Role of Pesticides in Produce Production, Preservation, Quality, and Safety 345
(tomatine) that are inhibitors of acetylcholinesterase [17], thus presumably providing
insect resistance but also posing risk to humans. Furanocoumarines (e.g., psoralen,
bergapten, and xanthotoxin) represent another important group of natural toxins and
probable human carcinogens produced mainly by umbelliferous plants, such as
celery or parsley. These phytoalexins are often present in plant tissues at relatively
high levels (10 to 1,000 mg/kg) compared to synthetic pesticide residues [18].
Various stress factors (stimuli) have been shown to induce phytoalexin synthesis,
such as mechanical injury, amount of light, attack by pests, or pathogen infection
[19,20]. Thus, by reducing plant stress from insect and other pest attack, weed
competition, and/or plant pathogen invasion, pesticides may decrease phytoalexin
production. More research studies should be done in this area to better evaluate this
premise and also to eliminate the possibility that pesticide application is one of the
potential stress factors [21].
11.2.1 INSECTICIDES
Of more than 800,000 of the existing insect species, about 10,000 species attack
crops and, when uncontrolled, can cause extensive damage and economic losses [2].
As the word implies, insecticides are designed to kill insect pests. Other chemicals
used in insect control include ovicides (e.g., petroleum oil), which are intended for
killing eggs of insects and mites; insect growth regulators (for disrupting molting
and other life processes such as chitin synthesis); insecticide synergists (such as
piperonyl butoxide for blocking insects’ natural detoxification system); pheromones
(for disrupting insect mating behavior); and attractants (for luring insects to a trap)
and repellents.
11.2.1.1 Major Classes of Insecticides and Their Modes of Action
Table 11.2 presents major chemical classes of insecticides along with their modes
of action and representative examples. Organochlorine and pyrethroid insecticides
are nonsystemic (contact pesticides), which means that they do not appreciably
penetrate plant tissues and therefore are not translocated within the plant. Neonic-
otinoids are systemic pesticides, and some of the organophosphates and carbamates
possess systemic and some nonsystemic properties [5].
The mode of action of most insecticides involves the stimulation or inhibition
of the transmission of nervous impulses [1,5]. The organophosphorus and carbamate
insecticides inhibit the enzyme acetylcholinesterase by its phosphorylation or car-
bamoylation, respectively. Acetylcholinesterase catalyzes the hydrolysis of the major
excitatory neurotransmitter acetylcholine to its inactive form choline, which ceases
the stimulation of the receptors. Thus, inactivation of this enzyme leads to accumu-
lation of acetylcholine at the nerve synapse and continuous transmission of nerve
impulses. Nicotine and neonicotinoids can mimic acetylcholine and act as its antag-
onists at the postsynaptic nicotinic acetylcholine receptors, causing overstimulation,
which leads to the insect’s twitching, convulsing, and, finally, dying. Pyrethroids
interfere with the operation of sodium channels that are involved in the propagation
of action potentials along nerves [22]. Most of the organochlorine insecticides act