||<p/>In apple growing areas, in the Netherlands, <em>Panonychus ulmi</em> Koch and <em>Adoxophyes orana</em> FvR. are the most important pests. Preliminary experiments had shown that <em>P. ulmi</em> could be controlled by predators, but these predators are killed by insecticide sprayings against other pests.<p/>As <em>A. orana</em> had already been cultured on an artificial medium, the application of the population sterilization technique was within the realms of possibility. However, the high population density of the species was a disadventage. But if this was counterbalanced by a poor capacity for dispersal, the application of this technique, on a small scale, would be possible. In a simulation model, it was demonstrated that only 100 immigrants per generation per ha, was already too high to make the technique feasible.<p/>Three modes of dispersal could be expected: flight, aerial transport of larvae with wind and transport of larvae together with plants and packaging materials. The last possibility was not investigated, it was assumed that it could be reduced to a great extent by appropriate measures.<p/>Dispersal by flight was studied in release-recapture experiments. The released moths were cultured on a meridic diet. For marking, either <sup><font size="-1">32</font></SUP>P or the dye Calco oil red D was added to the medium. In order to recapture the moths, light traps and sex traps were used. The experiments were carried out in several places, an open field, orchards, and hedgerows. The results obtained indicated that the movement from the release points was small. The greatest distance at which a male was recaptured in these experiments was 250 m. Only in the experiment in which 75500 males were released homogeneously throughout an orchard of 1.5 ha, were the distances, at which males were still recovered, somewhat greater, but the maximum distance was 435 m. For this particular experiment it was calculated that about 1 to 2 percent of the males had left the orchard. It is assumed that, for this kind of biotope, this a representative percentage.<p/>In light traps, in general, the numbers of males captured were higher than the numbers of females captured. This could be the result of, either a lower flight activity of the females compared to the males, or an eventual difference in the diurnal flight period of males an females. In laboratory experiments, evidence was obtained for both possibilities.<p/>Concerning larval dispersal by means of wind, only qualitative data were obtained. It was demonstrated that it occurs frequently and that characteristic behaviour elements are involved. The larva spins a silken thread when it drops off from a leaf. This thread can be broken by the force of the wind, exerted on the thread. It subsequently breaks near the end at which it is attached to the leaf. The buoyancy of the larva in the air depends on the length of the thread. It was demonstrated that wind velocities of more than 3 m per sec have an immobilizing effect on the locomotion of the newly-hatched larvae. It is likely that this behaviour protects the larva against being blown away from the leaf, with a very short thread. Experiments on phototaxis and geotaxis yielded indications that the behaviour of young larvae makes them vulnerable to this mode of dispersal. The behaviour of older larvae was different in this respect. It is likely that, because of the greater weight, the chances for aerial transport of older larvae are reduced. In a qualitative way, aerial transport can be important. At 20 °C and 70 % R.H. the newly-hatched larva can survive 6 hours starvation. With an air speed of 2 m per sec, transport over more than 40 km is possible. The quantative aspects will be determined by the vegetation that surrounds the egg-mass. In an orchard, a large proportion of the larvae will be sieved out from the air, by the surrounding trees.<p/><em>A. orana</em> has a wide hostplant range. In preliminary experiments the possibility of a variability of response towards different hostplants was investigated. No indications were found.<p/>When there is no variability in response towards apple, between <em>A. orana</em> in orchards and <em>A. orana</em> in other biotopes, it is important to know the population densities in these biotopes, especially in the fruit growing areas, where eventually the population sterilization technique is going to be applied. The population densities were estimated in 23 different places, hedgerows, lanes and woods, by releasing a known number of marked moths into the wild population. From the numbers recaptured by means of sex traps, the number of moths of the wild population was calculated, with the aid of the Lincoln-index. The density in woods was found to be lower than the average population density in orchards, but for some hedgerows higher densities were calculated. The calculated densities were highest when a great part of the hedgerows consisted of alder ( <em>Alnus</em> spp.) or hawthorn <em></em> ( <em>Crateagus</em> sp.).<p/>The conclusion of these studies was, that with regard to the dispersal of the adults, there is a fair chance that the population sterilization technique can be applied on a small scale, when the area is surrounded by a zone of 500 m, free from other populations of <em>A.orana.</em> If this is not possible, sterilized adults also have to be released in these populations. In this conclusion, the dispersal of larvae by wind, is not taken into consideration. It is assumed that quantitatively, this mode of dispersal is of minor importance.