This disclosure relates to an interfacial encapsulation process for making encapsulated beads particularly suited to enclose a fumigant effective against selected insect species. A typical insect species of concern is the fire ant. Fire ants are difficult to kill with ingested poisons. While ingested poisons may thoroughly decimate foraging worker ants, the use of a chain of tasters in the fire ant colony prevents ingested poison from reaching the queen of the colony, thereby protecting the colony. A particularly valuable volatile insecticidally effective poison includes volatile phosphoric or thiophosphoric acid esters. In addition to operating by ingestion, they provide a fumigant which is airborne, by-passing the tasters. Thus, if the bait is carried into the colony and opened in that closed environment, there is a much greater possibility that the bait will be effective to kill the queen of the colony. The fumigant is is commonly known as DDVP, a fumigant including as one ingredient dimethyl-2, 2-dichlorovinyl phosphate acid ester. There are other fumigant phosphoric or thiophosphoric acid esters which will be collectively referred to also as DDVP. As an example, several such insecticidally effective compounds or mixtures thereof are set forth in U.S. Pat. No. 4,094,970.
DDVP produces a toxic atmosphere for the ants in the colony particularly if the bait can be carried into the colony by foraging ants and is opened in that closed atmosphere. The ants are enticed to open it by incorporation of an attractant food comprising the shell. One suitable attractant food is soybean protein. Hence, the present apparatus contemplates fabrication of a small bead by means of microencapsulation wherein the shell is fabricated with a small measure of soybean protein, and the core is an edible mixed with DDVP. The amount of DDVP is sufficient to be effective in the atmosphere; that is, once the bait has been broken open, DDVP volatilizes sufficiently to have the fatal impact required for colony decimation.
The bead shell is an important factor in preparing the bead of bait material for eventual extermination of a fire ant colony. On the one hand, it should be frangible and broken easily by the insect so that the insect can readily bite through the surrounding shell. On the other hand, the shell must be impervious to water from the exterior, impervious to the core on the interior to prevent DDVP from weeping or saturating the shell material whereby premature fumigation might occur. If this were to happen, premature toxicant exposure might well warn away the foraging ants. Moreover, toxicant exposure might well poison foraging ants before they have the opportunity to deliver the bead into the colony. Thus, it is desirable that the ant finding the bead remove it as a result of foraging; this is a delicate balance wherein the bead must be tough enough to be handled, delivered by machine, and yet should be sufficiently easily opened by insect bite. Moreover, it should be sufficiently free of DDVP which might possibly weep through the wall of the shell and thereby defeat the highly desirable fumigant procedure for insect eradication.
One prior art structure is found in U.S. Pat. No. 4,094,970 setting forth a polyurethane system. Additional references are U.S. Pat. Nos. 3,492,380; 3,575,882; 3,270,100 and 3,577,515. By and large, they generally refer to solid carrier pelletization processes. It is submitted, however, that a quality insecticidal bait must have a toxicant which is enclosed within the bead formed by interfacial encapsulation and there should be an attractant food dissolved in the shell. That is, the attractant food material must in some fashion be in the shell to attract foraging insects. Otherwise, the foraging insects will have no interest in the bead and will ignore it.
The core in the bead is made with DDVP as the fumigant mixed with soybean oil. Separately, the shell ultimately formed should have an attractant such as soybean protein or oil in or on the shell to serve as an attractant. It must either taste or smell good to the foraging insect, sufficiently to cause the insect to carry the bead back to the colony or hive. In this light, it will be understood that the completed bead has the secure, impervious surrounding shell which encapsulates the DDVP. Fumigation does not start until the shell is actually broken open. In the meanwhile, the shell is sufficiently attractive to the foraging insect that it will be carried back to the hive or colony.
Interfacial polycondensation encapsulation is one procedure which enables this to be accomplished. As an example, an interfacial liquid body is defined as having two portions, one being an oil phase which forms droplets in water. The oil phase is added to the water to react with the oil phase at the droplet interface. The polycondensate system is defined by the two parts, one part being the oil phase and the other being the water phase. This method brings the two parts together, thereby achieving bead formation at the droplet interfacial area. With controlled stirring and the addition of either a surfactant or anti-foaming agent, bead size is controlled to form coated beads which are recovered and washed. By this procedure, beads of a selected size are formed and are recovered, washed and thereafter used. The interfacial polycondensate system obtains production of beads in the range of about 1.0 mm or smaller wherein the surrounding shell is impervious to DDVP enclosed therein.