Pheromone emission rates

A key feature of the Hereon dispensing system is the ease of regulating the pheromone emission rate from the dispenser. Thus, the emission rate may be adjusted by varying one or more of a variety of parameters (4) 1) thickness of outer layers of the dispensers, 2) pheromone concentration per unit area of the dispenser, 3) size (area) of the dispenser, 4) amount of flake applied per acre, 5) the polymer used to fabricate the dispenser, and/or 6) polymer stiffness. Duration of effectiveness may also be extended by increasing the thickness of the inner layer of the dispenser, in effect increasing the size of the pheromone reservoir of each flake. 

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. 

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)... 

A mathematical model for the transmission of a chemical signal has been developed by Bossert and Wilson (2). According to this model, the substance is transmitted through the air in accordance with simple laws of diffusion when the air is still and the substance cannot adsorb on or react with other substances. At any point away from the emitter, pheromone concentration is a function of 1) the rate of molecular emission, 2) the diffusion coefficient of the substance, 3) the distance from the source and 4) the time from the initiation of the emission. 

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