Pheromone from controlled-release formulation

The emission from a controlled-release formulation is generally limited by a diffusion process which is controlled by the concentration gradient across a barrier to free emission and the parameters of the barrier itself (3). The rate of release follows approximate zero order kinetics if the concentration gradient remains constant i.e., the rate is independent of the amount of material remaining in the formulation except near exhaustion. A large reservoir of pheromone is generally used to attain a zero order release. Most formulations, however, tend to follow first order kinetics, in which the rate of emission depends on the amount of pheromone remaining. With first order kinetics, In [CQ/C] = kt where CQ is the initial concentration of pheromone, C is the residual pheromone content at time t, and k is the rate of release. When C 1/2 CQ, the half-life, of the formulation is 0.693/k. Discussions of the theoretical basis for release rates appear elsewhere (4- 7)... 

The emission of a pheromone from a controlled-release formulation can depend on the diffusion through holes in the matrix or on the penetration of the compound through a wall or membrane by absorption, solution and diffusion (8). Thus variation in the parameters of the formulations, such as film thickness, particle size, solvent, pore dimensions, etc., alters the release rate. The design of the formulation must therefore take into account the effect of each variable on the emission rate in order to develop a system that is effective during the appropriate cycle of the target insect. 

In 1980, we reported the structure of the Comstock mealybug, Pseudococcus comstocki (Kuwana), pheromone as 2,6-dimethyl-l,5-heptadien-3-ol acetate (24, 25) and in 1981, we identified the pheromone of the citrus mealybug, Planococcus citri (Risso), as (lR-cis)-(+)-2,2-dimethyl-3-(l-methylethenyl)cyclobutanemethanol acetate (26). Both of these pheromones have been synthesized and are currently being used as baits in monitor traps in California and Texas. These compounds are more volatile than the pheromones for most other insects and therefore formulations for controlled release need to be modified from those described earlier. 

Blerl-Leonhardt, B. A. Release Rates from Formulations and Quality Control Methods In "Insect Suppression Using Controlled Release Pheromone Systems", Kydonieus, A. F. and Beroza, M. Eds., CRC Press, Boca Raton, FL, In Press. 

Field measurements of concentrations of disparlure in air under woodland plots treated with three different slow release formulations showed that all released pheromone for about one month concentrations decreased about 80 in the first five days and 90-98 over 35 days. Between 75 and 85 of the disparlure remained in the formulations after 35 days even though release had became very slow. Measurements with other formulations containing tetradecenol formate applied to corn showed these were more efficient but not persistent enough to control Heliothis Zea for more than one month. No satisfactory measurements of concentrations in vapor plumes from point sources were possible even though these may be as effective as broadcasts. Further field research is limited by sampling and analysis techniques and the need for better micrometeorological data. 

Satisfactory formulations of pheromones that afford a constant release rate over a long period of time are not likely to be achieved without the cooperation of the chemical industry. Controlled release of pheromones is discussed in the Proceedings of a recent symposium (623). Fortunately, several organizations in the U. S. have been interested in the possibilities of the use of pheromone formulations on a commercial basis. One company (605) has supplied formulations for several investigators, and in addition has carried out extensive field tests with their own personnel. It has developed the use of hollow fibers, which are simply microcapillary reservoirs that serve to contain a vaporizable material and mediate evaporation of the material into the atmosphere . The fiber wall is essentially impermeable. One end is sealed, and the release rate essentially depends on diffusion from the surface of the liquid-vapor interface to the open end of the hollow fiber. Fibers are supplied in two forms. A tape form consists of a parallel array of fibers on an adhesive tape this form is used to establish point-source evaporators. The chopped fiber form is used for dissemination with a ground rig or from the air. 

Brooks, T. W., and R. Kitterman Controlled release of insect pheromone formulations based on hollow fibers, and methods of applications. Paper presented at the June 26—29, 1977 meeting of the American Society of Agricultural Engineers. Available from Conrel Co., 110 A. St., Needham Hts., MA 02194. 1977. 

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