FOLIAR PERSISTENCE AND RESIDUAL ACTIVITY OF TEBUFENOZIDE AGAINST SPRUCE BUDWORM LARVAE

A field study was conducted to investigate the persistence of tebufeno zide in white spruce foliage. An aqueous suspension concentrate formul ation, RH-5992 2F, was sprayed over single trees at three dosage rates , 35, 70 and 140 g of the active ingredient (AI), in 2.0 litre ha(-1), using ground application equipment. Foliage was collected at differen t intervals of time up to 64 days after treatment and tebufenozide res idues were measured by high-performance liquid chromatography. Foliage was also fed to laboratory-reared 4th- and 6th-instar spruce budworm (Choristoneura fumiferana Clemens). The data indicated that tebufenozi de residues in foliage declined with time according to first-order kin etics. The average rate-constant and half-life of disappearance (DT50) were 0.0340 and 20.45 days, respectively. Larval mortality declined g radually, corresponding to the residues, but was still appreciable (49 to 70%) when the larvae were fed with foliage collected 64 days after treatment. The amount of foliage consumed by the larvae decreased whe n foliar residues of tebufenozide increased, thus indicating anti-feed ant activity of the chemical. The LD(50) values for both instars were similar and averaged c.25 ng per insect, but the LD(90) values were si gnificantly lower for 4th-instar than for 6th-instar, at 63.6 and 96.1 ng per insect respectively. This implies that, theoretically, at a fo liar concentration of 1.0 mu g tebufenozide g(-1) foliage (fresh wt), the spruce budworm larva needs to consume 65 to 100 mg of foliage in 1 0 days to cause mortality in about 90% of a population of the insect.