Biology Reference
In-Depth Information
rotenone-based products in the USA (Tanner et
al ., 2011). Chronic exposure to rotenone has
reproduced features of Parkinson's in mice
(Betarbet et al ., 2000; Inden et al ., 2011) by
causing extensive damage and tissue necrosis of
parts of the midbrain (Norazit et al ., 2010), as
well as increased infertility (Khera et al ., 1982).
On the other hand, its ef ect on cell growth and
cell cycle arrest is being investigated to be used as
a possible chemotherapy drug against cancer
(Fang and Casida, 1998; Goncalves et al ., 2011).
Although rotenone is rapidly biodegraded
by UV light, traces of the compound have been
detected in olives and other consumables from
organic farming (Cabras et al ., 2002; Cabizza et
al ., 2004). The lethal concentration of rotenone
against An . rumicis is around 0.05 ppm, and the
LD 50 for the housefl y Musca domestica is 0.30
ppm (Soloway, 1976). The control of disease
vectors using rotenone is unfeasible due to its
rapid decomposition in the environment,
requiring repeated applications, and due to the
chronic toxicity and ef ects that it may bring
upon populations. It is an example of a natural
insecticide that should not be considered safe or
environmentally sound for disease control.
the alkaloids cervadine and veratridine, which
are considered highly toxic to mammals (rat oral
LD 50 = 13 mg kg −1 ). These compounds af ect the
membrane sodium channels of the nerves
resulting in paralysis and mortality. Nicotine is
an alkaloid obtained from the foliage of the
tobacco plant Nicotiana tabacum . The WHO
classifi es it as a highly hazardous pesticide
(WHO, 2002) on account of its high toxicity to
mammals (mouse oral LD 50 = 3mg kg −1 and rat
dermal LD 50 = 50 mg kg −1 ) (Soloway, 1976;
Okamoto et al ., 1994). Nicotine is a synaptic
poison that binds to acetylcholine receptors
located at the neuromuscular junctions. Its
mode of action is similar to that of the
organophosphorates; other plant extracts
containing alkaloid insecticides with similar
mode of action include the wild tobacco
Nicotiana glauca and the calabar bean
Physostigma venenosum containing anabasine
and physostigmine, respectively. These com-
pounds highlight the fact that natural com-
pounds do not always equate to products of
good human or environmental safety, and
consumers should always use natural products
that are certifi ed by bodies such as the WHO or
the Environmental Protection Agency (EPA).
Alkaloid insecticides
Ryania, sabadilla and nicotine are three well-
known botanical pesticides containing alkaloids.
Alkaloids are generally toxic to vertebrates but
may be used as insecticides in low concentrations.
Despite their relative toxicity, organic farming in
many parts of the world continues to use these
insecticides for controlling crop pests (Isman,
2005). The half-life of these pesticides is much
shorter than those of synthetic compounds;
therefore relative safety can be assured if used
appropriately. However, the WHO considers
ryania and sabadilla to be obsolete pesticides as
they are no longer registered for pest control
(WHO, 2002).
Ryania is a biopesticide derived from the
wood of the South American bush Ryania
speciosa . Its application is mostly concentrated to
organic farming in the USA and India (Isman,
2005). The active compound is the alkaloid
ryanodine, the oral LD 50 for rats is 750 mg kg −1 .
Seed extracts of the South American lily
Schoenocaulon oi cinale contain sabadilla, a
botanical pesticide. The active compounds are
4.1.3 Repellents
A repellent is 'a chemical that prevents an insect
from reaching a target to which it would
otherwise be attracted' (Browne, 1977).
Repellents induce behavioural reactions that
result in insects avoiding contact with sources of
stimulation, whether after direct tarsal contact
or after encountering airborne particles. Many
plants contain natural repellent compounds
that are primarily for their defence against plant-
eating insects, but act as ef ectively against
mosquitoes and other vectors of disease (Moore
et al ., 2006). When leaves are damaged, plants
often produce volatiles that evoke strong
repellent responses in mosquitoes (Gatehouse,
2002; Logan et al ., 2010) due to their high
vapour toxicity (Lee et al ., 2001). Examples of
specifi c plant-derived components will be
discussed in this section. However, often study
results cannot be compared due to the use of
dif erent repellent testing methods (Maia and
Moore, 2011); therefore it is of vital importance
 
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