1984. lntraspecific competition between different aged larvae of Agromyzn frontella (Rondani) (Diptera: Agromyzidae): advantages of an oviposition-deterring pheromone. Can. J. Zool. 62: 2192-2 196. In laboratory studies evaluating the effects of intraspecific competition between Agromyza frontella larvae of different ages, individuals starting 24 or 48 h before other larvae within the same leaflet largely escaped the negative effects of competition. However, larvae starting development later experienced a higher incidence of mortality and reduced pupal weight. Thus an oviposition-deterring pheromone laid down by ovipositing A. frontella females would only have to remain effective for 24 h to confer a competitive advantage to the first individual developing within the leaflet. In two separate field tests this proved to be the case, with active pheromone remaining for >24 h under both hot and rainy conditions. There has been some doubt concerning the adaptive value of certain oviposition-deterring pheromones under field conditions because of their water-soluble nature. The results of these experiments would suggest that the required active life of an oviposition-deterring pheromone may be considerably shorter than previously thought, and thus their persistence, even for short periods, may be adequate to reduce intraspecific competition in natural populations. QUIRING, D. T., et J. N . MCNEIL. 1984. Intraspecific competition between different aged larvae of Agromyzcr frontella (Rondani) (Diptera: Agromyzidae): advantages of an oviposition-deterring pheromone. Can. J. ZooI. 62: 2192-2196.L'etude en laboratoire des effets de la competition intraspecifique entre des larves d'riges differents d'A~romyza frontella, a montre que les individus ayant eclos 24 ou 48 h avant les autres ii I'interieur du m6me foliole, ont largement Cchappe aux effets negatifs de la competition. Cependant, chez les larves ayant debut6 leur developpement plus tard, I'incidence de la mortalite a ete plus elevee et le poids des pupes plus faible. Donc une pheromone de marquage, comme celle deposee par les femelles d'Agromyza frontella apres la ponte, n'aurait qu'a persister pour une pkriode de 24 h seulement afin d'offrir un avantage competitif au premier individu se developpant h I'interieur du foliole. Au cours de deux tests separes, realises sur le terrain, l'activite de la pheromone s'est maintenue pour plus de 24 h sous des conditions de temperature elevke ou de pluie. La valeur adaptative de certaines pheromones de marquage en nature a ete mise en doute etant donne leur solubilite dans I'eau. Les resultats obtenus au cours de ces tests indiqueraient que la duree de vie active d'une pheromone de marquage pourrait 6tre considerabIement plus courte que ce qu'on a d'abord pense et qu'en consequence, sa persistance, m6me pour de courtes pkriodes, pourrait etre suffisante pour reduire la competition intraspecifique dans les populations naturelles.