Pheromone release rate

Early experiments in our laboratory were concerned with methods for sampling and analysis of TDAL from formulations (6), insects (7) and from the forest atmosphere (8). This work was largely founded upon concepts developed previously by Beroza e t al. (9, 10, 11). Since then, several other groups have applied these concepts to the measurement of a number of different insect pheromone release rates (12, 13). On the basis of our early findings, we were convinced that the existing laboratory techniques for release rate determination from formulations were inadequate. Laboratory tested formulations did not experience the extremes of climatic variation which are the norm in the field and consequently the release rate results were not transferable to field performance. 

Our first step was to develop a simple effective wind tunnel which allowed us to age formulations in the laboratory under very nearly natural conditions. Rather than attempt to control all climatic parameters - temperature, pressure, humidity, light intensity, wind speed, turbulence, etc. - which would have been a formidable engineering task, a very simple design was chosen in which only air speed, temperature and illumination were controlled The only real deficiency in this system is the lack of control of humidity. However, that aspect was addressed separately by measuring the pheromone release rate of each formulation under conditions of very high and very low humidity. To date, none of the candidates tested has shown a major humidity dependence. 

The pheromone release rate is insignificant at temperatures below 10 C. Dispensers exposed inside traps in the first part of May collected beetles during a two-month period, showing that they were releasing pheromones during the beetles main flight period. 

Bossert and Wilson (1963) showed that the ratio of the natural pheromone release rate to the behavioral threshold (Q/K) is a fundamental characteristic of different communication systems. In some communication functions such as with alarm pheromones it is imperative that the signal fade quickly. Organisms producing alarm pheromones generally have a high value of Q/K. Other chemical communication systems such as sex pheromones utilize a relatively persistent signal, characterized by low Q/K ratios. Q/K ratios in different organisms and communication systems are summarized by Matthews and Matthews (1978). 

Shapas, T. J. and Burkholder, W. E. (1978) Patterns of sex pheromone release from adult females, and effects of air velocity and pheromone release rates on theoretical communication distances in Trogoderma glabrum, J, Chem, EcoL, 4, 395-408. 

Release rates of acetate esters of (2S,3S,7S)-3,7-dimethylpentadecan-2-oll and (2S,3R,7R)-3,7-dimethyltridecan-2-ol 3 from polyethylene and cotton dispensers have been measured at different temperatures and loadings. Adjustment of the initial pheromone load on polyethylene for expected temperatures should permit the formulation of a constant release rate during the entire flight period. Successful field trapping trials for N. sertifer were carried out in Sweden, the Czech Republic, Italy, and Greece [29]. 

Pheromone propagation by wind depends on the release rate of the pheromone (or any other odor) and air movements (turbulent dispersion). In wind, the turbulent diffusivity overwhelms the diffusion properties of a volatile compound or mixture itself. Diffusion properties are now properties of wind structure and boundary surfaces, and preferably termed dispersion coefficients. Two models have dominated the discussion of insect pheromone propagation. These are the time-average model (Sutton, 1953) and the Gaussian plume model. 

Schal, C., Charlton, R. E. and Card6, R. T. (1987). Temporal patterns of sex pheromone titers and release rates in Holomelina lamae (Lepidoptera Arctiidae). Journal of Chemical Ecology 13 1115-1129. 

As in most Lepidoptera, spruce budworm males locate conspecific females by flying upwind along a pheromone plume. The blends and release rates of these pheromone components form an important part of a specific communication system for the species. Once the communication system of an insect is understood, especially the pheromone chemistry as it relates to male behavior, it can be used in a variety of ways. For example, pheromones can be used to detect the presence of an insect in an area, to remove males from a population by trapping or poisoning and in air-permeation techniques in which the controlled and continuous release of pheromone components in the forest can disrupt mating. The latter use of pheromones appears to alter the normal male behavioral responses to the natural pheromone (16). 

Pheromone releaser distributions and/or point-source release rates have pronounced effects on the disruption of trap captures of spruce budworm (39). Small field plot studies with hand-placed releasers in moderate- to-high density budworm populations indicate an increase in trap disruption as the point sources of the synthetic pheromone are increased in release rate and decreased in number per unit area (Fig. 1). Identical pheromone dosages per plot were present in each treatment. Optimizing the releaser spacing and point-source release rate is therefore important and implies the need for a formulation re-design. This effect of releaser distribution and point-source release rate on trap disruption and mating disruption has been demonstrated in several insect species (40). 

Ross D. W. and Daterman G. E. (1998) Pheromone-baited traps for Dendroctonus pseudotsugae (Coleoptera Scolytidae) influence of selected release rates and trap designs. J. Econ. Entomol. 91, 500-506. 

In most of the work, the physicochemical behaviour of the formulations was studied by gas chromatographic estimation at intervals of the residual pheromone remaining in formulations sprayed onto filter papers. The latter were of the silicone-treated "phase-separating" type as these best simulated a leaf surface. When the formulations were exposed in a laboratory wind-tunnel there was little pheromone loss other than by release, at least for the monounsaturated acetates used in most of the preliminary work, and such analyses provided accurate information on release rates under these conditions. However, use of this technique in the field showed that loss of pheromone was very much more rapid than under comparable conditions of temperature and windspeed in the wind-tunnel (2). These results were taken to indicate that there was significant loss of pheromone by degradation under field conditions. 

Release Rates of Pheromones from Hollow Fibers... 

Following a review of methods used to measure the release rate of pheromones from hollow fiber formulations two newly designed pieces of apparatus are described. Results of release rate studies obtained with the new apparatus are discussed. 

This method is, however, the most time consuming of the residue analysis methods. Other disadvantages are that the method is destructive as once the pheromone has been extracted from the device, it is useless. Since examination of the residue at various time periods is necessary to establish a release rate this necessitates a large sample size. 

Figure 1. Air flow apparatus for measuring release rates of pheromones from hollow fibers.

To determine the versatility of the apparatus for measuring release rates in general, the release of (acetyl-l-1 C)-gossy-plure was measured from rubber septa, a pheromone release device frequently used in field experiments. After 21 days of measurements, the septa were extracted and any residual activity in the apparatus was measured. The average recovery, detailed in Table II, was 97% indicating that the usefulness of the mini-airflow device was not limited to hollow fibers alone. 

The release rates of (acetyl-l- Cj-gossyplure shown in Figure 3 have a mean value of 3.80 yg/fiber/day. When the mini-airflow rate is compared to the mini-static air rate of 2.27 yg/ fiber/day, it appears that the correlation of release rate measurements by different methods is no closer to solution than it was before. Although the rates are still not in agreement, there no longer is any question of achieving an accurate mass balance at the end of a series of measurements by either method. Since all the pheromone can be accounted for by both the mini-airflow and the mini-static air methods, the differences appear to be related to the conditions of the experiments. 

The release of pheromone from hollow fibers as described by Brooks (6) is dependent upon several factors including the movement of air past the open end of the fiber. In the mini-airflow apparatus the open end of the fiber is in a stream of constantly moving air so that any released pheromone is immediately swept away from the end. In the static air apparatus the only air movement across the open end will be as a result of diffusion. Consequently the concentration of pheromone is more likely to build up at the open end of a fiber in a static air apparatus than at the open end of a fiber in an airflow apparatus with the result that the release rate is lowered in the static air apparatus. Work is now in progress to evaluate the effect of air speed on release rates. 

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)... 

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