D.G. Embree


Executive Summary

This is the second of a series of reports in response to the 1991 survey, the "Improved Pesticide Applications Wish List ". The survey was conducted by the Forest Pest Management Institute (now Great Lakes Forestry Centre, Canadian Forest Service, Dept. of Natural Resources, Canada) at Sault Ste. Marie, Ont.

Of the 52 "wishes" related to insecticides, and 59 similar "wishes" for herbicides, two which have been identified as priorities are addressed here:

Priority 1: Determine the effect of atmospheric stability and wind speed on pesticide offtarget movement and deposition/efficacy.

Priority 2: Determine the influence of temperature and relative humidity at time of application on pesticide off-target movement, and deposition/efficacy.

How fast a spray drop falls depends overwhelmingly on its size and shape, which in turn depends on the spray formulation, the type of spray nozzle, and the rate of evaporation. Its trajectory depends on the movement and density of the air it passes through, which is governed by the condition of the air mass, and in turn by the weather pattern and time of day. Whether the drop performs any useful purpose depends on it immediately, or eventually, coming in contact with what it is meant to kill.

Studies by the National Research Council (Crabbe et al. 1994) established that optimum success in pesticide application can be best achieved in unstable air conditions and low winds - and, in the case of aerial application, when spray planes fly beneath an inversion. In stable conditions, spray drops, dispensed at similar heights, travelled on average 35% further downwind than in unstable air. Work by Dickison and Foster (1995) supported these studies. Air stability was measured with specially equipped aircraft immediately before each of several spray operations. Deposit in unstable to neutral air was three times that in stable conditions, when otherwise similar pre-spray conditions were compared.

At the time of spraying, the influences of temperature and humidity on deposit and efficacy begin with the evaporation of descending spray drops and end with the immediate effect on the spray target. For insect pests these influences end with deposit. (Obviously extremes of temperature have short term effects on insect activity, in which case efficacy depends upon the persistence of the pesticide. Also, high humidities may dilute deposited water-based insecticides, spreading them over larger areas thereby increasing chances of contact by the insect, but reducing toxicity.)

For insecticides, success is best achieved with small drops (100 microns or less) as these are most apt to land on or near the insect pest. Drops containing water or volatile oils evaporate as they fall, and the smaller the drop, the quicker it slows down and floats on air currents. Reduction in drop size can be determined through a series of calculations based on hydrodynamics. Recent studies by Riley (1995), Riley and Smith (1994) and Teske (1996, 1997) showed that conventional calculations overestimate the amount of evaporation that occurs in small water based drops by 2-fold. As the amount of water in the drop is reduced, the remaining water molecules adhere to the nonvolatile components of the drop and resist evaporation. These results have ramifications for spray simulation models such as AGDISP, FSCBG, and PKPW. (See Appendices).

Drop size appears to be less critical for weed control. At the time of spraying, weather conditions should be such that the spray drop can penetrate the plant's cuticle while the plant is functioning in a fashion that will enable infusion of herbicide into vital areas. High temperatures are generally unfavourable. At low temperatures chemical reactions are retarded, and at higher ones the volatile herbicides, such as 2,4-D, can dissipate rapidly into the air. High humidity can slow the process while winds can increase it. Finally, differences between plant species is critical, resulting in variable responses to treatments.

Individual roles of wind, air stability, temperature and humidity in the dynamics of the spray cloud are reasonably understood. Their combined effects along with the capabilities of spray aircraft or ground spray equipment, height of spray release, the life styles of insect pests and physiologies of weed plants is a much more complicated concept and difficult to comprehend. Simulation spray models seem to be the best instruments to predict likely outcomes of the various combinations of factors occurring under field conditions, and the most promising means of devising future spray strategies for the wide range of conditions encountered in pesticide operations. For the models to work requires the development of practical methods to measure immediate pre-spray atmospheric conditions.