Studies on ecology and temperature relationships of Bemisia tabaci in Italy.

The whitefly Bemisia tabaci has been reported in southern ltaly since 1939, but it has been the object of major attention since the appearance of tomato yellow leaf curl geminivirus in tomato crops in Sardinia and Sicily, in 1989. Some of the biological characteristics of an ltalian biotype of B. tabaci, collected in Sardinia, were investigated in laboratory experiments by culturing the insects on Phaseolus vulgans and Vicia faba . The influence of temperature on the developmental time from egg to adult on P. vulgaris cv Saxa, at 12 h : 12 h (light : dark) photoperiod was established. The calculated lower temperature threshold for egg-to-adult development (a) was 12 °C, but the experimental lower threshold was between 15 and 16 °C. The minimum developmental time from egg-to-adult was 261 day-degrees [expressed as C(t-a)] for constant temperatures (t) ranging from 16 to 26 °C; the time for development of 50 % of the adults was 309 day-degrees. From 29 to 35 °C the developmental time remained constant, but mortality increased. Mortality from egg to adult reached a minimum at 26 °C, being about 56%. The survival of different stages at temperatures between 0 and 6 °C was studied on V. faba, which was used because of its good resistence to low temperatures. Eggs and nymphs were the more resistent instars, surviving for 4 but not 8 days at 0 °C (constant temperature), while adults died within 24 h at 0 °C. A survey for the presence of B. tabaci in ltaly revealed different degrees of establishment in the environment: glasshouses only, field crops, crops and weeds. All the areas where the whitefly occurred both in crops and weeds were characterized by an annual mean temperature above 16 OC and an annual number of frost days less than six . B. tabaci was found in field crops close to glasshouses in areas with an annual mean temperature between 14 and 16 °C. A map was derived showing the risk of establishment of B. tabaci in the open field in Italy.