Effects of age and height of onion (Allium cepa L.) plants on infestation thrips, Thrips tabaci Linderman (Thysanoptera: Thripidae) in Sokoto, Nigeria.

Three sets of onion (Allium cepa L.) crops transplanted at 4 week intervals were exposed to thrips, Thrips tabaci Lindeman (Thysanoptera, Thripidae) infestation at different times from February to May in 2001 - 2002 and 2002 - 2003. Results indicated that at 4, 8 and 12 weeks after transplanting (WAT), the oldest (12 weeks) and tallest (60 cm) plants had the highest thrips population of 240 thrips/plant and rose to 416 thrips/plant one week later. In the second and third generations, the middle aged plants (second oldest) had the highest thrips population of 608 thrips/plant and this was significantly different (P < 0.05) from the oldest and youngest, even though in some cases they were the second tallest. This was maintained at 5, 9 and 13; 6, 10 and 14 and 7, 11 and 15 WAT (first generation only). Plants exposed to thrips in March consistently had the highest thrips population. It was also observed that 40% of onion leaves constitute inner leaves, 50% intermediate and 10 % outer leaves. The percentages of thrips in those regions were 64, 33 and 3%, respectively. Therefore, it was obvious that the oldest crop had the highest population of thrips early in the season and later in the season. The second oldest crops continued to support the highest number of thrips irrespective of the period of the year.