Pesticide Formulations and Other Parameters Affecting Dose Transfer

Chapter 21

Pesticide Formulations and Other Parameters Affecting Dose Transfer Franklin R. Hall

Downloaded by CORNELL UNIV on October 11, 2016 | http://pubs.acs.org Publication Date: June 24, 1988 | doi: 10.1021/bk-1988-0371.ch021

Laboratory for Pest Control Application Technology, Ohio State University, Wooster, OH 44691 The biological effect of a pesticide, properly timed, depends upon the toxicological properties of the AI, its formulation, the concentration and pattern of the dose at the point of action. Monosize droplet atomization techniques allow a precise examination of droplet/plant/pest relationships. The effects of droplet size, pattern, and formulation changes on the dose transfer process in various insects and mites involve delivery, impingement, retention, toxicity, behavioral and resistance phenomena. Separation of drop size, concentration, and formulation effects can be useful in understanding toxin activity and developing accurate parameters for specific targets and crop protection agents. High speed cinematography, computer image analysis, and fluorescence photography were used to study atomization, droplet formation and transport, deposition and deposit formation phenomena with various pesticide/additive combinations. Predictive responses in spatial disruption and age structure of specific pests are essential to the design of useful formulation/ application protocols of each AI. Pesticides provide innumerable benefits for the control of various pests which destroy almost 33% of all food crops. However, the use of such agents has also resulted in significant costs to public health and the environment (1). In general, the amount of agrichemicals released into the environment has risen 1900% in the 50 year period between 1930 and 1980 (2). Although the improved efficacy of the more recent pesticides has allowed a reduction in use rates as low as a few grams per hectare, the capability to effectively deliver these smaller amounts of agrichemicals to specific targets has become increasingly difficult to achieve (3). 0097-6156/88/0371-0260$06.00A) ° 1988 American Chemical Society

Cross and Scher; Pesticide Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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Spray application is a complex and dynamic process involving many interdependent components. As summarized by Young ( 4 ) , the factors of atomization, transport to target, impaction, deposition formation, movement in/on plant, and biological effect are influenced by each other, the external factors of environment and operating conditions, target properties, as well as the formulation, and the active ingredient (Al). Improvements in pesticide use efficiency will require a more intensive study of the physical and physiochemical parameters controlling droplet dynamics, deposition on defined targets and biological response (Figure 1). The biological effect, properly timed, depends upon the toxicological properties of the Al, its formulation, the concentration and pattern of the dose at the point of action. Droplet deposition and biological response of pests and diseases have complex relationships, sometimes not well correlated and Hi si op (5), Graham-Bryce (6) and Ford and Salt (7) suggest that there is a lack of basic understanding of the dose transfer processes. Graham-Bryce (6) discussed the increasing efficacy of modern chemicals and suggested that further improvements in efficacy may not be as rewarding to agriculturists as unlocking such secrets as biological availability and developing bio-targeting via improved delivery methodology. Geissbuhler et al. (8) predicts that future activities for research in agrichemicals will be governed by such things as 1) advances in the knowledge of crop biochemistry and pest biology, 2) decreased successes in conventional approaches, 3) increased use of electronic information and data development and transfer, and 4) increased economic and ecological pressures leading to a modified crop technology and regulatory environment. As a consequence of this environment we will see 1) biotechnology becoming an increasing component of research, 2) more "biorational" designs, 3) more sophisticated evaluations, and 4) development of targeted oriented delivery systems. Restructuring the placement of a pesticide (i.e., as close to the target as possible) is clearly fundamental to good pest management. As Courshee (9) concluded, the actual target needs to be defined in terms in both space and time. The proportion of pesticide which finally reaches the target and the form available to the pest must be enhanced if we are to increase the efficiency of pesticides. With respect to insects, increased knowledge of the biology should reveal the stage of greatest vulnerability and a greater understanding of insect movement and probability of impingement or encounter (with a residue) within a crop structure (i.e., canopy). This understanding will dramatically increase our ability to select or design more accurate schemes, improve timing or develop resistance management, some of which could only be implemented if improved application technology existed. Under current conditions, savings in pesticide inputs are not being fully realized. While pathogens and weeds (i.e., Phytphthora; Amaranthus spp., respectively) also elicit a number of difficulties in targeting, one of the major problems resides in identifying the specific target site and subsequent protocols. Matthews (10) has


262

PESTICIDE FORMULATIONS: INNOVATIONS AND DEVELOPMENTS

Influences

Secondary effects

physical properties, operating conditions

Techniques used

• ATOMIZATION Droplet collection


drift atmospheric and operating conditions —

TRANSPORT TO TARGET Flash and high-speed cine photography

evaporation

spray and surface properties

IMPACTION

atmospheric" conditions

N

) retention. M

Quantitative analysis

\ >'

DEPOSIT FORMATION deposit and surface properties. -

reflection

'

atmospheric conditions

i

dynamic spreading / static spreading

Optical microscopy

Scanning electron microscopy

5

MOVEMENT IN/ON PLANT uptake

BIOLOGICAL*^*^ EFFECT Figure

1.

The many interactions of f o l i a r atomization (modified a f t e r 4 with permission).



21. HALL

Dose Transfer

summarized our problems with regard to insects, pathogens and weeds and the parameters of equipment selection, modifications, spray volumes, and drop sizes. It is apparent that much has been accomplished in defining these principles. However, there remains many opportunities for improvement in the selective placement of toxicants within the crop environment. Graham-Bryce (6) succinctly identified one of our major problems of placement efficiency when comparing lab and field toxicities of DDT and deltamethrin (Table I). While obviously gaining an enormous increase in chemical efficacy, deltamethrin loses a significant amount of this efficacy advantage (vs. DDT) when placed in a field situation. Additionally, while we know in general terms, an optimum range of droplet sizes for selective targets (Table II), we lack in many instances the ability to deliver that pesticide to that target (10). Additionally, we also have a great deal yet to learn about the optimum placement strategies and criteria for our major pests (5). Additional guidance for nozzle selection has been obtained with the issuance of more practical recommendations for various crop applications (Southcombe, personal communication). The BCPC Nozzle Selection Handbook is an attempt to better educate the user about preferred spray qualities for specific targets. Foliar target areas may be much greater than land areas in terms of surface area, although pesticide recommendations are still generally expressed in terms of 1/ha. Some researchers have attempted to relate dosages per plant surface area while others have selected spray volumes based upon orchard tree row volumes (10-12). With the increasing pressures from ecologists, economists, and environmentalists (i.e., groundwater contamination), it is now clearly up to the crop protection specialists to delineate these parameters which would allow more precise delivery and usage of agrichemicals. Clearly, availability to the target, physicochemical properties, and a fundamental understanding of the biological characteristics of a population (tolerance) and some explanation of substrate specificity of the target process and resistance mechanisms will be fruitful areas for needed research. Atomization of a pesticide and its subsequent fate in the environment will depend in part upon the properties of solubility, volatility, partition characteristics and stability. Considerations of these properties are reviewed and discussed by Hartley and Graham-Bryce (13) and more recently by McCann and Whitehouse (14). Past studies of pesticide formulations have tended to focus on reliability, ease of handling, and safety. Clearly these are important, but Graham-Bryce (6) succinctly illustrates that enhancement of biological activity via formulation and application research deserves more attention than it has thus far received. While applications made at different dosages may give similar areas of deposit, the rates of volatilization in a given environment may be the same. However, the loss represents a greater proportion of the toxicant applied at the lower dose. Therefore, losses may become more serious for more active compounds applied at lower rates (14). In order to pursue a fundamental understanding for optimizing formulation/


263

264

PESTICIDE FORMULATIONS: INNOVATIONS AND

DEVELOPMENTS

Table I. Comparison of intrinsic activities and application rates for representative insecticides (after Graham-Bryce, 1983)

Insecticide DDT Dimethoate Deltamethrin


a

Typical application rate, g/ha 1000 500 20

Relative application rate 50 25 1

Relative lethal dosage a

1600 1039 1

Mean value calculated from data for four species; Phaedon

cochleariae, Anopheles stephensi, and Musca domestics. Table II.

Choristoneura

occidentalism

Optimum droplet size ranges for selected targets (after Matthews, 1979)

Target

Droplet sizes (urn)

Flying insects Insects on foliage Foliage Soil (and avoidance of drift)

10-50 30-50 40-100 250-500

application effects on dose transfer, there first must be a definition of the droplet pattern in space and time required for an optimum biological effect. Thereafter, application and formulation methodology designed for a specific distribution and release rate can be devised. Heretofore, the comprehension of volume, droplet size, and formulation interactions on biological effects have been partly hidden because of the wide range of droplet sizes produced from most conventional nozzles. The development of controlled droplet application (CDA) nozzles has lessened the range of droplet sizes and some success has been obtained with the use of laboratory monosized droplet devices. It is these devices which offer the unique opportunity to study the effects of formulation on the biological effect as it is influenced by droplet size, pattern of distribution, and concentration (15). Examination of Dose Transfer Examination of the biological effect may also be divided into more or less immediate and well defined direct effects (mortality) and those which are classified as sublethal effects. With the development of the synthetic pyrethroids and other new chemistry, the behavioral changes which occur and contribute to significant population effects, especially under low volume (small droplet)



21. HALL

Dose Transfer

265

conditions, must be studied if we are to develop a better understanding of the dose transfer process. The study of pest behavior and how insects or mites move about their environment, find food, avoid being eaten, etc., is both challenging and important. The adaptations, requirements and mechanisms of gaining experience for various insect and mite pests all show a wide diversity of behavior. Understanding the mechanisms underlying behavior requires first understanding the behavior itself. The observations can be assembled into a series of quantitative parameters including 1atency - time to onset; frequency - number of occurrences; duration - length of time of pattern; and intensity - number of acts/unit time. We need to know how sublethal levels of toxicants can affect pest behavior and whether such changes are a meaningful contribution to overall crop protection. Consequently, as we attempt to use more potent toxicants and target them specifically, the measurement of how a pest acts and reacts to a toxin becomes more important. In an attempt to better understand the interaction of drop size, distribution, and dosage on the efficiency of the dose transfer process, systems for generating uniform drops and providing various patterns of distribution have been developed and utilized in these studies (Figure 2) (16,17). Coupling video technology to the systems enhances our ability to better understand pest behavior in response to droplet encounters, i.e., tobacco budworm, Heliothis virescens F, cabbage looper, Trichoplusia ni (Hubner), and twospotted spider mite, Tetranychus urticae Koch (16). Video techniques, documented over time, allow a further elucidation of the behavioral responses of pests to a toxin. For example, video recordings can document the detection process by a pest, and when this feature is diminished by formulation, droplet drying (oil vs. water), or residue decline (time or rainfall). Droplet sizes and oviposition of 7. urticae (Figures 3 & 4) appear to be related in that decreasing sizes of droplets increase the effect with decreasing amounts of bifenthrin (18). In recent tests with fenpropathrin (19), placement of toxicant was accomplished with an automated microsyringe, calibrated to deliver 1-5 ul droplet, 5-1 ul droplets, and 10-1/2 ul droplets per 1.3 cm diam discs. Five adult 7. urticae females were placed on each disc (8 replicates) after droplets had dried and feeding, aversion, and oviposition responses were observed after 48 hr. One other observation clearly denoted in Table III, particularly with feeding responses, is that as the same amount of toxicant is delivered in an increasing number of smaller droplets, the response is generally greater than with fewer but larger droplets/disc. The normal random search behavior of mites thus appears to allow more frequent encounters with the toxicant and hence a more effective transfer. Initial interest in dispersal responses of mites was intensified after studies by Iftner and Hall (20) showed that 7. urticae adults could not only detect, but also respond to deposits of pyrethroids (Table IV). It is not yet known whether this displacement, in response to residues of pyrethroids, is a result of (1) mere increased movement


266

PESTICIDE FORMULATIONS: INNOVATIONS AND DEVELOPMENTS

RMS VOLT METER

LIQUID ORIFICE J ASSEMBLY STROBE^


FREQl JENCY ME1rER

0 - 6 0 KVDC SUPPLY O O

CHARGING RING DROPLET

AUDIO OSCILLATOR

OSCILLOSCOPE

Figure

2.

Schematics

o f components used t o produce a n d charge uniform s i z e drops.

120

Figure 3.

2

E f f e c t o f number of 200u drops/cm o f b i f e n t h r i n on T. urticae egg production a t 48 hr.


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Dose Transfer

267


100

2

Figure 4. E f f e c t of number of 120u drops/cm of b i f e n t h r i n on T. urticae egg production at 48 hr.

Table III.

Fenpropathrin and 7. urticae

Treatment & Number of droplets 2.4 EC*> 1 - 5 ul 5 - 1 ul 10 - 1/2 ul Check a

Maine

Avg % in water 0.5 ab 2.5 c 2.75c 0.00 a

behavior - 48 hr

Avg number eggs/disc 27.88 b 5.88 a 4.38 a 45.88 c

a

Avg number feeding scars/disc 136.88 b 18.13 a 9.38 a 282.50 c

i n Ar»s»lt

significantly different using DNMRT (P

Pesticide Formulations and Other Parameters Affecting Dose Transfer

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