K. van Frankenhuyzen, N. Payne, L. Cadogan, B. Mickle, A. Robinson
The work reported here addresses the question as to how much scope exists for improvement of B.t. spray efficacy for control of the spruce budworm by altering spray droplet size distribution and application volume. Laboratory evidence suggests that droplets in the physically optimum size range (<80 um) contain at best an LD50 and that feeding inhibition caused by ingestion of sublethal droplets has a negative effect on dose acquisition and subsequent mortality. We explored the hypothesis that the provision of an efficacious dose per unit foliage and subsequent dose acquisition can be enhanced by using coarser spray atomization to increase the dose per droplet. This objective was addressed by comparing differences in spray deposition, dose acquisition, and spray efficacy for Foray 76B applied at 30 BIU/ha using fine and coarse atomization and at 60 BIU (2 x 30)/ha using coarse atomization.
Spray application. Four 50 ha blocks with predominantly balsam fir were located along the shores of Bird Lake in Nopiming Provincial Park in southeastern Manitoba. Prespray larval densities ranged from 8 to 18 larvae per 45-cm tip. Three blocks were treated with Foray 76B at actual application rates of 38 BIU/ha applied in 1.9 L using fine atomization (~9,900 rpm; VMD: 69 um, NMD: 31 um, treatment 30-F), 38 BIU/ha in 1.9 L using coarse atomization (~4,200 rpm; VMD 173 um, NMD 68 um, treatment 30-C), and 58 BIU/ha in 2 x 1.5 L using coarse atomization (~5,800 rpm, VMD 157 um, NMD 63 um, treatment 60-C). Sprays were applied on June 2 and 3 in early morning under favorable conditions, using a Cessna 188 equipped with four AU4000 Micronair atomizers calibrated to deliver 20 L/min with a track spacing of 45 m. Spray application was preceded by a week of warm sunny weather (>25 °C) which continued for another 10 days after application with no rain. Larval development was at 50-60% fourth instar at the time of spray application.
Aircraft navigation was aided by using a differential global positioning system (DGPS). Tethersonde flights were used to obtain relevant meteorological data at aircraft flying height. The position of the sample trees used for deposit assessment was surveyed with DGPS and were plotted relative to recorded flight lines in the UTM Grid system to reveal variations in track spacing and swath overlap. Application rates were calculated from flow rate and GPS ground speed and were plotted for each of the sprays to examine variability in application rate in the vicinity of the sampling sites.
Spray dispersal was assessed by estimating droplet densities, size spectra, and volumetric (fluorometric) deposition at 20 stations in each block, corresponding to locations that were used for larval sampling and monitoring of dose acquisition. Each sampling station consisted of a halyard system that enabled positioning of samplers at midcrown level. Samplers included two aluminum combs (fluorometric assessment), two Kromekote card combs (droplet size and density), and two balsam fir branch tips (droplet size and density). Two additional balsam fir branch tips were used to compare Health Canada's Quantitative Redox Assay (QRA), a spore-based method, and Entotech's immunoassay-based Deposit Assessment Method (DAM kit), for deposit assessment.
Coarse atomization of Foray 76B resulted in a VMD of 110-160 um, and an NMD of 4080 um, (corrected droplet sizes) on both K-card combs and needles, as compared to a VMD of 60-80 um, and an NMD of 25-30 um, in the fine atomization treatment. Droplet densities on both K-cards and foliage was about two times higher in the 30-F than in the 30-C block.
Coarse atomization of 1.9 L/ha yielded an average density of 0.4 droplets per cm2 of needle as compared to 0.8 in the fine atomization treatment. However, despite the lower density of droplets, coarse atomization resulted in the same total dose per unit foliage as fine atomization (30-F versus 30-C), as indicated by all assessment methods (fluorometric, DAM, and QRA). Increasing the application rate to 58 BIU/ha by applying 2 x 1.5 L with coarse atomization resulted in a roughly proportional increase in droplet densities as well as the dose deposited per unit foliage. The total dose deposited per unit foliage as estimated by the DAM (measurement of toxin) and QRA (measurement of viable spores) methods showed a weak correlation with the volumetric deposit assessment obtained from the aluminum combs (Pearson correlation coefficient, r = 0.42 for QRA and 0.49 for DAM). However, there was a higher correlation (r = 0.81) between the DAM and QRA estimates of spray deposit on the colocated fir tips.
Dose acquisition was monitored by collecting 10-15 larvae per branch from each of 2 branches per tree (20 trees per block) 2 days after application. Larvae were placed on artificial diet for 3 days at 25 °C to monitor recovery from feeding inhibition (production of frass) and fate of the larvae (dead without frass production = B.t. lethally-dosed; dead with frass = other mortality factors; alive = survivors). From each branch 5 shoots were collected for spray deposit assessment by the QRA method to explore correlation between the proportion of lethally-dosed larvae and residual spray deposits.
Coarse atomization enhanced dose acquisition, but only marginally so. In the 30-F block, 40% of the larvae collected 2 days after spray had obtained a lethal dose, as compared with 50% in 30-C. However, increasing the application rate (and the resulting dose deposited) did not further increase the percentage of lethally-dosed larvae (50% in 60-C). There was a weak positive correlation between the residual dose on foliage collected 2 days after spray and the proportion of lethally-dosed larvae (r=0.38).
Spray efficacy was assessed by taking two prespray and postspray midcrown branches from each of 60 sample trees per spray block (and 96 trees in the untreated control). The increase in dose acquisition between 30-F and 30-C was not reflected in spray efficacy. The % reduction in larval populations did not differ significantly and was 60% in 30-C and 74% in 30-F. Increasing the application rate in the coarse atomization treatment increased the level of population reduction to 80%. All treatments significantly reduced defoliation relative to the control. Average % reduction in larval density per tree was not correlated with either the proportion of lethally-dosed larvae or day-2 residual spray deposits as measured by QRA.
In conclusion, the results of this study suggest that even with an application volume as low as 1.9 L/ha, the VMD of the deposited droplet size spectrum can be doubled from ~80 to ~160 um without reducing the dose delivered to the target foliage. Application of 38 BIU in 1.9 L/ha with coarse atomization delivered the same dose per unit foliage as with fine atomization but in fewer and larger droplets. This dose distribution slightly improved dose acquisition but the improvement was not reflected in treatment efficacy. Increasing the application dose with coarse atomization significantly increased the dose deposited and treatment efficacy, but without a demonstrable increase in dose acquisition.