organisms of exposure to insecticides or vermicides, such as avermectins. Bush flies (Musca vetustissima) breeding in the dung of cattle treated for nematode control with Avermectin B1 are significantly more asymmetric for two morphometric wing characters than flies breeding in the dung of untreated cattle. Fluctuating asymmetry has been used as a measure of environmental quality. For example, water quality has been assessed by comparing the amount of asymmetry in aquatic insects reared in polluted and clean water. In industrially polluted waters, particular bloodworms (larvae of chironomid midges) may survive but often exhibit gross developmental abnormalities.

However, at lower levels of pollutants, more subtle effects may be detected as deviations from symmetry compared with clean-water controls. In addition, measures of developmental effects on non-target insects have been used to assess the specificity of biocides prior to marketing. The technique is not completely reliable, with doubts having been raised about interpretation (variation in response between different organ systems measured) and concerning the underlying mechanism causing any responses measured.



Chapter 6