NATIVE LARVAL PARASITOIDS ASSOCIATED WITH TUTA ABSOLUTA (MEYRICK) IN GREENHOUSE TOMATO CROPS OF SOUTHERN SARDINIA.

Since its first detection in Sardinia (Italy), Tuta absoluta (Meyrick) has been reported as a major pest of greenhouse tomatoes. In recent years, however, a tendency toward a progressive reduction of tomato borer infestation levels has been observed. The reasons behind this decline are probably diverse, including both the increase in growers' ability to manage the pest and the adaptation of native predators and parasitoids to the new prey/host. In order to assess the species composition of the parasitoid complex associated with T. absoluta larvae in Sardinian greenhouse tomatoes, a two-year (2010/11) survey was conducted in one of the island's major horticultural areas (Pula, Cagliari). An estimate of the levels of parasitism caused by native wasps was also carried out. The occurrence of larval parasitoids of T. absoluta was detected in approximately half of the crops monitored. The average parasitism rate recorded in tomato plants infested by tomato borer larvae was 1.3%. Moreover, while no parasitism was observed in 76.4% of the samples collected, only 3.0% of the samples showed a parasitism rate exceeding 10%. The highest rates were recorded in spring and summer, with a peak in July (4.8%). Of 159 adult parasitoids recovered from infested leaves, 62.9% were found to belong to a Necremnus sp. near artynes (Walker), 34.6% to a Necremnus sp. near tidius (Walker), and 2.5% to Neochrysocharis formosa (Westwood) (Hymenoptera: Eulophidae). These results show that some native parasitoids have adapted to T. absoluta in the study area. Although parasitoid abundance in tomato crops appeared to be low, their contribution for the control of tomato borer infestation could possibly be enhanced through the application of conservation biological control measures.

An epidemiological survey of TYLCD in southern Sardinia (Italy).

Tomato yellow leaf curl Sardinia virus (TYLCSV) and Tomato yellow leaf curl virus (TYLCV) are among the most virulent pathogens of greenhouse tomatoes in Sardinia (Italy). To investigate the relationship between seasonal population trends of the vector Bemisia tabaci and the dynamics of TYLCD spread to susceptible crops, between May and October 2007 we carried out a survey in a tomato growing area located in the south of the island. On three farms specialized in the production of fresh market tomatoes we monitored, outside commercial greenhouses, the following parameters related to TYLCD epidemiology: mean weekly catches of the whiteflies B. tabaci and Trialeurodes voporariorum on yellow sticky traps, ratio between the two whitefly species and proportion of B. tabaci adults carrying TYLCSV/TYLCV in adult samples collected on hosts not susceptible to the disease, proportion of tomato plants infected by TYLCSV/TYLCV after a two-week exposure to open field conditions. Generally speaking, the flight activity of whiteflies increased during spring, reached a peak in May or June and gradually declined in summer. At the beginning of the survey, T. vaporariorum was found to be the prevalent species, but after a shift in composition of whitefly populations during July, B. tabaci became predominant. While the percentage of vector adults carrying the viral agents of the disease was relatively high up to July, with maximum values ranging between 14 and 25%, during the following months it decreased to less than 5%. The incidence of TYLCD in the plants exposed outside the greenhouses showed a similar trend in the sites surveyed, with two peaks roughly coinciding with the beginning and end of summer. Therefore, two distinct phases of TYLCD spread were observed: from spring to midsummer when the disease was transmitted by low 8. tabaci populations with relatively high proportions of virus carriers; from midsummer to autumn, when the disease was spread by larger vector populations with low percentages of individuals carrying the viruses. Further studies are necessary to gain a better understanding of the interactions among B. tabaci biotypes, TYLCSV/TYLCV and their hosts.

Seasonal trends of whitefly populations in a Mediterranean tomato growing area.

The whiteflies Bemisia tabaci and Trialeurodes vaporariorum and their associated viruses constitute a major threat to tomato crops in the Mediterranean region. Continuous host availability and mild climate are thought to be among the factors contributing to the outbreaks of whitefly-related problems in this area. We carried out a year-long survey to investigate the relative contribution of different plants, agricultural and not, and indoor/outdoor crops as hosts of the two whiteflies and the tomato yellow leaf curl disease (TYLCD) in a multi-crop system typical of tomato growing areas in southern Sardinia (S. Margherita di Pula, Cagliari, Italy). For this purpose, during 2005 we monitored whitefly population trends in different plots of a horticulture farm, evaluated seasonal changes in the infestation density of the two pests on the most represented host species and assessed the incidence of TYLCD on tomato crops and susceptible weeds. Whitefly catches on yellow sticky traps were found to be higher inside et along the external perimeter of greenhouses compared to open field crops or uncultivated areas, thus suggesting significant adult movement between indoor and outdoor patches. In most plots flight activity increased between late spring and late summer, peaking in July. The number of immatures of the two whitefly species showed similar dynamics, but while T. vaporariorum reached the highest densities in greenhouse tomato crops in June, peak levels of 8. tabaci were recorded between July and August in outdoor horticultural crops and weeds. The occurrence of TYLCD was detected all year round on weed hosts, but the highest number of infected plants was observed in June on long cycle tomato crops. The present survey has demonstrated the contribution of non-agricultural plants the maintenance of tomato yellow leaf curl disease in the study site. However, tomato crops established in summer as major reservoirs of TYLCD-associated viruses and presumably played a key role in the spread of the disease to autumn tomato crops. The implementation of a crop-free period between successive susceptible crops, or at least the avoiding of whitefly movement through greenhouse openings and the complete destruction of plants after final harvest, would probably reduce the severity of TYLCD epidemics in this area.

Biological agents for whitefly control in Sardinian greenhouse tomatoes.

To evaluate the effectiveness of alternative options for biocontrol of whiteflies in greenhouse tomatoes, an experiment was carried out during the cropping season 2005-2006 in one of Sardinia's major horticultural districts (S. Margherita di Pula, Cagliari, Italy). Twelve long-cycle and 17 short-cycle tomato crops (8 autumn and 9 spring crops) were surveyed. All of them were treated for insect pest control at the beginning of the growing season, but in 19 out of 29 cases whitefly natural enemies were also released (BCA greenhouses), at least four weeks after the last treatment. The following release programmes were tested: on autumn crops, 1 Macrolophus caliginosus and 12 Eretmocerus mundus/m2; on long-cycle crops, 1 M. caliginosus (released in autumn or spring) and 24 Encarsia formosa/m2 or 48 E. formosa/m2; on spring crops, 1 M. caliginosus and 24 E. formosa/m2 or 48 E. formosa/m2. The cost of each option was fixed at approximately 0.25 Euros/m2. The remaining greenhouses were maintained as controls (no BCA greenhouses). While whitefly and mirid populations were monitored monthly, whitefly species composition and mortality of immature stages were estimated at least twice during the growing season. On short-cycle autumn crops, the release of M. caliginosus and E. mundus produced negligible results in terms of Bemisia tabaci control. On long-cycle and spring crops, even though in June mortality rates in BCA greenhouses were found to be 2- to 3-fold higher than in no-BCA greenhouses, Trialeurodes vaporariorum population growth was not significantly affected by natural enemies. Among the beneficials tested, E. formosa proved to be the most effective; E. mundus and M. caliginosus did not establish well, probably owing to the persistence of insecticide residues, scarce prey availability and intense plant de-leafing. The presence of indigenous natural enemies of whiteflies was observed in most sites, but in general they contributed little to biological control. The present experiment showed that in Sardinian tomato greenhouses the use of beneficial insects may result in inadequate biocontrol of whiteflies. In particular, the application of cultural practices which may disrupt the establishment and development of predator and parasitoid populations should be critically reexamined.