Insecticide resistance is the ability of a strain of insects to survive doses of insecticides that are lethal to most individuals in a population of the same species. Factors that accelerate resistance development in insect pest populations include:
- Rapid life cycle
- Frequent and widespread use of the same type of insecticide
Insecticide resistance in a population arises through a continuous selection process. Some individuals possess biochemical defences against a specific insecticide. With constant exposure, susceptible insects die, and resistant ones survive, reproduce, and pass on the resistant trait rapidly if the population breeds quickly.
After a few years of DDT use, insects like houseflies, mosquitoes, lice, and fleas became highly resistant. The prolonged residual action of organochlorine chemicals contributed to this resistance by eliminating susceptible individuals, leaving only those with inherent detoxification abilities to survive and pass on genetic traits to the next generation.
Insect Growth Regulators
The term “Insect Growth Regulator or IGR is a chemical that controls insect pests by disrupting their development and reproduction. This is done by:
Juvenile Hormone Analogues (JHA) are chemicals that closely resemble the natural juvenile hormone and interfere with normal glandular function, leading to either death or an inability to reproduce successfully.
Chitin Synthesis Inhibitors (CSI) are chemicals that disrupt the formation of chitin and the insect moulting process.
Although Insect Growth Regulators (IGRs) act more slowly compared to traditional insecticides, they hold significant potential in urban pest management.
Juvenile Hormone Analogues
Hydroprene controls cockroaches. Exposing immature cockroaches to this juvenile hormone analogue causes developmental issues like deformed wings and darker body colour, resulting in sterile adults. Effective control may take 3-5 months. Since hydroprene does not affect adult cockroaches, it’s advised to combine hydroprene with a traditional insecticide. Periodic application every four months is suggested.
Methoprene controls a variety of insects that undergo a complete metamorphosis (e.g. mosquitoes, flies, grain pests, and fleas). In flea control, methoprene disrupts the moulting process of immature stages, preventing pupal formation or inhibiting adult emergence from the pupa, thus breaking the generational cycle.
Pyriproxifen. This mimics the structure of the natural juvenile hormone of insects and disrupts their growth. This is used to manage fleas and cockroaches.
Fenoxycarb. Although it is chemically a carbamate, it functions as an IGR by disrupting the inhibition of larval moulting.
Chitin Synthesis Inhibitors
These substances interfere with the normal development of the insect’s cuticle by inhibiting chitin synthesis. Chitin provides structural strength to the insect cuticle. Immature insects exposed to CSIs typically die within one or two moulting cycles because they fail to produce a new cuticle. Since they do not affect adults, they are frequently combined with an adulticide for more comprehensive pest control.
Examples of CSIs include:
Triflumuron used in controlling insects such as cockroaches and fleas
Hexaflumuron used as a bait for controlling subterranean termites
Diflubenzuron used in the control of a range of insect pests including cockroaches, fleas and ants.
Advantages:
- Pest population declines overtime
- Low mammalian toxicity/ non-toxic to plants
- Effective in minute quantities
- Target specific and safe for natural enemies
Disadvantages:
- Takes a long time to work/ slow mode of action
- Only kills certain life stages of pests
- Relatively expensive
- They are chemicals so there is a possibility of build-up of resistance
Miscellaneous Insecticides
Fipronil
It is a chemical from the phenylpyrazole family. This is non-repellent and has strong insecticidal properties even at low concentrations. It is mainly used for controlling cockroach infestations.
Mode of action
Fipronil is toxic to insects by contact or ingestion. It blocks the GABAA-gated chloride channels in the nervous system which prevents the absorption of chlorine ions, leading to hyperexcitation of the infected insect’s body and muscle and eventual death.
Hydramethylnon
This is a pyrimidin-2-one hydrazone compound. This insecticide is primarily used as a bait to manage ants and cockroaches.
Mode of action
Hydramethylnon works as a metabolic inhibitor by clocking the normal processes in the insect that makes Adenosine Triphosphate (ATP). this leads to a depletion of the energy necessary for movement and life processes. It is commonly labelled as a ‘slow-action’ or ‘delayed-action’ insecticide because it takes several days to cause death. In ant control, this delayed effect can be beneficial, facilitating the widespread distribution of the active compound throughout the colony.
Fumigants
Fumigants are pesticides characterised by their volatility, acting as toxic gases within enclosed spaces. Certain commonly employed insecticides demonstrate a fumigant effect, releasing vapours in confined areas that insects inhale, such as dichlorvos strips or mothballs used in a closet. This topic, however, focuses on a specific category of pesticides commonly known as fumigants. This group includes several extremely toxic and hazardous compounds.
Fumigants are typically stored as pressurized liquids in industrial cylinders. When dispensed, the pressure drop causes the stored liquid to vaporise, although in cold conditions, a vaporizer may be necessary. Materials suitable for fumigation are often those requiring penetration of insecticidal action, such as treating timber or grain to eliminate pests. To create an enclosed space for the poisonous fumes to act over a specific duration, this can be achieved by placing the items in a pressure chamber (where a vacuum may be created prior to fumigation), covering them with appropriate plastic sheeting, or conducting fumigation in relatively airtight conditions.
Insects subjected to fumigation are eliminated by inhaling poisonous vapours. The dosage of fumigants depends on both concentration and exposure time. Temperature influences insect respiration and the diffusion rate of fumigants, making it a crucial factor in fumigation applications. Fumigations are typically avoided at temperatures below 15°C.
Certain fumigation chemicals are so dangerous that individuals handling them need a specific license. Licensed fumigators undergo specialized training to acquire the necessary knowledge for safely managing fumigations.
Fumigant regulations vary between states and even within them, leading to inconsistency. Some states require licensing for certain fumigants, while others do not. Different user categories within a state may also have varying licensing requirements. For instance, commercial urban operators might need a license, while those who work on agricultural or horticultural operations might not. Given this disparity. It’s crucial for potential fumigators to:
- Verify their eligibility to use specific fumigants, usually by contacting the state Department of Health.
- Confirm that the fumigant is registered in the state for the intended purpose.
- Strictly adhere to label directions relevant to the state.
Fumigants, commonly used for insect pest control, are inhalation poisons, with complex and diverse modes of action. They typically exhibit broad toxicity across various life forms, including humans.