International Congress BIOLOGICAL PRODUCTS - Gruppo di ...

BOTANICAL INSECTICIDES:
REFLECTIONS AND PERSPECTIVES
John E. Casida
Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy and Management,
University of California, Berkeley, California 94720-3112, USA
Importance and History. Insects are our main competitors for a limited supply of food and fiber. Pests destroy
about one-third of the world food supply during growth, harvesting and storage, even with the use of pesticides and
other control measures. The insecticides have evolved from inorganics to botanicals to synthetic organics. The inorganic
insecticides have almost disappeared from use on crops for reasons of inadequate effectiveness and safety. The
botanicals and synthetic organics have many similarities yet important differences and limitations. The author presents
some reflections and perspectives on these relationships from a half century of personal experience in insecticide
research.
Five Major Botanicals. There are five major botanical insecticides, i.e. pyrethrum, rotenone, nicotine, neem
and ryanodine. Pyrethrum extract from the flowers of the white daisy Chrysanthemum cinerariaefolium is the most
important. The countries of production include Kenya, Tanzania and Australia. The effectiveness is enhanced and the
economics improved by adding piperonyl butoxide for stabilization from metabolic detoxification in insects. Pyrethrum
is also important because its active ingredients, the pyrethrins, provided the model for the synthetic pyrethroids which
now constitute 20% of the market value of all insecticides. Rotenone from the roots of Derris elliptica and
Lonchocarpus utilis has for 150 years been used to control chewing insects and as a piscicide to kill unwanted fish.
Rotenone has been the prototype for Complex I respiratory inhibitors but not for economically successful structural
modifications. Home gardeners recommend rotenone use. Nicotine, the alkaloid of tobacco plants (and cigarettes), has
declined in use for decades but serendipitously served as the mode of action but not structural prototype of the new
synthetic neonicotinoids of high effectiveness and apparent safety. Neem has important local uses in India and China
and the extract with azadirachtin active ingredient is an effective crop protectant acting as an antifeedant. Ryania is the
ground stemwood of Ryania speciosa. The supply is limited but the effectiveness is high for lepidopterous larvae.
Supply and Composition. One of the limitations of botanical insecticides is consistent supply with adequate
standards of effectiveness and safety. This is a constant challenge even with the best producer organizations. Pyrethrum
once had an allergen now removed by a defined cleanup procedure. We analyzed cubé resin (the commercial rotenone
supply) and found 11 stilbenes and flavonoids and 29 rotenoids (including one that incorporated chlorine from the
solvent extraction conditions). Nicotine has been extensively studied except perhaps its environmental degradation
products. Neem contains many active ingredients; we described nimbolide and epoxyazadiradione as cytotoxic agents.
The principal active ingredient of Ryania was long described as ryanodine until we found that 9,21-dehydroryanodine
was of even greater importance.
Toxicology. Botanicals are more difficult to evaluate toxicologically than synthetic insecticides. They usually
have one or two major active ingredients (e.g. pyrethrins I and II in pyrethrum extract) but there may also be many
related bioactive compounds (e.g. in neem). Unless highly purified they also have hundreds to thousands of other
extractives or natural products which can vary with the plant source and processing. Critical evaluation of toxicology
requires production and use samples of consistent composition. Toxicology studies on pyrethrum extract meet current
standards but those for other botanicals are lacking in scope, although this does not necessarily mean hazards are
involved.
Residues of botanical insecticides are rarely a known problem. The pyrethrins photodecompose rapidly to
compounds of greatly reduced neurotoxicity. The same applies to the rotenoids and ryania components. Natural
products are generally considered to be easily metabolized. They have few or no halogens or refractory substituents.
Quite the contrary they have phenol, alcohol and ketone substituents, and alkene moieties that are amenable to ready
degradation. The analyses are more difficult than with most synthetics: the lack of halogens limits the analytical
detector sensitivity; multiple components and related but inactive analogs complicate the process.
Mode of Action and Resistance. Mode of action is an important aspect of insecticide use, safety and
toxicology. The same targets are involved in the most part with the botanicals and synthetics. Major insecticides act on
four principal targets in the nervous system.
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