Bionomics Of The Sugarcane Top Shoot Borer, Tryporyza Nivella ( Fab. ) ( Pyralidae: Lepidoptera)

Abstract

The bionomics of the top shoot borer, Tryporyza nivella (F.) of sugarcane have been carried out as follows: i) The systematics has been reviewed and the identi­fication has been confirmed. ii) The biology of the pest was studied both in the field and under laboratory condition. The female lays eggs in masses on the undersurface of sugarcane leaves. Newly­hatched larva bores through the midrib and enters into the spindle causing 'dead heart' or 'shot-holes'. A single larva infests a single cane only and there it passes its larval and pupal stages. The moth comes out of the cane through the exit hole. The top shoot borer passes five instars during its larval stage. The instars were identified by regular sampling of the infested canes and confirmed by the application of Dyar's law. Occurence of five broods in a year was confirmed. :i:H.) Seasonal and regional (11 north-western sugarmills zones of Bangladesh) incidence of the pest have been surveyed. The incidence in all the mills zones was always minimum in the first brood. It increased up to brood number four anddecreased in the last or fifth brood. The incidence was the highest at Panchagarh among all the eleven study areas. It is also evident that the relationship between temperature and humidity is not independent but the two variables inter­act on the percentage of infestation. iv) Varietal behaviour (succeptibility to pest attack, reducing ability for loss in yield and recovery)of the four promising varieties Isd-16, Isd-17, Isd-2/54 and L.J.C. has been assessed. Variety Isd-16 was the least succeptible to top shoot borer while L.J.C. was the most succeptible one. Infestation of the top shoot borer produces a remarkable reduction in the length, circumference and weight of all the varieties. It has also a detrimental effect on the sucrose recovery in all the varieties studied. The loss was highest (35.598%) and the lowest (6.28%) in Isd-17 and L.J.C. respec­tively. v) Efficiency of mechanical control practice has been evaluated. Without mechanical control the incidence of T • nivella was 4.80 - 55.80% in the grower's field and it was 4.64-13.45% in all other study areas where usual mechanical control practice was adopted. But this practice adopted properly in the present investigation efficiently reduced the incidence and it was 4.20-6.60% only. vi) The natural enemies (parasites) of top shoot borer prevailing in the study areas have been identified and their effectiveness in controlling the pests in different broods at di-fferent sugarmill zones have been observed. Biology of the most common three parasites (Elasmus, Stenobracon and Topobracon sp. ) has been studied. --------