«: AGROCHEMICALS: FATE IN FOOD AND THE ENVIRONMENT PROCEEDINGS OF A SYMPOSIUM, ROME, 7 - 1 1 JUNE 1982 JOINTLY ORGANIZED BY IAEA AND FAO l^J I N T E R ...»
Initial studies were addressed to the development o f thermoplastic-based controlled release molluscicides, algicides and insect larvicides where agent emission is in water. Since the diffusion-dissolution method is inoperable and carrier-type systems are inadequate (as well as expensive) for emission beyond a few months, the remaining economic choice was to develop a long-term leaching system.
It was discovered that water soluble co-leachants in thermoplastics gradually solvate into an external water medium, leaving a porosity network for further water egress and subsequent contact and partitioning o f the agent. A secondary key discovery was also needed before very efficient leaching systems could be developed. Porosigens and many chemical agents are generally, although not IAEA-SM-263/43 TABLE I. TEMEPHOS RECIPE 
always, solids and thus matrix incorporation is not o f dispersed molecules, but o f molecular aggregates. Such aggregates are relatively large and cannot traverse intramolecular voids normal to polymers at rates conducive to the desired dispenser surface emission. It was found that free volume is dramatically increased by using polymer blends wherein the several polymers have a disparity in melt index.
Consequently, through proper adjustment o f free volume and use o f one or more porosigens a new class o f long-term controlled release dispensing systems was developed. The concept has been extended from emission in water to emission in soil, air and living organisms.
3. A Q U A T I C LARVICIDES
Initial larvicide work was aimed at the mosquito. The agents used were temephos, which has been cleared for use in potable water [31 ], and tributyltin fluoride, a potent non-persistent molluscicide and highly selective insect larvicide [32, 33]. Early formulations have been disclosed elsewhere and the test results described [25, 27, 33]. Proper selection o f the porosigen in terms o f processing compatibility and water solubility, porosigen/porosigen and porosigen/agent ratios leads to desired release rates.
Controlled release temephos is commercially available as E C O P R O ™ 1707 (Environmental Chemicals Inc., Barrington, Illinois, USA). It has been produced in various dispenser geometries: sinking pellet, suspending strand, suspending chip, bimodal (mud floating) pellet, etc. tailored to the environmental conditions [28, 34, 35]. The formulation shown in Table I has efficaciously released temephos for over 3 years (Table II) in repeat challenge laboratory tests.
228 C A R D A R E L L I and C A R D A R E L L I
Ethylene vinyl acetate co-polymer; manufactured by U.S.I. Chemicals, Inc.
a k 0,0-diethyl-o-(isopropyl-6-methyl-5-pyrinidinyl) phosphorothioate, used as 50% powder;
from Ciba-Geigy Corp.
2-( l-methylethoxy)phenol methylcarbamate, used as 70% active powder; from Mobay
Field evaluations have been extensive [ 3 5 ] and are generally favourable.
Several dispenser types are on the commercial market, e.g. an anchored floating strand for use against catch-basin mosquitoes and an anchored floating chip used by the World Health Organization for mosquito control in potable water reservoirs.
4. INSECT ADULTICIDES
Air release systems have been developed that rely basically on insect contact with the pesticide. In one system the agent, an organophosphate or carbamate insecticide, is coupled with an additive, soy oil or lecithin, that serves both as a volatile porosigen and an attractant in a polymer alloy to ensure adequate free volume . Over 18 months effective control o f cockroaches and ants has been demonstrated, as well as considerable promise f o r use against Silver fish.
Several recipes are shown in Table III. Effective and rapid control o f Blatella germanica (cockroach) and Formica fusca (black ant) for at least 18 months has been noted.
This technique, using either a pheromone or other attractant, should be readily extendible to agricultural usages in crawling or flying insect control.
230 CARDARELLI and CARDARELLI
5.1. Aquatic herbicides Aquatic herbicides keyed to the control o f unwanted plant life in navigable waterways and irrigation systems were first developed in 1969 . It was discovered that the butoxyethanol ester o f 2,4-dichlorophenoxyacetic acid (2,4-D BEE) was soluble in natural rubber and other elastomers. Field and laboratory evaluations showed long-term (in excess o f three years) efficacy . However, processing, and thus manufacturing costs, are high and this material has never achieved commercialization. In follow-up studies it was noted that other esters and amines o f 2,4-D could be used, as well as a number o f other herbicides.
T w o important facets o f this effort lie in demonstration o f the versatility o f use o f polymeric materials, i.e. the wide variation possible in dispenser design, and discovery o f the so-termed 'chronicity phenomenon'.
5.2. Chronicity phenomenon
Conventional pesticide treatment methodology consists o f application o f a relatively large amount of the chemical agent to the target habitat. The quantity used depends on the agent o f choice, the target species and the nature o f the environment to be intoxicated. In use, conventional herbicide dosages vary from less than 1 ppm to over 30 ppm. Normal dosages are 'massive' in the sense that the amount applied is always far in excess o f the amount required if the lethal dose per individual target is multiplied by the number of target organisms to be destroyed.
Of necessity large quantities o f the given pest control agent are used in order to overcome dispersal through water flow, resulting in a rapid drop in concentration, and detoxication processes arising from the chemical reaction with dissolved or suspended matter, hydrolysis, solar radiation, etc. The effective half-life o f a given pesticide molecule in a natural water b o d y may be but a few hours. During this restricted exposure time the target species must absorb a lethal dose. If it were necessary to maintain a high toxicant concentration in the water course treated, say 5 ppm or greater, the volume of controlled release material necessary would be prohibitive. Consequently, one o f the problems addressed early in the development o f controlled release pesticides was to determine the application dosages wherein the target species population is exposed continuously to the agent present rather than periodic exposure as with conventional technology.
It has long been presumed that the Concentration X Time (CT) relationship is a valid method o f determining the pesticide application dosage. That is, if target exposure at 2 ppm for 6'h provides a lethal dose for some segments o f the IAEA-SM-263/38 231 population, then 1 ppm for 12 h or 4 ppm for 3 h are equally effective .
Universal acceptance o f this doctrine is convincing evidence as to at least an approximate reliability. Obviously there are upper limits based on respective kill mechanisms. Snails exposed to 1 ppm copper ion concentration under laboratory conditions succumb within 6 h to acute intoxication. At 100 ppm copper ion concentration the same species, Biomphalaria glabrata, does not die in 0.06 h (3.6 min). Sufficient time is necessary for the movement o f copper ion into the molluscan target, transport through internal tissue and accumulation o f a lethal dosage at sites o f cidal activity.
The critical question as regards controlled release is whether the CT relationship holds at dosage levels practical to the emission o f an agent from a polymeric matrix. If the necessary dosage is 2 ppm for a given plant species to succumb to a given herbicide, and the practical emission rate will provide only 0.01 ppm/d, then 200 d would be required for control. Obviously such a formulation would have severely limited value, if any.
Fortunately the CT relationship is not valid at ultra-low dosage levels.
A number o f laboratory experiments were performed to determine dosage levels necessary to destroy various aquatic weeds through herbicide emission from a polymeric matrix. During the course o f this activity it was noted that the CT presumption was not valid when a given target was exposed to continuous herbicide stress over a long duration o f time at very low concentrations .
Continuous water plant exposure to aquatic herbicide dosages in the 0.1 to
0.001 ppm concentration range, as continuously maintained by release from a dispensing polymer granule, will kill a wide variety o f plants within several weeks [39—41 ]. This phenomenon has been also observed with snail and mosquito species exposed to ultra-low, but continuous, molluscicide and larvicide dosages [4, 25].
The chronicity phenomenon has been observed with the herbicides noted in Table IV and evaluated against the various major aquatic weeds listed in Table V.
Most o f the herbicides evaluated against aquatic weeds are in agricultural use.
The phenomenon is aptly described by the data given in Table VI. This refers to one o f numerous 56-d laboratory evaluations published in full .
Assessment o f the chronic intoxication syndrome has been initiated. The working hypothesis is that the target organism's biochemical response to xenobiotic entry is not triggered in that the ultra-low agent concentrations involved are subthreshold. That is, normal penetration barriers are not erected as the target's defence mechanism fails to detect toxicant ingress. This level varies for different species, but is generally below 0.1 ppm water concentration.
5.3. Controlled release terrestrial herbicides In the summer o f 1980 several controlled release terrestrial herbicides based on a thermoplastic dispensing system were prepared and evaluated .
232 CARDARELLI and CARDARELLI
T A B L E IV. HERBICIDES EXHIBITING THE CHRONICITY PHENOMENON
A small 40' X 200' weed-grown field was plowed and disced to a depth o f approximately 6 in.1 All existing weeds and grass were raked o f f before the field was seeded with common dandelion. Sections o f 30' X 10' were marked o f f.
Each section contained three 10' X 10' plots. One plot was treated at a high dosage, another received a low dosage and the third plot served as the control.
Growth o f the weeds was checked weekly or when weather permitted. During each examination period the number of all weeds (except dandelions) were counted in five 1 ft 2 segments selected randomly. 2 Average weed heights were also noted.
The formulations evaluated are shown in Table VII [43, 4 4 ] and the results in Tables V I I I - X.
Formulations o f bromocil and 2,4-D acid were also evaluated in these preliminary studies. Results indicate the feasibility o f the controlled release methodology against terrestrial weeds, although optimum compounds have yet to be developed. It was observed that the dichlobenil formulations remained active not only during the summer o f 1980 but carried over with similar efficacy throughout the 1981 growing season.
T A B L E VI. LT VALUES F O R Myriophyllum spicatum EXPOSED TO VARIOUS
HERBICIDES A T A CONSTANT DOSAGE(five replicates X three plants per replicate)
Water-soluble salts o f trace metals have been successfully incorporated in thermoplastic matrices and are currently undergoing extensive field evaluation in Australia. Salts and oxides o f zinc, iron, copper, molybdenum, boron, cobalt, manganese, magnesium and selenium have been evaluated . Typical formulations are shown in Table XI. A laboratory evaluation o f several o f the controlled release zinc compounds with soy bean plants is presented in Tables XII and XIII.
The soil content in each growth pot was 1300 g and soil containing less than
0.01 ppm soluble zinc came from the state o f Arizona.
7. IN-FLIGHT MICRO-ENCAPSULATION
A major advance in the micro-encapsulation o f pesticides and pheromones was first announced in 1978 . In previous technology micro-capsules were, and still are, processed within a manufacturing facility and commercially distributed at a given mesh size and agent concentration. It was discovered that co-acervation was possible from an agent/thermoplastic/solvent system during flight . That is, a liquid solution is sprayed by conventional air or ground spray equipment and during flight solvent evaporation initiates co-acervation with a dry micro-capsule impinging on the foliage or soil. By a proper mix o f low and high molecular weight carboxylated polyacrylates it is possible to formulate envelope surface tack sufficient for adhesion to foliage even under adverse climatic conditions.
236 CARDARELLI and CARDARELLI
T A B L E VIII. CONTROLLED RELEASE TERRESTRIAL HERBICIDE D A T A(weeds/ft2 )a
Agents usable in this application consist o f a number o f trace nutrients, especially zinc compounds, insecticides, acaricides, several nematicides, fungicides such as benomyl pheromones, and various terrestrial herbicides. Several recipes are given in Table XIV.
Cross-linking occurs with volatilization o f ammonia and the solvent system.
Once cross-linked the formed micro-capsule is insoluble in water at pH values below
9.5. During flight micro-spheres are formed o f remarkably narrow size range, about 20 to 40 ц т dependent on nozzle parameters, and n o fines are observed, thus dramatically reducing the spray drift problem.
Test results with the first commercialized product, a horse barn spray, have shown an increase in effectiveness from \ d with conventional repellent/insecticides to 19 to 28 d using in-flight encapsulation.
Controlled release pesticides used as anti-foulants, molluscicides and larval insecticides in the public health area, and bait/contact toxicant systems have shown considerable efficacy, as evidenced by the increasing number o f commercialIAEA-SM-263/38 <
Ethylene vinylacetate co-polymer.
a Low density polyethylene (U.S.I. Chemicals Co. code MN 718).
c ^ Porosigen.