As previously mentioned, there are three different classes of larvicides: bacterial larvicides, insect growth regulators and spinosad’s. Bacterial larvicides are fermentations of solid sand solubles of whole bacterium containing insecticidal crystal proteins (ICP), which bind to the midgut disrupting digestion and fluid balance. Eventually, the midgut will rupture leading to septicemia and death. Insect Growth Regulators (IGR) are purified compounds that act as a juvenile hormone analog that is specific to insects that undergo complete metamorphosis. The IGR mimics high levels of juvenile hormone to prevent larvae/pupae from developing into the next instar or pupal stage. The last class is the spinosads, which are metabolites of bacterial fermentation Spinosyn A and Spinosyn D. Spinosads act as neurotoxins specific to invertebrates by binding to the nicotinic acetylcholine receptors, which causes excitation of nerves leading to involuntary muscle contractions and death.
Resistance: What is it? Why is it important?
‘A heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species’ – Insecticide Resistance Action Committee, IRAC
Insecticide resistance within mosquito control is a constant and growing problem. Unfortunately, our mosquito control toolbox is dwindling and only have two classes of adulticides available: Organophosphates and Pyrethroids. If a population develops resistance to one of these actives, then those products can no longer be used effectively to mitigate disease or nuisance problems and reduces the options for control tremendously. Due to the lack of chemistries of our adulticides, we must place a larger emphasis on larvicides. As shown above, we have five different active ingredients to treat larval habitats. Rotating between chemistries regularly and utilizing all the tools in the toolbox will prevent resistance from developing.
Similar to adulticides, overuse or sublethal dosing of larvicides can cause mosquito populations to develop tolerance and eventually resistance to some active ingredients. Lysinibacillus sphaericus, Spinosad, and (S)-methoprene have all been documented in academic and operational literature to have caused resistance in populations. However, the “magic bullet,” or Bacillus thuringiensis israelensis, has been used to combat resistance to these products and eventually return mosquito populations to larvicide susceptibility. According to the EPA, there is no evidence of documented resistance to Bti in its 30+ years of use, which is likely due to the synergistic nature of the four protein toxins, also known as the ICP’s.