The present invention relates to biological insecticides and, more particularly, to novel microencapsulated insecticidal pathogens for application to vegetation.
Concern over the toxic side effects of chemical insecticides on man as well as on the environment has led to the development of biological insecticides and, more particularly, insecticidal bacteria, viruses and fungi. Such pathogens, while deadly if ingested by the target insects, are generally harmless to non-target insects as well as to man and other forms of wildlife. Thus, viruses are known which are effective against the bullworm Heliothis zea and the tobacco bullworm H. virescens, the gypsy moth Lymantria dispar, the Douglas fir tossock moth Orgia pseudotsuoata, the European pine saw fly Neodiorion sertifer and both alfalfa and cabbage loopers Autographa californica.
Although viruses such as those described above as well as certain bacteria and fungi are highly effective against their target insects, their effectiveness is generally short-lived. More specifically, ultraviolet rays from the sun decrease the potency of the biological preparations thus necessitating virtually daily applications of the preparations to susceptible vegetation. Quite clearly, this need to repeatedly apply the insecticidal preparations substantially diminishes their cost effectiveness vis-a-vis conventional chemical insecticides which do not require so many repeated applications.
To prevent ultraviolet deactivation of insecticidal viruses, for example, formulations have been prepared containing both virus and a sunscreening agent. The presence of sunscreening agents does retard viral deactivation under laboratory conditions. However, results have been less than satisfactory when virus/sunscreening agent formulations were tested in the field. More specifically, when formulations containing both virus and sunscreening agent were dispersed in the field, the sunscreening agent was no longer in close enough contact to the virus to be effective at reducing the exposure of the viruses to the damaging effects of ultraviolet light.
Attempts have been made to provide formulations wherein an insecticidal virus and a sunscreening agent are maintained in close contact after dispersion of the formulation onto vegetation. Thus, C. M. Ignaffo and O. F. Batzer described "Microencapsulation and Ultraviolet Protectants to Increase Sunlight Stability of an Insect Virus" in the Journal of Economic Entomology, Volume 64, Number 4, pp. 850-853, August, 1971. Ignaffo et al attempted to use microencapsulation, as one of several ways, to increase the sunlight stability of the polyhedral inclusion bodies of the Heliothis nucleopolyhedrosis virus. Thus, samples of virus alone or virus and sunlight protectants were prepared for encapsulation. Among the sunlight protectants were Buffalo Black, Carbo-Jet Black, cellulose, carbon, aluminum powder and aluminum oxide. Among the microencapsulating walls were ethylcellulose and gelatin. Microcapsules containing virus and sunlight protectant were found to be more stable than virus alone. However, nonencapsulated mixtures of virus and sunlight protectant were found to be equally as stable as the corresponding microencapsulated mixtures.
Since only laboratory tests are discussed by Ignoffo et al, it is not surprising that their conclusion was that nonencapsulated mixtures of virus and sunscreening agent were as stable as the corresponding microencapsulated mixtures. Nonetheless, it is quite clear that the microencapsulated formulations of Ignoffo et al are not suitable for application to fields. In the first place, encapsulating materials such as the ethylcellulose employed by Ignaffo et al are not sticky and thus, will readily fall off the leaves of the plants which they are supposed to protect from insecticidal damage. Another problem is that the encapsulating materials are not environmentally stable. For example, the gelatin coating used is readily broken down by the environment. Finally, some of the sunscreening agents, e.g., the carbon black, are not desirable in terms of their appearance.
Attempts have also been made to provide formulations wherein an insecticidal bacteria is microencapsulated. Thus, E. S. Raun and R. D. Jackson in "Encapsulation as a Technique for Formulating Microbial and Chemical Insecticides" in the Journal of Economic Entomology, Volume 59, Number 3, pp. 620-622 (1966) describe encasulation of the bacterium Bacillus thuringiensis which is a pathogen of the European corn borer. The encapsulated bacteria were found to be as effective as standard clay granular formulations. The specific encapsulating agents are not discussed nor is the presence of a sunscreening agent disclosed. Accordingly, such formulation could be expected to suffer from a number of the disadvantages described above, i.e., ultraviolet breakdown of the pathogen, lack of stability in the field, inability to adhere to a treated plant and inability of the capsule to be broken down in the stomach of an insect.
More recently, Fogle et al in U.S. Pat. No. 3,541,203 developed a protected virus composition for insect control. The composition includes (1) a virus to be used as an insecticide, (2) protecting materials which aid in prevention of the deactivation of the viruses and which prolong the effectiveness of the virus, and (3) preferably a polymeric material to bind the light absorbing compound and the virus particles together as minute particles.
To maintain the virus composition, once applied, in the desired location on the leaves of plants, the sprayable composition includes, along with minute particles of the virus and light absorbing material in a binder, a liquid carrier vehicle which has dissolved therein a "sticker material". Where the polymeric binder material is substantially water-insoluble, the liquid vehicle is water or an aqueous solution and the sticker material is a water-soluble polymeric material such as methyl cellulose or polyvinyl alcohol. In another embodiment wherein the polymeric material is slightly water-soluble or water swellable, an aqueous vehicle need not include a sticker material because the polymeric material is slightly swollen by the aqueous vehicle and serves as its own sticker.
One suitable polymeric binding material is cellulose acetate phthalate which becomes soluble in water at a pH of about 7 and might more easily release the virus for use in infecting insects after being ingested by the insect. More specifically, cellulose acetate phthalate is believed to operate in an enteric fashion in the digestive tract of insect larvae wherein material in the digestive tract is alkaline.
To bind the admixture of sunscreening agent and virus together with an ethylcellulose polymeric material, Fogle et al combined the admixture with 2% ethylcellulose in toluene. While agitating the mixture, polybutadiene was added and the mixture poured into a vessel containing a petroleum distillate which caused the ethylcellulose to solidify to yield very small particles of ethylcellulose polymeric material having substantially homogeneously enclosed within the particles the admixture of carbon and polyhedrosis virus. It was then necessary to wash the capsules with additional petroleum distillate to completely remove residual amounts of the liquid polybutadiene material.
The microencapsulating process as well as the microcapsules obtained by Fogle et al suffer from a number of disadvantages. More specifically, the polymers forming the walls of the capsules are not always capable of retaining the sunscreening agent within the interior of the capsule. This of course diminishes the stability of the insecticidal preparation since the loss of sunscreening agent makes the pathogen more susceptible to the damaging effects of ultraviolet light. Another problem with the microcapsules of Fogle et al is that highly toxic materials are employed in their preparation and cumbersome washing steps are required for their removal.