The greening of pesticide-environment interactions: some personal observations

Abstract

Background: Pesticide-environment interactions are bidirectional. The environment alters pesticides by metabolism and photodegradation, and pesticides in turn change the environment through nontarget or secondary effects.

Objectives: Approximately 900 currently used commercial pesticides of widely diverse structures act by nearly a hundred mechanisms to control insects, weeds, and fungi, usually with minimal disruption of nature's equilibrium. Here I consider some aspects of the discovery, development, and use of ecofriendly or green pesticides (i.e., pesticides that are safe, effective, and biodegradable with minimal adverse secondary effects on the environment). Emphasis is given to research in my laboratory.

Discussion: The need for understanding and improving pesticide-environment interactions began with production of the first major insecticide approximately 150 years ago: The arsenical poison Paris Green was green in color but definitely not ecofriendly. Development and use of other pesticides has led to a variety of problems. Topics considered here include the need for high purity [e.g., hexachlorocyclohexane and polychloroborane isomers and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T)], environmental degradation and the bioactivity of resulting photoproducts and metabolites, pesticide photochemistry (including the use of structural optimization, photostabilizers, and photosensitizers to achieve suitable persistence), the presence of multiple active ingredients in botanical insecticides, the need to consider compounds with common mechanisms of action, issues related to primary and secondary targets, and chemically induced or genetically modified changes in plant biochemistry. Many insecticides are bird, fish, and honeybee toxicants, whereas herbicides and fungicides pose fewer environmental problems.

Conclusion: Six factors have contributed to the greening of pesticide-environment interactions: advances in pesticide chemistry and toxicology, banning of many chlorinated hydrocarbons, the development of new biochemical targets, increased reliance on genetically modified crops that reduce the amount and variety of pesticides applied, emphasis on biodegradability and environmental protection, and integrated pest- and pesticide-management systems.

Figure 1

Paris Green, the first green pesticide, was green in color only (definitely not ecofriendly). Label from 1867 package. Reproduced with permission from Getty Images.

Figure 2

Photosensitizers accelerate insecticide residue loss on bean foliage exposed to sunlight as illustrated for (A) DDT (25 ppm) containing triphenylamine (50 ppm; time dependence) and (B) dieldrin (10 ppm) containing rotenone (concentration dependence, 1‑hr exposure; error bars are SDs; photosensitization was due to chromanone moiety). hv, light energy.

Figure 3

Toxic photoproducts formed during insecticide photodecomposition on bean foliage illustrated by (A) heptachlor and (B) fipronil. The graph is adapted from Hainzl and Casida (1996). Potencies (micromolar IC₅₀ values) are for the γ-aminobutyric acid_Areceptor noncompetitive blocker site of mouse or rat brain membranes (Hainzl and Casida 1996; Lawrence and Casida 1984). hv, light energy.

Figure 4

Pesticide-induced changes in plant biochemistry. (A) Secondary metabolites induced in six crops by acifluorfen with 48-hr sunlight exposure; pisatin in peas; glyceollins in soybeans; hemigossypol in cotton; phaseolin in beans; xanthotoxin in celery; and N-feruloyl-3-methoxytyramine in spinach. Error bars are SDs. Adapted from Kömives and Casida 1983. (B) Neonicotinoid insecticides induce salicylate-associated responses in plants. (C) Safener induces herbicide-detoxifying enzyme and cofactor in corn but not in weeds.