ABSTRACT

A discussion of types of carriers and sizes of carriers used for mosquito granular larvicides. A brief discussion of pelleted mosquito larvicides.


In this discussion we will attempt to describe what granular larvicides are, the types of granular larvicides, how they are measured for size, and the various types of carriers that are used to formulate granular larvicides. This will be done in the context of mosquito control and may or may not apply to other uses. We will discuss the differences between granular and pelleted larvicides.

DESCRIPTION

Granular larvicides are dry formulations of pesticides that are not diluted with a liquid such as oil or water before use. They are used as purchased from the manufacturer or as formulated by the commission. They normally contain low concentrations of active ingredient, typically 0. 1 to 5.0 per cent by weight. Application rates can be as high as 100 pounds per acre, but typically are in the range of two to twenty pounds per acre.

GRANULAR LARVICIDES

Granular larvicides are products in which the active ingredient is applied to the surface of a carrier (the granule) and bound to the surface of that carrier with some type of binder or sticking agent that will keep most of the active ingredient on the carrier in transit, but will allow the active ingredient to release when the granular is applied to the larval habitat, which is water.

Normally the active ingredient releases from the surface of the granular within minutes of application to water, however, some granular formulations are designed to release the active ingredient over several hours to prolong the time the active ingredient will be available for larvae control.

With some carriers, such as Biodac, the carrier may actually absorb some of the active ingredient and then release the active when the granule is applied to water. Typical carriers are ground corn cobs, clay (such as Celatom), sand, and Biodac.

Biodac is a recycled paper product that is formed into various size granules. It appears to absorb pesticides well and early research in Cape May, NJ and Salisbury, MD indicates that it may be a new viable carrier for mosquito granular larvicides.

Sand is silica or quartz granules that are quarried and sorted for size. Celatom is stone that is mined and crushed. If sand or Celatom are used for a carrier, dedusted formulations should be used to reduce drift from the target area and minimize operator exposure.

The active ingredients that are currently available on granular carriers are Temephos and Bti. Methoprene can be mixed with sand by the mosquito district, but is not currently commercially available as a purchased product.

There are several ways to apply granular larvicides. When applying any pesticide make sure to use proper protective equipment. The simplest way to apply granules is to broadcast them by hand. Many commissions use a modem version of the horn seeder. Some commissions use a "belly grinder" which is a shoulder carried crank operated granule dispenser. These types of application are useful for small area treatments when the person applying the product can get next to the breeding habitat.

If larger areas need to be treated, backpack blowers are often utilized. This type of equipment will spread the granules 30 or 40 feet from the equipment. The Seacoast Area Mosquito Control and others have mounted several backpack blowers on ATV's which allow them to treat relatively large areas of salt.

When hundreds or thousands of acres of breeding sites need to be treated quickly, aircraft are used to apply the granules. Both fixed wing and rotary wing aircraft are used with excellent results.

The size of the carrier is often expressed in mesh size, such as 4-8, 10-14, or the size specifications may be cumulative and list the various sizes of particles that comprise a certain specification. The size of the carrier used is based on research and years of field experience in using various sizes. As an example of a size for corn cobs, a 4-8 granular is a granular that will have particles no larger than a 4 mesh per inch screen but not smaller than an 8 mesh per inch screen. By the same token a 10-14 mesh granular will not have particles larger than a 10 mesh per inch screen or smaller than a 14 mesh per inch screen.

Since there are more and smaller openings in a 14 mesh screen than their are in an 8 mesh screen, the 10-14 mesh granular will be considerably smaller in size than a 4-8 granular. The size of the granule in itself does not make it better or worse than another size.

Small granules will provide better coverage of an area than large granules, because there will be many more small granules per pound that there are large granules. However, with aerial applications small granules may drift more than large granules. You should also consider that with the same carrier, a small granule will have less weight and density than a large granule and may not penetrate dense vegetation as well as a large granule.

The above statements are true for granulars made from the same material, but different carriers have different densities. Based on typical specific densities, and using the same size granule, you can expect the penetration of sand to be better than Celatom or Biodac or ground corn cob, Celatom to be better than Biodac or ground corn cob and Biodac to be better than ground corn cob.

PELLETED LARVICIDES

Unlike granular larvicides on which the active ingredient is applied to the surface of the carrier, pelleted larvicides are homogenous blends of the active ingredient carrier and other inerts that may protect the active ingredient or enhance the activity of the pelleted material. The carrier may be calcium sulfate, plaster of paris, or finely ground corn cob held together with some type of glue. Pellets are normally extruded and have an appearance similar to pelleted rodenticides or rabbit food. They are typically 1/8" in diameter and approximately 1/4 "long. Pellets are normally quite dense and, therefore, penetrate foliage very well. Since there is normally very little dust with pellets, drift is not a problem and the pellets will not cling to vegetation.

Because the active ingredient is incorporated throughout the pellet, it will not fall off during transport. Special expensive equipment is needed to manufacture pellets, so they are only available from manufactures. To my knowledge no mosquito districts are producing commercial quantities of pellets.

Pellets are normally formulated to dissolve or slough at a relatively slow rate, and therefore offer the advantage of prolonged release of the active ingredient, thereby, giving the user larvae control over an extended period of time.

Pelleted larvicides normally are more expensive than granular larvicides, but the increased cost should be balanced against fewer applications and having the active ingredient present and available prior to breeding. Most manufacturers state that pelleted material can be applied as a pre-hatch treatment.

Application rates for pelleted material vary from 2.5 to 20 pounds per acre. Pellets may be applied with the same type of equipment that is used for granular larvicides, however, because pellets are normally larger than granules, there have been reports of difficulties applying pellets with aircraft application systems. Some manufacturers are evaluating the possibility of manufacturing shorter pellets that will flow through standard equipment more efficiently. The active ingredients that are currently available in pellets are Temephos, Bti, and Methoprene.