Despite the hazards of conventional insecticides,
some use is unavoidable. However, careful chemical choice and application
can reduce ecological damage. Although I don’t know if application by
donkey or even walking through your spray would be the best method. Carefully timed doses can be delivered at
vulnerable stages of the pest’s life cycle or when a pest population is about to explode
in numbers. Appropriate and efficient use requires a thorough
knowledge of the pest’s field biology and an appreciation of the differences among available
insecticides. I’m Dr. DeBusk and in this video, I’m
going to talk about factors important in a pesticide, modes of action, and synthetic
insecticides. Natural insecticides are covered in another
video. A pesticide is any material (natural, organic,
or synthetic) used to control, prevent, kill, suppress, or repel pests. For arthropod pests, insecticides are used
for insects and miticides or acaricides are used for mite control. Of utmost importance when it comes to pesticides
is the effectiveness of insecticides. They can treat an insect problem while it
is in progress, reducing numbers to insignificant levels. Another factor is that insecticide action
is rapid, usually taking effect within hours and alleviating the problem within a few days. In particular, rapid action allows this tactic
to manage a problem by dampening population peaks before major losses are sustained. Insecticides should be economical, at least
in the short run, compared with many other pest management tactics. In terms of only the crop, cost/benefit ratios
regularly are on the order of four to five dollars return for every dollar invested in
insecticide applications. Finally, insecticides should offer ease of
application. People with minimal experience or training
should be able to apply them effectively. An array of chemicals have been developed
for the purposes of killing insects. These enter the insect body either by penetrating
the cuticle, called contact action, by inhalation into the tracheal system, or by oral ingestion
into the digestive system. Most contact poisons also act as stomach poisons
if ingested by the insect. Toxic chemicals that are ingested by the insect
after translocation through the host or plant are called systemic insecticides. These are useful especially in managing scales
since their hard waxy covering make it difficult for insecticides to work. Fumigants used for controlling insects are
inhalation poisons. Chemical insecticides generally have an acute
effect, and their mode of action, that is, their method of causing death, is through
the nervous system, either by inhibiting acetylcholinesterase (an essential enzyme for transmission of nerve
impulses at synapses) or by acting directing on the nerve cells. Most synthetic insecticides, including pyrethroids,
are nerve poisons. Other insecticidal chemicals affect the developmental
or metabolic processes of insects, either by mimicking or interfering with the action
of hormones, or by affecting the biochemistry of cuticle production. Chemical insecticides may be synthetic or
natural products. Most synthetic insecticides are broad spectrum
in action, that is, they have non-specific killing action, and most act on the insect
(and incidentally on the mammalian) nervous system. This also means that these will kill not only
the pest but the natural enemies. Organochlorines are stable chemicals and persistent
in the environment, are fat-soluble, and accumulate in mammalian body fat. Their use is banned in many countries and
they are unsuitable for use in IPM. You may have heard of some of these. Pesticides included aldrin, chlordane, DDT,
lindane, and dieldrin. Organophosphates may be highly toxic to mammals
but are not stored in fat and, being less environmentally damaging and non-persistent,
are suitable for use in IPM. Anyone using organophosphates or carbamates
should get blood tests periodically to ensure that they are not absorbing pesticides since
they are acetylcholinesterase inhibitors. They usually kill insects by contact or upon
ingestion, although some are systemic in action, being absorbed into the vascular system of
plants so that they kill most phloem-feeding insects. Organophosphates include pesticides such as
chlorpyrifos, acephate, malathion, and diazinon. Because they are non-persistent, their application
must be timed carefully to ensure efficient kill of pests. Carbamates usually act by contact or stomach
action, more rarely by systemic action, and they have short to medium persistence. Pesticides include aldicarb, carbaryl, and
methomyl. Pyrethrins were originally derived from East
African chrysanthemum flowers and were shown to have insecticidal activity. Beginning in the 1970s, synthetic pyrethroids
came into the market for agricultural purposes as they were synthesized from pyrethrin derivatives. Their desirable features provide a quick knockdown
of insects at low rates, relatively low mammalian toxicity, and improved stability in outdoor
environments, which has increased their marketability in agriculture. They are effective against a wide range of
insect and mite pests and may be mixed with other pesticides for a broad spectrum of pest
control. Pyrethroids may be mixed with piperonyl butoxide,
a synergist, which enhances the effect of the active ingredient. Their mode of action is interference with
transmission of nerve impulses. Although nicotine-based insecticides have
been phased out for reasons including high mammalian toxicity and limited insecticidal
activity, the new-generation of nicotinoids or neonicotinoids, which are synthetic pesticides
modelled on natural nicotine, have a large market, in particular the systemic insecticide
imidacloprid, which is used against a range of insect pests. Neonicotinoids selectively target the nicotinic
acetylcholine receptors in the insect central nervous system and cause paralysis and death,
often within a few hours. These pose a lower threat to mammals and the
environment, and can be applied at water-soluble applications or seed coatings as a systemic
pesticide. They control soil insects that feed on roots,
above- and below-ground phloem-feeding bugs, and leaf and stem-feeding beetles. Neonicotinoids are toxic to bees so bee precautions
must be taken when applying, such as minimizing drift, applying pesticides when plants are
not in bloom, and avoiding applications when bees are around. The phenylpyrazole insecticide fipronil is
a contact and stomach poison that acts as a potent inhibitor of gamma-aminobutyric acid
(GABA) regulated chloride channels in neurons of insects, but is less potent in vertebrates. However, this poison and its degradates are
moderately persistent and one photo-degradate (a breakdown product produced by exposure
to light) appears to have an acute toxicity to mammals that is about 10 times that of
fipronil itself. Although human and environmental health concerns
are associated with its use, it is very effective in controlling many soil and foliar insects,
for treating seed, and as a bait formulation to kill ants, vespid wasps, termites and cockroaches. Fipronil in baits is taken back to the nests
of social insects by foraging workers and can kill the entire colony. This is also common in flea medicines for
animals. Insect growth regulators (IGRs) are compounds
that affect insect growth via interference with metabolism or development. They offer a high level of efficiency against
specific stages of many insect pests, with a low level of mammalian toxicity. The two most commonly used groups of IGRs
are distinguished by their mode of action. Chemicals that interfere with the normal maturation
of insects by disturbing the hormonal control of metamorphosis are the juvenile hormone
mimics. These synthetic analogues of juvenile hormone
halt development so that the insect either fails to reach the adult stage or the resulting
adult is sterile and malformed. Examples of pesticides include fenoxycarb,
methoprene, and pyriproxyfen. The chitin synthesis inhibitors such as buprofezin
and diflubenzuron, prevent the formation of chitin, which is a vital component of insect
cuticle. Typically, the affected insects shed the old
cuticle partially or not all, and, if they do succeed in escaping from their exuviae,
their body is limp and easily damaged as a result of the weakness of the new cuticle. In addition to the chemical and physical properties
of insecticides, their toxicity, persistence in the field, and method of application are
influenced by how they are formulated. Formulation refers to what and how other substances
are mixed with the active ingredient, and largely constrains the mode of application. Insecticides may be formulated in various
ways, including as solutions or emulsions, as unwettable powders that can be dispersed
in water, as dusts or granules that are mixed with an inert carrier, or as gaseous fumigants. The formulation may include abrasives that
damage the insect cuticle and/or baits that attract the insects. For example, fipronil often is mixed with
fishmeal bait to attract and poison pest ants and wasps. The same insecticide can be formulated in
different ways according to the application requirements, such as aerial spraying of a
crop versus domestic use. In conclusion, synthetic insecticides can
be a part of integrated pest management, although some of these insecticides can affect natural
enemies and pollinators or have mammalian toxicity. Careful planning and use is important when
incorporating these pesticides into your program.