Soil erosion is a major environmental threat to the sustainability and productive capacity of agriculture. During the last 40 years, nearly one-third of the world's arable land has been lost by erosion and continues to be lost at a rate of more than 10 million hectares per year. With the addition of a quarter of a million people each day, the world population's food demand is increasing at a time when per capita food productivity is beginning to decline.

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The paper by Arrow et al. and the responses are offered below without comment. It is hoped that the forum makes a useful contribution to the debate.

Can. Ent. 116: 1097-1 107 (1984) Superparasitism and multiple parasitism of the pea aphid, Acyrthosiphonpisum (Harris), by the solitary hymenopterous parasites Aphidius smithi Sharma & Subba Rao and Prnon pequodorum Viereck were studied in the laboratory. In intraspecific competition, an older larva usually eliminated a younger one; exceptions to this rule depended on the relative age difference and developmental stage of the competitors. In interspecific competition, P. pequodorum was intrinsically superior to A. smithi. regardless of the latter's age. The stage of host parasitized and the number of competing larvae had no significant effect on the outcome of the contest. A. smithi females discriminated against aphids previously parasitized by a conspecific female or by P . pequodorum, the degree of oviposition restraint increasing with the time between ovipositions. P. pequodorum females attacked aphids already parasitized by A. smithi, regardless of the age of the Aphidius larva, but avoided oviposition when the aphid contained a Praon larva or embryo. The rate of embryonic development is shown as a major factor in contest-type competition between first-stage larvae. dkveloppement des comp6titeurs. Dans les cas de compktition interspkcifique, P. pequodorum s'est rtvklk intrinskquement supkrieur a A. smithi, peu importe I'dge de ce dernier. Le stade de l'h6te parasitk et le nombre de larves concurrentes n'ont pas eu d'effet significatif sur le rksultat de la comp6tition. Les femelles d'A. smithi ont discrimink les hbtes dCjB parasitks par une femelle conspkcifique ou parP. pequodorum, le degr6 restriction de l'oviposition augmentant avec le temps ecoulk entre les ovipositions. Les femelles de P. pequodorum ont attaquC les pucerons dCjB parasitks par A. smithi peu importe I'dge de la larve d'Aphidius, mais ont 6vitC la ponte dans un puceron contenant une larve ou un embryon de Praon. La vitesse de dCveloppement embryonnaire apparait comme facteur d'importance majeure dans la comp6tition de type concurrence entre larves de premier stade.