Biopesticides - Pesticides: Evaluation of Environmental Pollution - page 595

19.8.1 Pyrethrum
Pyrethrum refers to the oleoresin extracted from the dried flowers of the pyrethrum daisy,
Tanacetum cinerariifolium (Asteraceae). The flowers of the plant are harvested shortly after
blooming and are either dried or powdered or the oils within the flowers are extracted
with the solvents. There are three esters of chrysanthemic acid and three esters of pyre-
thric acid. The extracts usually have an active ingredient content of about 30%. These
active insecticidal components are collectively known as pyrethrins. Two pyrethrins are
most prominent, pyrethrin-I and pyrethrin-II. The pyrethrins have another four different
active ingredients, Cinerin I and II and Jasmolin I and II. Pyrethrin compounds have been
used primarily to control human lice, mosquitoes, cockroaches, beetles, and flies. Some
“pyrethrin dusts,” used to control insects in horticultural crops, are only 0.3%-0.5% pyre-
thrins and are used at rates of up to 50 lb/A. Other pyrethrin compounds may be used in
grain storage and in poultry pens and on dogs and cats to control lice and fleas.
The insecticidal action of the pyrethrins is characterized by a rapid knockdown effect,
particularly in flying insects, and hyperactivity and convulsions in most insects. These
symptoms are a result of the neurotoxic action of the pyrethrins, which block voltage-
gated sodium channels in nerve axons. As such, the mechanism of action of pyrethrins
is qualitatively similar to that of DDT and many synthetic organochlorine insecticides. In
purity, pyrethrins are moderately toxic to mammals (rat oral acute LD 50 values range from
350 to 500 mg/kg), but technical grade pyrethrum is considerably less toxic (~1500 mg/kg)
(Casida and Quistad 1995).
Pyrethrins are especially labile in the presence of the UV component of sunlight, a fact
that has greatly limited their use outdoors. A recent study indicated that the half-lives of
pyrethrins on field-grown tomato and bell pepper fruits were 2 h or less (Antonious 2004).
For many years, world production of pyrethrum was led by Kenya, with lesser quanti-
ties produced in Tanzania and Ecuador. In the past 5 years, Botanical Resources Australia,
with plantings in Tasmania, has become the second largest producer in the world (30% of
world production at present). Pyrethrum produced in Tasmania is qualitatively similar to
that produced in East Africa and elsewhere.
19.8.2 Neem Pesticides
The key insecticidal ingredient found in the neem tree is azadirachtin, a naturally occur-
ring substance that belongs to an organic molecule class called tetranortriterpenoids
(Grace-Sierra 1990). Azadirachtin is just one of more than 70 limonoids produced by the
neem tree. It occurs in all parts of the neem tree, but is concentrated in the kernel. The con-
centration of azadirachtin in neem kernels varies with environmental factors and genetic
makeup, but may be as high as 10 g/kg of seed kernels. A single tree yields about 2 kg of
kernels each year. It is a powerful insect antifeedant and growth regulator. It is structur-
ally similar to insect hormones called “ecdysones,” which control the process of metamor-
phosis as the insects grow from larvae to pupae to adults. Metamorphosis requires the
careful synchrony of many hormones and other physiological changes to be successful,
and azadirachtin seems to be an “ecdysone blocker.” It blocks the insect's production and
release of these vital hormones. Insects then will not molt, thus breaking their life cycle
(National Research Council 1992; AgriDyne Technologies 1994).
Azadirachtin may also serve as a feeding deterrent for some insects. Depending on the
stage of life cycle, insect death may not occur for several days. However, upon ingestion
of minute quantities, insects become quiescent and stop feeding. Residual insecticidal