Control of Pests by Mineral Oil
Mineral oils sprayed onto citrus trees can control a wide range of pests, including mites, aphids, psyllids, leafminers and scale. The control effect of the oil is mainly by suffocation, when the oil moves into the spiracles (breathing holes) of the insects. Recently, it has been shown that the oil also controls insects by modifying their behavior. For example, female leafminers do not lay eggs on leaves where there are oil deposits. Mineral oil may also reduce the level of transmission of virus diseases by aphids.
Advatanges of Oil Spray
Oils have several advantages compared to chemical insecticides. Their main benefit is that they are broad-spectrum. They control a wide range of pests, as well as fungus diseases such as black spot and greasy spot. They are even effective against the eggs of aphids, mites and some moths.
Furthermore, they are easy to handle and relatively safe to the grower applying them. They dissipate quickly after spraying. Mineral oils also kill any algae growing on the trees and fruit, leaving them clean. This improves the appearance, and thus the marketability, of the fruit.
Drawbacks of Oil Spray
The main disadvantage of mineral oil is that it can injure the tree (phytotoxicity). Growers have to balance the desired control effect against the risk of harming the tree. In recent years, new oils have been developed which have a smaller risk of damage to the tree. It has also been realized that reducing the concentration of the oil in the spray from 2% to 1% means there is less risk of damage the tree, but the oil is just as effective.
Another problem is that the oil kills by contact. It has to be sprayed directly onto the eggs or bodies of insects. This means that applications have to be thorough and timely. Insects migrating onto a tree recently sprayed with mineral oil are not usually harmed by any oil residues.
Use of Mineral Oils in Tropical Asia
Heavier oils seem to be more effective than light ones under tropical conditions. Recently, experiments in Sarawak have found that more frequent sprays using an even lower concentration of mineral oil (0.5%) did not cause any phytotoxicity in the trees. Since the climate is warm, the growth rate of the tree is faster and the oil quickly disappears after spraying.
See this article on Easy Do It Yourself Pest Control Tips