Microencapsulated insecticides

Figure 5.4 Principle of insecticide microencapsulation by interfacial polymerization [28].

Microencapsulation with PCL using the solvent evaporation method can be experimentally difficult. For example, PCL was the only polymer of five that failed to yield spherically shaped microcapsules using this technique (82). The insecticide Abate has been incorporated into PCL (21% loading) by the solvent separation method in a comparative study, PCL afforded good-quality microspheres although poly (methyl methacrylate) microcapsules were smoother and had fewer defects (83). 

Dragan D, Carpov A. 1987. Microencapsulated organophosphorous insecticides. III. Some aspects on the volatilization of malathion and disulfoton from aqueous suspensions of microcapsules. Microencapsul 4 97-105. 

Growth regulators -animal [GROWIH REGULATIONS - ] (Vol 12) -encapsulation of [MICROENCAPSULATION] (Vol 16) -as insecticides [INSECT CONTROL TECHNOLOGY] (Vol 14) -plant [GROWIH REGULATIONS - ANIMAL] (Vol 12)... 

Other significant uses of PCBs included heat exchangers and hydraulic fluids. Prior to controls PCBs were also used in adhesives, coatings, plasticizers and inks for microencapsulating dyes for carbonless duplicating paper as extenders in pesticide formulations and catalyst carriers in olefin polymerizations to impart hydrophobicity to materials and surfaces in bactericide formulations (combined with insecticides), and in immersion oil for microscopes. Mixed with chloronaphthalenes, PCBs were also used in wire and cable insulation in the mine and shipbuilding industries (ref. 80, p. 455). 

Uses Methyl parathion is a contact insecticide and acaricide used for the control of boll weevils and many biting or sucking insect pests of agricultural crops. It kills insects by contact, stomach, and respiratory action. The formulations include dusts, emulsifiable concentrate, ULV liquid, microencapsules, and wet-table powders. 

Recent studies Indicate that microencapsulated Insecticides do not represent a unique or special hazard to beneficial Insects, In particular bees. 

The widely used organophosphate Insecticide methyl parathlon was the first material to be formulated as a microencapsulated pesticide. This formulation, sold under the tradename PENNCAP-M Insecticide (a registered trademark of Pennwalt Corporation), consists of nylon-type microcapsules which contain the active Ingredient. The capsules are suspended In water and typically have an average particle size of approximately 25 microns (fifty percent by weight of the capsules have a particle size of 25 microns or more). Upon application by conventional spray equipment the water evaporates, and the active Ingredient Is slowly released over an extended period of time. 

Our first objective was to determine whether microencapsulated methyl parathion Is unique In Its property to be carried back to the hive by bees. To that end a mixture of three commonly used insecticides along with MMP was applied to a plot of blooming rape. The agents were azlnphos-methyl (Guthlon), parathion, and carbaryl (Sevin). By using a mixture on a single plot the effects of variation In bee visitation were eliminated and the tendencies to be carried to the hive could be measured by the relative residue levels in the pollen samples. Five applications were made over a period of seventeen days. Pollen samples were collected from hives placed near the field after two, three, four, and five successive applications approximately two days after each application was made. The application rates were doubled for the last two applications. The data are shown In Table I. 

Another area of concern is the residual life of a pesticide once it is brought back to the hive. Microencapsulated methyl parathion was at one time believed to represent a special hazard because of its controlled release feature. Thus methyl parathion from MMP was reported to persist in stored pollen for up to 17 months.( ) Unfortunately, little is known about the persistence of insecticides in honey bee combs etnd the subsequent effects of their residues on the honey bees. Carbaryl has been shown to persist for at least eight months in colonies ( ) euid permethrln for at least seven months.O) Recently, USDA researchers at the University of Wisconsin studied samples from two bee kills that apparently Involved methomyl and MMP applied to sweet com. Samples were collected to determine, among others, whether methomyl persisted in combs. Analysis demonstrated that eight months after the insecticide application, residues of 0.03 ppm of methyl parathion and 0.03 ppm of methomyl ( 5) remained, even though the latter is considered to be a short-residual pesticide. 

A recent innovation in in-situ microencapsulation is the development of acid-triggered release of pesticide from the microcapsules [12]. Diols and aldehydes are reacted to form an acid labile acetal moiety. The acetal is then reacted with isocyanate to create a prepolymer. The prepolymer is a polyisocyanate cmitaining the acid labile moiety and suitable for in-situ shellwall polymerization. The prepolymer is dissolved into a pesticide, emulsified into water, and shellwall formed in-situ. Under alkaline or neutral pH conditions in a container, the insecticide is safely contained in the microcapsules. Acid could be added to the spray tank to rapidly release capsule contents prior to application. Alternate shellwall chemistry for in-situ microencapsulation utilizes etherified urea-formaldehyde prepolymers in the oil phase that are self-condensed with acid catalyst to produce encapsulating aminoplast shellwalls [13]. This process does not have the problem of continuing CO2 evolution. Water-soluble urea-formaldehyde and melamine-formaldehyde prepolymers can be selected to microencapsulate water or aqueous solutions [14]. 

Another innovation in in-situ microencapsulation is aminoplast shellwalls containing base-cleavable ester moiety [15]. Polyols reacted with diacids that contain thiol or hydroxy functionality produce crosslinking groups. These crosslinking groups along with urea-formaldehyde prepolymer are dissolved into the pesticide and the in-situ microencapsulation process is completed. The resultant microcapsules may contain an insecticide that is safer to handle under acidic conditions yet will rapidly release the insecticide in the alkaline gut of an insect. Alternately, base could be added to the spray tank to rapidly release capsule contents prior to application. 

Formulations which make the insecticide readily available and easy to detach from the surface, such as suspensions, wettable powders and microencapsulated preparations, provide the most active residual deposits, although oil solutions or emulsions are effective and frequently used- Application is usually through standard hydraulic pumps or compression sprayers (knapsack sprayers), where a tank pressure of about 20 psi ( 138 kPa) can be maintained. 

Microencapsulation techniques have been developed to enhance the residual activity of the formula ions by placing a thin chemical shell around the synergized pyrethrins. This allows for a sustained release of the insecticide by diffusion through the shell wall for 30 to 60 days. 

K. Hirech, S. Payan, G. Camelle, L. Brujes, J. Legrand Microencapsulation of an insecticide by interfacial polymerisation ... 

Fig. 6.6-37 Description of the action of a microencapsulated two-effect agrochemical product [6.6.3.1] a) microcapsule surrounded by a disinfectant film, b) disinfectant evaporation, c) diffusion of the insecticide through the wall and dissipation of the active substance...

Microencapsulation by interfacial polycondensation is a usefiil method to microencapsulate a liquid core material. Especially, polyurea and polyurethane microcapsules have been extensively investigated in various industries [209]. For example, aliphatic hexamethylene diisocyanate (HMDI) and aliphatic ethylene diamine (EDA) have been used to prepare polyurea microcapsules containing insecticide called diazinon [210]. A urea linkage is formed immediately by the reaction between an amine and an isocyanate group (see Figure 4.31), and a polyurea is synthesized by the reaction between an amine with two or more amine groups and an isocyanate with two or more isocyanate groups. 

Bingham, G. Gunning, R.V. Gorman, K. Field, L.M. Moores, G.D. Temporal synergism by microencapsulation of piperonyl butoxide and-cypermethrin overcomes insecticide resistance in crop pests. Pest Management Science 63, 276-281, 2007. 

Toxic chemicals such as insecticides may be microencapsulated to reduce the possibility of sensitization of factorial person. 

Fumigants may be acaricidal, nematicidal and/or fungicidal in their action. Some insecticides with relatively high vapour pressure can be applied in soil granule form to act as soil fumigants, for example, dichlorvos and sulphotepp. Microencapsulation with polyureas can be used to prolong their action [35]. 

Microencapsulation. Microencapsulation (qv) has now been commercially practiced for more than 30 years, following the first application of the technology to carbonless copying paper. Pesticide formulations based on microcapsules appeared in 1974 with the product Penncap-M containing the insecticide methyl parathion (4). Since then many microcapsule suspension formulations have been introduced and form the major group of CRF. 

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