Scale insects resistance

The development of resistant strains of an insect to a given insecticide is not new. Melander (7) in 1914 pointed out that the San Jose scale in Washington had developed a resistance to lime-sulfur sprays. Recently Babers (1) of the Bureau of Entomology and Plant Quarantine brought together an excellent evaluation and summary of the literature dealing with the development of insect resistance to insecticides he lists 111 references to work on this phenomenon. 

Starting in 1908 in the orchards of the Pacific northwest, the cases of resistance before World War II and the era of the synthetic organics involved the HCN used against scale insects on citrus, the arsenicals used against orchard caterpillars and cattle ticks, and tartar emetic applied against the tiny insect pests called thrips as now, many instances originated in California (Table I). The resistance mechanisms were investigated in two of... 

While there are some real problems to be solved which involve residues, insect resistance to insecticides is the really big problem. Curiously, the opponents of pesticides fail to measure it in its full dimensions. Resistance of insects to insecticides is a truly growing problem. As long ago as 1908 the repeated use of lime-sulfur sprays in orchards in Clarkson Valley, Washington, selected out a resistant strain of the San Jose scale, which spread and reached southern Illinois orchards in 1920. Then there followed three species of scale insects... 

Forgash (1984) has reviewed the early development of resistance in insects. Resistance toward the older general biocides used as insecticides is not so common, but in addition to the lime sulfur resistance in San Jose scale, we have seen several other examples. 

Figure 1. The parasltold Aphytls melinus parasitizes a California red scale Insect. A. melinus Is being selected for pesticide resistances. (Photo 1 Jack Kelly Clark, U.C. Cooperative Extension.)...

To-date, over 180 wild, tuber-bearing species of Solarium are known (i) and while many of these have been screened for insect resistance over the past ten years, the first large-scale systematic efforts to examine wild Solarium germplasm for insect resistance were initiated by E. B. Radcliffe at the University of Minnesota and reported in a series of papers first appearing in 1968 (. These studies led to the identification of several excellent sources of resistance to aphids and leafhoppers. Prior to this time, very few species were known to resist attack by... 

Refers to any of a class of amorphous solids or semisolids. Natural resins occur as plant exudations (e.g., of pines and firs), and are also obtained from certain scale insects. They are typically yellow to brown in color, tasteless, and translucent or transparent. Oleoresins contain essential oils and are often sticky or plastic other resins are exceedingly hard, brittle, and resistant to most solvents. Resins are used in varnish, shellac, and lacquer and in medicine. Synthetic resins, e.g., bakelite, are widely used in making plastics. Refer to Varnish and Shellac. 

Particle removal by impaction is evident in many common situations one example is the accumulation of dirt on the leading edges of blades of an electric room fan. The dustier the house and the less frequently the fan is cleaned, the more pronounced this buildup will be, up to some maximum thickness where the mechanical strength of the dirt layer becomes insufficient to resist aerodynamic forces. Other examples of impaction on a larger scale are crushed insects on automobile windshields and headlights (see Fig. 4-30) and the accumulation of snow on the upwind side of a tree trunk during a snowstorm. 

The Stepwise Progression of Resistance. Resistance was first noted for insects, when Melander reported in 1914 that San Jose scale on apples in Washington State had become insensitive to lime-sulfur (33). This was greeted with some skepticism, and was followed by scattered reports, limited to insects, over the next 30 years. By 1946, resistance was documented for only 11 species (40). 

Because of these problems, and also the development of DDT-resistant strains of insects, DDT was banned in the United States in 1973 and has since been banned in other countries in the developed world. Alternative insecticides are available, such as organophosphates, but they cost at least three times as much as DDT. The developed world has the luxury of choosing to use these, as it can pay more for the food which they help protect and for a pleasant environment. However, in the Third World the nations resources are so small that even DDT can barely be afforded. It is therefore still used on a large scale in these countries. 

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