The effect of undersowing cabbage with white clover on thrips infestation and flight activity.

In 2001-2003, the levels of infestation of thrips in cabbage monocrops and cabbage/white clover intercrops were compared. The flight activity of thrips was monitored using blue sticky traps and white water traps to obtain a better understanding of population dynamics of thrips. Plant samples were taken to record the number of thrips on cabbage. Over the years of observations, the highest number of thrips was collected in blue sticky traps on cabbage undersown with white clover. In the period from 15th June to 5th July 2001, the number of thrips collected in blue sticky traps in the monocropped cultivation and intercrops with white clover was on similar low levels. Next, the number suddenly increased to 372 thrips/trap in monocropped cultivation and 509 thrips/trap in the intercropped cultivation. During the period of the highest peak of thrips activity, which was on 17th July, there were 650 thrips/trap and nearly the same number was noticed for both types of cultivations. After this period, until the end of vegetation, the greater number of thrips was noticed for the traps placed in the intercrops. Additionally, in 2001 the thrips were collected in white water traps. Using this type of traps, 480 total thrips/trap were collected in the monocropped cultivation and 819/thrips/trap in the intercrops during the whole vegetation season. The percentage participation of Thrips tabaci Lind. caught in white water traps was 24.4% in the monocropped cultivation and 15.4% in the intercrops. In 2002, during the period from the middle of June to the third decade of July, significantly higher number of thrips was collected in blue sticky traps placed in the cabbage with white clover. The number of thrips collected during the period of the mass flights, which means in the first decade of July was over twice as many thrips in the traps placed in the intercropped cultivation (1316 thrips/trap) as in the monocropped cultivation (589 thrips/trap). In 2003, during the whole vegetation period, the number of thrips collected in blue sticky traps placed on the plots where cabbage was cultivated with white clover was evidently higher. In this year two peaks of the thrips flight activity were recorded: the first on 16th July and the second on 5th August. On both occassions, the number of thrips collected in blue sticky traps placed in the intercropped cultivations was about twice as high as in the monoculture cultivation. In 2001-2003, the thrips feeding on cabbage in the monocropped and intercropped cultivations were observed mainly in July and once again in August. The number of thrips on cabbage was low, only in 2002 this number was higher. In 2001, the number of thrips on cabbage in both types of cultivations was on similar level. The highest number of thrips was observed during the peak of thrips flight activity, which was in the middle of July. In years 2002-2003, despite the higher number of thrips collected in blue sticky traps placed in the intercropped cultivations, the number of pests collected from the cabbage undersown with white clover was lower than in the monocropped cultivation. In 2002, the period of the most intensive occurrence of thrips on cabbage was overlapping with the period of mass flight activity of thrips. During this period, a little higher number of thrips was noticed on cabbage in the intercropped cultivation (3.4 thrips/plant) than in the monocropped cultivation (3.2 thrips/plant). In 2003, the highest number of thrips on cabbage in both types of cultivations was noticed before the first significant peak of thrips flight activity. Whereas in the first decade of August, when the same high number of thrips collected in blue sticky traps was again noticed, no increase in the number of thrips feeding on cabbage was observed in both type of cultivations. Over all years of observations, despite the higher number of thrips collected in blue sticky traps in the intercropped cultivation, this number was always lower on the cabbage undersown with white clover. The most dominant species in both cultivations was Thrips tabaci Lind. Its percentage participation in the collected material was 83.1% in the monocropped cultivation and 76.6% in the intercropped cultivation.