Synergized pyrethrins

Bates, Hewlett, and Lloyd (I) found that both piperonyl butoxide and SKF 525A, the ester of 2-diethylaminoethyl 2,2-diphenyl-w-pentanoate, synergized the action of pyrethrins on insects of species of the lesser mealworm beetles and houseflies but both antagonized the action of malathion. SKF 525A is known to increase the effects on mammals of drugs of various types and has been shown to synergize pyrethrins. 

The use of pyrethrins in heavy (nonvolatile) oil films to control moths in stored products was practised prior to the discovery of synergists (Potter, 1938), typically using solutions of 1.3% pyrethrins. These have now been replaced by synergized formulations, which are more cosh effective, e.g. 0,3% pyrethrins + 3.0% PBO 0-25% bioresmethrin + 0.25%. PBO. Applications for this purpose are from mist, fog or UI.V sprayers adjusted to give a coarse droplet size (>80 pm) which provides a surface deposit on stacked commodities when applied at dosage levels equivalent to 4 mg synergized pyrethrins per m. ... 

Dove (1947a, b, 19491 and Dove and MacAllister (1947) introduced the idea that PBO could play a major role in synergizing pyrethrins for the control of stored products insects. This was commercially developed into a combination admixture grain protectant and a warehouse and mill spray in the early 1950s, as reported by Dove (1951. 1952a, b, 1953, 1958). 

The increase in successful treatments with synergized pyrethrins in the USA led lo applications by Bulk Handlers in Australia, and trials in South Africa (Jouhert, 1965 Joubert and DeBoer, 1968 Joubert and du Toil, 1968). The South African work concluded that a water-based spray at a dosage of 1.25 ppm of pyrethrins 12,5 ppm PBO afforded excellent protection to maize against insect infestation for a year. 

In South Africa, Jouborl and du To it (1968) recommended alternating synergized pyrethrins with cheaper insecticides, such as malathion, as a precaution against the selection of insecticide-resistant strains. The South African authorities. predicting the onset of resistance to malathion, insisted that synergized... 

A mixture of malathion plus synergized pyrethrins was introduced into New South Wales, Australia. Its efficacy was studied in commercial stores by Des-marchelier et at. (1979). who found that treatment of stores in the South Dubbo region in 1977, with 18 gl malathion plus 3gl of synergized pyrelhrins resulted in 255 infestations (1 live insect or more) of R dominie a and T. iasiu-neum. of which only 17 wrere heavy (more than I insect per 5 kg). This compared favourably with the 614 infestations, of which 34 were heavy, reported for the same stores in 1976. when 18 g t 1 malathion alone was used. 

Greening (1980) also compared the efficacy of synergized pyrethrins with and without the addition of malathion when applied to newly harvested grain during 1976. The results after 17 months on-farm storage in New South Wales revealed that both wheat and barley were completely protected by 3 ppm pyrethrins 27 ppm PBO, The addition of malathion was deemed u n necessary. 

Products based on synergized pyrethrins offer the physician, fanner, pet owner and veterinarian safe and effective control of insect pests without the inclusion of toxic additives, and without restrictions on repeat treatments. Since PBO is not a cholinesterase inhibitor, it may be used in conjunction with flea or tick dips, collars and oral medications. 

Head louse infestations periodically cause disruptions at schools aii over the world. The treatment with pyrethrins and PBO is relatively cheap, and effective. The most common formulations contain either 0.15% or 0,3% pyrethrins synergized with 3.0% PBO formulated into non foaming shampoos (water- based pm-sized emulsions) that are applied directly to the affected area and repeated after i week to kill any newly hatched lice. The pediculicidal and ovicidal efficacy of synergized pyrethrins arc enhanced in shampoo formulations as a result of the surface-tension-lowering action of the formulation and are preferable to direct applications of lotions. 

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. 

Commercial formulations using these techniques have extended the residual activity of synergized pyrethrins to German cockroaches under field conditions to 1 month (Bennet and Rea. 1978). This feature, coupled with low mammalian... 

Tests in 1984 with synergized pvrethrins in a microencapsulated polymer system in an isopropauol-bused spray demonstrated the benefits of the system in prolonging the residual effects of synergized pyrethrins and resulted in the development of a successful commercial product (MacDonald and Miller, 198b). 

---