M. M. PEREIRA
Escola Superior Agrária de Beja, Praceta Rainha D. Leonor, 7800-Beja, Portugal. E-mail: mmpereira@mail.pt;
A. M. MEXIA
Instituto Superior de Agronomia, Tapada da Ajuda, 1399 – Lisboa, Portugal.

Some species of spider mites (Tetranychus spp.) are maize pests in some areas of Europe where corn is an important food source. Those species have the potential to develop high populations that may result in significant yield reductions. Feeding damage is therefore cumulative; mite feeding reduces chlorophyll content of leaves and may interfere with vascular tissue.
In 1998, a field survey was conducted, at Baixo Alentejo (Portugal), in order to evaluate abundance and spatial distribution of mites in 3 maize fields. These maize fields were sampled on a weekly basis, from tassel to grain-filling growth stages, when mites densities can increase rapidly and the crop is the best (and probably the only one) host for this pest.
Each field sampling consisted of 3 groups of fifty leaves and the maize growth stage was recorded each week. The leaves were randomly selected and removed from each third of the plants (lower, middle and upper thirds); the samples were identified with the field, date and the third of plant which was sampled. Each leaf was carefully observed under a dissecting microscope and the number of mites (females and males) per leaf and the number of infested leaves were recorded.
The average rate of plant damage was also determined, for each field under study, and for each plant third, using a 1-10 rating scale: 1= 1-10 % of the leaf area damaged by mite feeding to 10= 91-100 % of the leaf area damaged.
The abundance and the within-plant distribution of these spider mites were described and the average plant damage rating was determined for each field.

Index terms: Tetranychus spp., spider mites, spacial distribution, plant damage rating, maize.