Cotton production in Australia is vitally dependent upon the safe and efficient application of

pesticides. Timely and of insecticides, often only possible using aircraft, is required to maintain adequate control of pest species such as heliothis spp. However if pesticide droplets are transported downwind away from a sprayed area, significant contamination of susceptible areas can occur. For example, contamination of pastures due in part, to endosulfan spray drift, led to significant disruption of the Australian beef cattle industry during 1998.

Previous research (Woods et al. 2001), quantified typical downwind insecticide deposition

profiles for ultra low volume (ULV) and low volume (LV) application of insecticides and

showed that a combination of application parameters could reduce spray drift values. Subsequent to this work, the Australian Pesticide and Veterinary Medicines Authority (APVMA) introduced a regime for the 199912000 cotton season that stipulated that the insecticide endosulfan should be applied by aircraft using Large Droplet Placement (LDP) techniques. Fundamentally, this application technique specified the application of sprays from agricultural aircraft using droplets with a Volume Median Diameter (VMD) greater than 250 pm, water volumes greater than 30 Llha and the use of spray booms where the distance between the two outermost nozzles did notexceed 65% of the wingspan. Used in conjunction with appropriate management strategies, it was postulated that a droplet spectra with a VMD greater than 250 pm could reduce downwind drift levels as a result of the inherent higher droplet sedimentation velocities.

LDP technology requires the use of relatively large water volumes (30 Llha and above)

compared to ULV (3-5 Llha). However aqueous droplets can evaporate rapidly, particularly

when conditions are hot and dry (eg. >30•‹C and RH < 40%). As droplets evaporate they become smaller and may travel longer downwind distances thus increasing the potential for spray drift. One solution is to add an anti-evaporant adjuvant (AEA) to the formulation. Many materials are available which are capable of modifying pesticide behaviour eg. wetters (surfactants which increase droplet spreading and penetration), thickeners (which increase initial droplet size) and stickers (which help droplets adhere to leaf surfaces). Most AEAs are thought to work by adding a skin to the water droplet thereby slowing down the rate of evaporation. There are many claims made regarding the ability of such adjuvants to modify the spray drift behaviour of pesticides.

Some claims are unsubstantiated and rigorous evaluation is required. With increased reliance on water based application from aircraft there is thus a need to investigate the evaporative process and optimise LDP technology for the aerial application of insecticides in cotton.

This project was therefore undertaken to quantify the performance of aircraft nozzle systems,

establish if any, the reductions in spray drift resulting fiom the adoption of LDP technology,

quantify the influence of certain adjuvants and investigate the impact of atmospheric stability on the downwind dispersal of droplets.