New research has shown that an old method controls insects in the garden without using poisonous sprays that may be hazardous to the gardner and the family.
Researchers at the University of California, Berkeley, found that soap and water or detergent and water sprays can keep pest insects at nondamaging levels. In the home garden where 100 percent insect control is seldom necessary, these sprays were found to be effective against arthropods (includes insects, crustaceans, arachnids and myriapods).
The research was done by Wayne S. Moore, staff research associate, Carlson S. Koehler, urban pest management specialist, and Joseph C. Profita, laboratory assistant, all of Cooperative Extension Service.
The utility of soap sprays for insect control was demonstrated as early as 1842, according to their report of the research. During the latter half of the 19th and early 20th centuries, whale-oil soaps, and more commonly fish-oil soaps, were an important part of insect control terminology.
As the more effective synthetic organic insectcides were developed, research on soap as an insecticide was largely discontinued; but soap continued to be used, principally by home gardners and in other small-scale situations.
Today there is renewed interest in soap sprays, the report says, yet directions for use are often vague and confusing, and the literature is unclear as to how effective soaps are. To obtain a more accurate estimate of the potential for soap spray on ornamental plants. Berkeley conducted a series of replicated trials in 1978.
Each trial included a pesticide known to be effective against the pest, and an untreated check. The soap and detergent solutions were Ivory Liquid dishwashing detergent.Acco Highway Plant Spray soap (containing by volume 38.5 percent coconut oil soap, 1.1 percent lanolin, and 0.3 percnet EDTA), Shaklee's Basic H, Fels Naptha laundry bar soap, and Tide detergent. The pests and plants sprayed were a mixed aphid population on tobira, acacia psyllid, Psylla uncatoides; on Sydney golden wattle, Acacia longifolia, and citrus red mite, Panonychus citri; on Mexican orange; Choisya ternata.
Treatments were applied as coarse sprays until runoff with polyethelene plant misters. Soap sprays are known primarily for their ability to physically dislodge insects from plants when applied at high pressure. In addition, other researchers have suggested that insects might be asphyxiated by soap sprays.
The aphid population crashed several days after treatment, but in the other trials, counts were made at both one and seven days after treatment.
The soap and detergent solutions achieved a substantial reduction of the three insects tested. Some of the solutions provided a level of control similar to that of the pesticide standard.
In general, the more concentrated solutions provided more satisfactory control than those less concentrated.
Some soaps and detergents tested are not reported here. Over-all, the soaps were not more effective than the detergents. Solutions derived from dry formulations performed well, but they were not as easy to use as liquid soaps and detergents. Bar soaps required chipping and boiling to make a solution, and some jelled when left to cool overnight.
Another problem was encountered when solutions were prepared from bar shaving soap.Though it was fairly effective, excessive lather clogged spray nozzles and made it difficult to apply.
More concentrated solutions provided more effective control but increased the potential for plant damage. Leaf burning, along the margins or in patches, was the most common phytotoxicity sympton. Some solutions left unsightly white residues on certain plants. The dry formulations left more unsightly deposits.
Moreover, it has not yet been determined whether there is a harmful buildup of soap and detergent materials in the soil when repeated applications are made over a long period.
In a total of eight experiments conducted during 1978, the soap and detergent sprays produced high mortality of all arthropods tested except spittlebugs. The mites, aphids, psyllids and thrips used in these experiments are all soft-bodied, sucking arthropods, the kind known to be particularly susceptible to soap sprays. Soap sprays also had been advocated, before the advent of synthetic organic insecticides, for control of whiteflies and scale insects.
The effects of soap and detergent sprays on beneficial insects have not been adequately studied. Observations indicate adult snake flies and convergent ladybird beetles die when contacted with the sprays. However, it is thought that these sprays are less damaging to natural enemy populations than are most synthetic organic insecticides.
Because these sprays do not have the residual properties of synthetic organic insecticides, and do not provide the same consistently high level of control, repeated applications, thoroughly applied, are indicated. In addition, phototoxicity may reduce their applicability on certain plants.