INTRODUCTION

Techniques of manipulation of agroecosystems have an important role among the strategies of integrated pest management (IPM) (van den Bosch & Telford, 1964) and its successive developments, i.e. vegetational management (Altieri& Letourneau, 1982) or ecosystem management (Speight, 1983). This technique is a complex agronomic practice leading to diversification of the agroecosystem which creates a less suitable environment for the development of the phytophagous populations (Delucchi, 1997). In this context the natural enemies are augmented either by removing and mitigating adverse elements or by providing lacking requisites (DeBach, 1974).

It appears evident that to reach these aims more must be known about the ecology of the various guilds of useful, harmful, and innocuous insects or at least about the main species. Such knowledge must necessarily concern not only crops, but also all the surrounding territory in relation to it. Numerous species, in fact, spend only a part of their life in cultivated fields needing other habitats for activities such as nesting, reproduction, overwintering, or for simple refuge (Maini, 1995).

In relation to chrysopids and other lacewings, attention has been directed for years more towards the field release of artificially reared individuals (augmentation method) than towards the above aspects. Ridgway & Kinzer (1974) and Ridgway & Murphy (1984), reviewing the use of lacewings in biological control, only once mention environmental manipulations speaking almost exclusively about the food attractants. These techniques are now well developed but their results depend on the natural populations in neighbouring areas, and on which more information is also needed.