June 2002 Larviciding
The old days of covering everything with one pesticide, such as waste-oil mixed with diesel fuel, are gone. Mosquito control, thankfully, has become an activity of prescription applications of environmentally compatible products, by a competent applicator, in a properly timed manner.
Larviciding is a general term assigned to the process of killing mosquitoes by applying natural agents and biological or chemical pesticides designed to control larva and pupa, the aquatic stages of the mosquito life cycle. Larviciding was implemented as a malaria control in the early 1900's and has become established as an essential part of an integrated pest management (IPM) approach in the control of mosquitoes.
The reason for larviciding is to kill the mosquito while it is still in a concentrated environment, before it emerges as an adult, disperses, and starts the breeding and blood-feeding phase of its life cycle. Effective larviciding results are not always easy to achieve. Accuracy of a larviciding application is extremely important and timing of the application is critical to effective control results. Dose rates must be sufficiently high to kill mosquitoes but sufficiently low to minimize non-target effects. Manufacturers of larviciding products are aware of this dilemma and have strived over the past 20 to 30 years to create products that are specific to mosquitoes and their life cycle of development.
There are many products and formulations used for “larviciding.” The choice depends on how the product is to be applied, the habitat to be treated, the life stage of the mosquito, and specie(s) being targeted. Products range from biological controls (living control agents), biological pesticides, insect growth regulators, chemical pesticides, and surface films.
In South Walton we use biological controls as a first choice if it is appropriate for the situation. We raise mosquito fish, Gambusia affinis, a small live-bearing native fish, and place them in areas that are manmade, permanent water-holding areas such as drainage retention areas, drainage ditches, ponds, livestock watering troughs and abandoned swimming pools.
Another biological control agent we use is Bacillus sphaericus (Bs). It is commonly occurring, spore-forming bacterium found throughout the world in soil and aquatic environments. The strain used for mosquito control produces a protein endotoxin, which, when ingested by the mosquito larva, upon digestion, becomes activated and breaks down the gut lining of the insect and kills the larva. This bacteria is specific to mosquitoes and basically, a form of germ warfare against mosquitoes. This bacteria will recycle in the environment for up to six weeks, provided water and mosquito larva are present in the environment. We use this product in semi-permanent water situations, 2 to 3 months, such as roadside swales, ditches, drainage retention areas, woodland pools and seasonal ponds.
Biological pesticides are toxins produced by living organisms. In South Walton, as in almost all mosquito control programs, BTI, Bacillus thuringiensus israelensis, is used for many situations we encounter. BTI is grown commercially in large fermentation vats. Upon sporulation, the delta-endotoxin is produced as a protein crystal. This crystal contains five different endo-toxins. Because of this diversity of toxins, BTI has been used in mosquito control since 1980 with no resistance occurring amongst the mosquito or black fly species it has been used against. This product, like Bs, is very specific to mosquitoes and black flies. We use BTI in roadside swales and other temporary water holding situations. This product does not recycle, is active for about 48 hours and must be applied during the early larval stages to be most effective.
Insect growth regulators (IGRs) do not kill the mosquito but interferes with the insect’s development. We use methprene, a synthetic mimic of a naturally occurring insect hormone called juvenile hormone (JH). Methoprene is sold under the trade name of Altosid and comes in many formulations and dosages. The application of Altosid into mosquito habitat, maintains a level of juvenile hormone in the environment, which keeps pupa from metamorphosing (changing) into the adult stage. Since pupa don’t eat, they eventually use up body stores and starve to death. We use Altosid in areas of known volume, such as catch basins. The Altosid XR briquet we use is applied every 4 months, to catch basins holding water.
Chemical pesticides used for larviciding are contact pesticides, which, when applied to larva habitat, kill the insect by exposure. Chemical pesticides are indiscriminate, they will kill most arthropods, fish, amphibians, reptiles, etc. if the dose rate is high enough and exposure long enough. The chemical pesticides used for mosquito control tend to be quite effective against arthropods but are minimally toxic to amphibians, reptiles, birds or mammals.
The chemical larvicide we use is Abate, the trade name for Temephos, which is an organophosphate compound. This product kills mosquito larva as well as other insects in the treated habitat. However, still, we don’t want to kill non-targets even if they are “just” insects. To minimize non-target effects we use Abate in floodwater mosquito habitat, areas that fill with water, produce a brood of mosquitoes and then dries up. Because mosquitoes are one of, if not the first insect to colonize an “unstable”, recently flooded area, Abate is very effective. We pre-treat an area we know produces mosquitoes when it is dry. When it floods the product is there, the mosquito eggs hatch, the larva is exposed to the pesticide and dies. Once such an area starts supporting mosquito predators we switch to BTI or Bs to treat these areas so as not to affect the establishment of natural predators.
Surface films are materials that interfere with the aquatic stages of the mosquito from getting air. While in the aquatic stage mosquitoes must get oxygen from the atmosphere with the use of a siphon or breathing tube they push through the water surface and then can stay suspended at the surface because of water tension. Surface control agents are of two types: oils that clog the siphon tube and essentially smothers the insect or monomolecular surface films (MSF), an alcohol based compound, that lowers the surface tension of the water. By removing the surface tension of the water, the mosquito can no longer stay suspended at the surface to obtain air and essentially drowns. The MSFs are very safe to the environment and non-target organisms.
In summary, there are many products with different formulations, dosages, and modes of action. The key to their effectiveness is choosing the product that creates minimal disruption to the environment, is safe for non-targets, and most importantly, the application is properly timed. |