EFFECT OF TRACER DYES ON INITIAL DEPOSITS AND PERSISTENCE OF BACILLUS-THURINGIENSIS SUBSP KURSTAKI TOXIN AFTER APPLICATION OF 2 COMMERCIAL FORMULATIONS ONTO SPRUCE TREES

The effect of two tracer dyes [Erio Acid Red (EAR) and Acid Black 48 ( AB-48)] on initial deposits and persistence of Bacillus thuringiensis subsp. kurstaki (Btk) toxin (delta-endotoxin) was studied after sprayi ng two commercial formulations, Foray(R) 48B and Foray(R) 76B, over po tted white spruce [Picea glauca (Moench) Voss] seedlings, at a dosage rate of 30 billion international units (BIU) per ha. Spray was applied using a spinning disc atomizer calibrated to deliver droplet sizes si milar to those utilized in ultra-low-volume (ULV) treatments in operat ional insect control programs. The sprayed seedlings were left outdoor s at the Sault Ste. Marie laboratory for 18 days under natural conditi ons of sunlight, wind and rainfall. Initial deposits and persistence o f delta-endotoxin protein in spruce foliage were determined by immunoa ssay [enzyme-linked immunosorbent assay (ELISA)] quantification of the delta-endotoxin. The total protein (inactive plus active) and delta-e ndotoxin (active protein) concentrations in the two formulations were determined by a gravimetric procedure and by ELISA respectively. The i nitial deposit levels of the toxin on foliage were not markedly affect ed by the addition of either of the two tracer dyes, and showed only a narrow range of 1521 to 1625 ng/g foliage (fresh weight) for Foray 48 B, and 1789 to 2056 ng/g for Foray 76B. However, the persistence of th e toxin was significantly influenced by the presence of the dyes. The toxin persisted in foliage only for 7 d post-spray when the EAR dye wa s added to Foray 48B, compared to 10 d when no dye was added. The aver age half-life (DT50) of disappearance was 17.4 h for Foray 48B with EA R, and 20.9 h when no dye was present. In contrast, the situation was reversed in Foray 76B, since the duration of persistence was 10 d when EAR was added to Foray 76B, compared to 7 d when no dye was added. Th e average DT50 was 27.9 h for Foray 76B with EAR, and 22.2 h without t he dye. Persistence was the longest (14 d) when the AB-48 dye was adde d to Foray 76B, and the DT50 was 44.9 h.