Soap-Detergent Sprays: Now days, soap-detergent sprays are available in the market that are designed for the control of plant pests. These sprays have found to be highly effective against different small soft-bodied arthropods, like aphids, young scales, whiteflies, psyllids, mealybugs, and spider mites. Large insects, like - caterpillars, sawflies, and beetle larvae are normally immune to soap sprays. However, some large insects like boxelder bugs and Japanese beetles, are vulnerable.
With their minimal adverse effects and reactions on other organisms, soaps are considered as selective insecticides.
Pesticidal soaps are soaps, which are specially manufactured to improve specific pesticidal or pest-deterring characteristics, while remaining relatively benign otherwise. The lipophyllic ends of these soap molecules penetrate the insect cell membranes. Short-chain fatty acid soaps are used as moss-killers and in general use contact herbicide products. Soaps based on long chain fatty acids have insecticidal properties and are made using natural products, such as coconut oil, which is processed to concentrate certain chain lengths of the fatty acids. The toxicity of fatty acids to insects and fungi increases with increase in chain length to a length of 10 carbons and then decreases. Unsaturated fatty acids with 18 carbons have also found to be superior in insecticidal properties. Ammonium soaps have known to be effective as wildlife deterrents while sulfur soaps as fungicides.
The same properties, which give pesticidal properties to soap, can also make them irritating to handle and damaging to plants.
How soaps and detergents kill insects
Not more people properly understand how soaps and detergents skill insects. In most of the cases, control is achieved as a result of the disruption of the cell membranes of the insect. The application of soaps and detergents may also damage remove the protective waxes, which cover the insect, thereby causing death through excess loss of water.
Soap and detergents function stringently as contact insecticides, without any residual effect. Therefore for effective action of soaps and detergents they should be applied directly to, and completely cover and wet the insect.
According to tests, soaps and detergents can be used to control a variety of insects and plant pests. Small and soft-bodied arthropods, like aphids, mealybugs, psyllids and spider mites are most sensitive to soap. Some characteristics of soaps that make it highly appealing for insect control include -
- Ease of use
- Less expensive
- Selective action
- Need to wet the insect during application
- Absence of any residual effectiveness
- Potential to damage some plants
Some household soaps and detergents can also be effectively used as insecticides. Particularly, some brands of hand soaps and liquid dishwashing detergents have proved to be highly effective for this purpose. Although, these products are less expensive, there is an increased risk of plant damage with these products. All clothes-washing detergents and dry dish soaps should not be used on plants, as they are very harsh. Some soaps and detergents are poor insecticides and so it is essential to identify safe and effective combination of soaps and detergents for insect control requirements. Irrespective of usage of the soaps and detergents for insect control requirements, they should always be applied diluted with water and generally at a concentration of around 2 to 3%.
One of the most serious drawbacks of using soap and detergent sprays as insecticides is their potential to cause plant injury, known as phytotoxicity. Some plants are susceptible to these sprays and therefore may be seriously damaged. For example, plants such as hawthorn, sweet pea, cherries and plum are sensitive to soaps. Portulaca and certain varieties of tomatoes are sometimes injured by insecticidal soaps. The risk of plant damage increases with the use of homemade preparations of soaps or detergents. If the homemade preparations of soap and detergents are required to be used, it is advised to test the sprays for phytotoxicity problems on a small part of plant a day or two before treating an extensive area of the plant.
Plant injury can be reduced by diluting the sprays, more than the 2 to 3 percent, as suggested on label instructions. To reduce leaf injury, plants should be washed within a couple of hours after applying the spray. Limiting the number of applications can also reduce the leaf damage as the damage can be accumulated with repeated application. However, due to the short residual action, repeat applications may be required at relatively small intervals (every four to seven days) to control certain pests, like spider mites and scale crawlers. Also, the application should be thorough and entirely wet the pest. This generally means applying the spray to the undersides of leaves and other protected areas. Insects such as aphids within curled leaves cannot be completely wetted and hence cannot be controlled.
Environmental factors may also affect the usage and effectiveness of soaps. In general, soaps (but not synthetic detergents) are affected by the presence of hard water minerals that may result in chemical changes thereby producing insoluble soaps (soap scum). Insect control decreases, if hard-water sources are used. The quality and effect of application can be improved by applying the spray early or late in the day, i.e. at a time when the drying is not overly rapid.