Deposition, distribution, persistence and fate of bacillus thuringiensis variety kurstaki (BTK) in pecan orchards following aerial and ground appucations to control pecan nut casebearer larvae

Abstract

Aqueous mixtures of Bacillus thuringiensis Berliner var. kurstaki (Btk) were applied over three plots located in a pecan orchard in New Mexico, USA at a dosage rate of 39.6 billion international units (BIU)/ha and volume rates of 9.4 L/ha (Plot 1), 935 L/ha (Plot 2), and 47 L/ha (Plot 3). Plots 1 and 3 were sprayed aerially using a Cessna 188 aircraft fitted with Micronair AU 5000 atomizers and D8–46 disc‐core (hydraulic) nozzles, respectively. Plot 2 received ground application using a Wilbur‐Ellis air‐blast sprayer. Spray deposit patterns on artificial samplers and live pecan substrates were determined by quantifying the δ‐endotoxin at three different levels in the tree canopy and at ground level using enzyme‐linked immunosorbent assay (ELISA). Droplet density (droplets/cm²) and size spectra were also assessed to evaluate the uniformity of coverage at the three canopy levels using horizontal and cylindrical (vertical) Kromekote card samplers.

Droplet density and size spectra on the Kromekote cards, and deposits on artificial samplers and pecan substrates varied both within and among the plots. They were higher on the horizontal compared to the vertical artificial samplers. The droplet data on artificial samplers indicated phase separation and non‐uniform distribution of Btk particles. Analysis of spray deposits on the samplers showed definite gradients in deposit levels, decreasing from the upper to lower canopy in the aerial treatments, and from lower to upper canopy levels in the ground application. Canopy coverage, indicated by the ô‐endotoxin levels at the mid‐canopy target sites (viz., nutlets and budlets), decreased in the order: Micronair > air‐blast > hydraulic spray systems. Aerial application with hydraulic nozzles provided larger droplets at ground and canopy levels, with greater uniformity of deposits; however, the deposits at the target sites were relatively low. The δ‐endotoxin was found to be short‐lived and the average dissipation half‐life values for the deposits on the artificial samplers, pecan foliage, nutlets and budlets from the mid‐canopy were 24.4, 17.9, 14.3 and 16.5 h, respectively.

Key words:

• Btk spray
• Micronair atomizer
• hydraulic nozzles
• air‐blast sprayer
• droplet density
• droplet size spectra
• phase separation
• canopy penetration
• vertical distribution
• ô‐endotoxin
• ELISA
• DT₅₀