ĐIỀU TRA MẬT ĐỘ VÀ ẢNH HƯỞNG CÁC MỨC GÂY HẠI CỦA BỌ NẸT (Thosea obliquistriga Hering.) ĐẾN NĂNG SUẤT DONG RIỀNG TẠI HƯNG YÊN VÀ VÙNG PHỤ CẬN

In recent, years caterpillar (Thosea obliquistriga Hering) attacking Canna (Canna edulis Ker) at Hungyen province and its surrounding increased remarkably. The study was conducted in Khoai Chau district (Hung Yen Provice), Thanh Tri and Thach That (Ha Noi) in 2008 and 2009. The survey on density dynamic of the caterpillar showed that first appearance of the insect pest was at 120-130 days after planting of canna at low density (0.3-0.5 larva/leaf) in July 2008 and 0.12-0.14 larva/leaf in May 2009. Then its density was increased and reached highest density at 190-240 days after planting with 3.58-4.32 larva/leaf in September 2008 and 2.06 -3.84 larva/leaf in August larva/leaf). Afterward its density decreased and was 0.06 - 0.36 larva/leaf at harvesting time in October 2009. The insect density at stage of canna 3-6 leaves althought carterpillar damage 10-70% leaves but it can not result in significant redution in yield at the stage 7-8 leaves they damage 30-70% foliage and cause yield reduction from from 18,71-40 %.