The development of spider mite populations (T urticae and T cinnabarinus) as well as plant injuries on different croton cultivars were studied. Observations were conducted in commercial and experimental glasshouse conditions. The highest spider mite populations were noticed on cultivars Norma and Petra, the lowest on Golden Sun. Tetranychus urticae was the dominant species in commercial glasshouses. Croton cv. Norma was severely injured by T urticae. The leaf damage index (LOI) for this cultivar was almost 4 after sixteen weeks of mite infestation.

The influence of PGPR on the susceptibility of glasshouse cucumber and tomato to spider mites was studied. It was found that plants treated with PGPR suppressed the development of mite populations on both studied crops. The effect was more evident for cucumber, as compared to tomato plants. Mite population was smaller by more than 40% on the leaves of the susceptible cucumber cultivar, Corona growing in the presence of rhizobacteria, as compared to that without bacteria. The development of spider mite population on the bacterized susceptible tomato cultivar, Romatos was more inhibited on young leaves (43%) than on older ones (34%). As the injury of plants progressed the level of inhibition was decreased.

Experimental cucumber and tomato plants were cultivated under glasshouse conditions with or without PGPR. Young plants were infested with spider mites (T urticae - cucumber and T cinnabarinus - tomato). Leaves were analysed for protein and amino acid concentration. In infested leaves of the cucumber cultivar, susceptible to spider mites (Corona), an important decrease in protein content occurred in both bacterized and nonbacterized plants. Amino acid content was higher in mite infested leaves and the presence of PGPR did not influence this phenomenon. The content of proteins was also studied in the leaves of the less susceptible cucumber cultivar (Aramis). Spider mite feeding caused a small decrease in their concentration. The presence of bacteria in the root system caused an increase of soluble proteins in the leaves. In infested leaves of the highly susceptible tomato cultivar (Rornatos), cultivated without bacteria, an evident increase of amino acid content was found opposite to plants with PGPR. The protein concentration was also increased in injured leaves of this cultivar but the presence of bacteria in the root system of mite infested plants caused a decrease of these compounds in their leaves. Injured foliage of the less susceptible tomato cultivar (Sionka) had a lower protein concentration in bacterised plants as compared to those nonbacterised and both controls.

Experiment was conducted twice: in spring of 1999 and 2000. Two greenhouse cucumber varieties (Aramis and Corona) with different susceptibility to spider mires were employed to compare the harmfulness of the two-spotted spider mite and the carmine spider mite. Effects of the rwo-spotted spider mite and the carmine spider mite feeding either on the fruit production or on the content of some organic compounds were measured. Moreover, development of spatially separated populations of the two-spotted spider mite and the carmine spider mite was analysed. Two abiotic parameters were taken into account: temperature and sunlight duration. Increment of the carmine spider mite population density was faster during the season of higher temperature and sunlight duration (spring of 2000). Differences in the development of rwo-spotted spider mite were not observed. The increase of fruit weight was closely correlated with increase of densiry of spider mires. The feeding of relatively low populations of the carmine spider mite (up to 5-7 mobile stages per leaf) stimulated the weight increase of fruits collected from Aramis and Corona varieties. The feeding of rwo-spotted spider mite and the carmine spider mire affected the distribution of organic compounds produced in plants. In quire severely infested leaves (2-3 damage level of De Ponti scale) rota! amount of sugars and phenolic compounds increased as compared ro control leaves.