Field studies were carried out in order to assess the populations of thrips on cotton leaves, by developing a sampling program to estimate the density with an acceptable level of precision. Thus, in a cotton field (1.1 ha) in central Greece, 45 leaves were collected (one leaf per cotton plant) at 10‐day intervals from May to September, for four consecutive growing seasons (1995–98). Five species of thrips were found: Frankliniella intonsa was the most abundant species followed by Thrips angusticeps, Thrips tabaci, Frankliniella occidentalis and Aeolothrips intermedius. Although considerable differences were observed in the population fluctuations among species, the highest population densities, for all species found, were recorded in July and August. As indicated by Taylor's power law estimates, all species presented aggregated distribution among sampling units. As this type of spatial pattern indicates, the accuracy obtained in estimating mean population density increased with the increase of the mean. Furthermore, the increase of the mean caused an exponential decrease in sample size. However, the precision level is acceptable only in high mean values, while at the same time the benefit from an increase in sample size is of no practical value.

The predatory effect of adult ladybird Coccinella septempunctata L. on adults of thrips, Thrips tabaci Lindeman, and whiteflies, Trialeurodes vaporariorum (Westwood), was examined in controlled environment chambers, on tomato leaves, in transparent small plastic cages at proportions of 1/(10 + 10), 1/(20 + 20), 1/(30 + 30) and 1/(40 + 40) predator/number of thrips and whiteflies. We conclude that C. septempunctata could be used with success for the biological control of thrips and whiteflies in greenhouse crops, with almost the same effectiveness as for aphids, at predator/prey proportions near 1 : 30. Additionally, it was found a prey preference for T. tabaci in comparison with T. vaporariorum. According to the model used, effective predation is correlated with predator/prey ratio rather than to prey preference.

The predatory effect of adult female Orius niger (Wolff) (Hem., Anthocoridae) on the larvae I–II and adults of two injurious thrips, Frankliniella occidentalis (Pergande) and Thrips tabaci Lindeman (Thysan., Thripidae), was examined in June 1997, in controlled environment chambers (temperature 22 ± 0.2°C, 65 ± 3% RH, a 16 h light : 8 h dark photoperiod and light intensity 9000 Lux). This study took place on sweet pepper leaves (Capsicum annuum L.) in transparent small plastic cages at proportions of 1/5, 1/10, 1/20 and 1/30 predator/number of thrips for larvae I–II and adults separately for each thrips species. O. niger was proved an effective predator for the immature stages (larvae I–II) and for the adults of the two thrips species. We concluded that O. niger could be used with success for the biological control of thrips in greenhouses crops.

The predatory effect of female adults of Coccinella septempunctata L. (Coleoptera: Coccinellidae) on aphid Macrosiphum euphorbiae (Thomas) was examined under laboratory conditions, in controlled environmental chambers. This study took place on single rose leaves in transparent small plastic cages at varying proportions of predator/number of aphids. Coccinella septempunctata proved to be an effective predator for the biological control of M. euphorbiae under controlled conditions in closed environments. It is proposed to use starved adult females of C. septempunctata in biological control programmes in greenhouses, as a specialized predator of M. euphorbiae in proportions near 1 : 35 predator/aphids. The percentage of aphids that may escape was correlated to the hunger satiation of the predator and the total number of aphids, and the theoretical models describing the predator's behaviour under controlled conditions were defined.

The objectives of this study were to determine the depth of penetration into the soil by Frankliniella occidentalis (Pergande) and Thrips tabaci Lindeman (Thysan., Thripidae) in cucumber and tomato crops in greenhouses. A metal sampling apparatus sampled for the two species of thrips at five levels of soil depth (0-10 cm), over seven dates of sampling, each in eight replications. In general, thrips were found to be in greater numbers in the first 2 cm of soil, with the number of insects decreasing with depth increasing. In the depth of 8-10 cm, no insects were found. Number of thrips found in the soil under cucumber plants was greater in comparison with the tomato samplings. Frankliniella occidentalis was found in greater numbers in comparison with T. tabaci. The date of sampling showed a significant interaction with soil depth, with the number of insects found increasing from the first to the last date of sampling.