Horticulture Mineral Oils (HMOs) – Benefits, when & how to use

Horticultural Mineral Oils have become popular as a way to control pest problems on fruit trees in Kashmir valley. HMOs are highly refined oils that are known by many names, including, spray oils, dormant oils, supreme oils, superior oils, or white mineral oils. HMOs are mainly sprayed to kill San Jose Scale and European Red Mite insects, but it also helps to keep other insects such as aphids, thrips and mealybugs at bay to a large extent. Dormant oil sprays are also effectively used to combat the over-wintering stages of the pests. They can be used in the fall and winter after leaf drop or in the spring while the branches are still bare before they experience any spring growth.

MODE OF ACTION

The primary mode of action of horticultural mineral oils is to kills insects by suffocating them in absence of oxygen. The oil blocks the air holes (spiracles) through which insects breathe, causing them to die from asphyxiation. Mineral oil creates a thin layer around the surface of eggs of insects where it stops the gaseous exchanges and disrupts the metabolism of eggs. In some cases, oils also act as poisons and interfere with normal metabolism of the insect. HMOs may also cause hardness in insect eggs which make it difficult for the eggs to hatch. Mineral oils form a layer on the plant parts and prevent the settlement of the newly hatched insects. HMOs also dissolve the external waxy layer on the insect body causing dehydration. HMOs also disrupt the ability of some insects to feed, thus starving them to death.

Benefits of using Horticultural Mineral Oils:

1. Safe on other environmental elements and negligible toxicity to humans and other vertebrates.
2. Friendly to natural enemies of pests (beneficial insects)
3. Reduce the harmful effect on the environment.
4. Easy and safe to use.
5. Less in costs when compared with the traditional pesticide chemicals
6. No record of the development of pest resistance. Since the mode of action is mechanical rather than chemical, there is less likelihood of insects developing resistance to the oils.

WHEN SHOULD DORMANT OILS BE APPLIED?

Proper timing of application is critical for the success of dormant oils. The window for application extends from bud swell to when leaves just start emerging. Dormant oils should be applied when a plant is dormant i.e. before leaves or flowers show signs of breaking dormancy; that is, before bud break. A common mistake is to apply dormant oil sprays too early (on the first warm day in February) before insects are actively respiring and susceptible to the oil’s suffocating effects. It is better to wait as close to the bud break as possible before applying oil sprays. For summer use, oils are effective against insects that are soft-bodied and slow-moving. Oils will not control late instar immature or adult stages. Effective scouting to discover pest populations in the early instar stage will be necessary for effective control of pests.

HMOs should be applied when a plant is dormant. They can be used in the fall and winter after leaf drop or in the spring before flower buds begin to open. Since recommended application rates and temperature ranges differ between dormant oils, it is imperative to read each product’s label and follow directions carefully. Failing to do so could result in plant leaves burning or essentially suffocating a plant. A good rule of thumb is to avoid spraying on sunny days (even during cooler weather) and avoid spraying when temperatures are freezing or close to freezing. The spray is not effective in low temperatures and coverage will also be uneven. Also, avoid applying oils when severe freezing trends are expected within the following 3 to 4 days. Apply in the early morning or late afternoon. The ideal temperature range for application is between 4 and 21°C, with the day of application expected to stay above 10°C for at least 24 hours. Spraying on drought-stressed plants should be avoided as drought-stressed plants are more susceptible to oil damage. Conversely, applying during very humid conditions reduces the rate of evaporation and can also cause burning. Apply when rain is not predicted for the next 24 hours. Do not apply if a sulfur-based pest control product has been applied within the previous 30 days as the oil and sulfur combination can be toxic to plants.

PRECAUTIONS WHILE APPLICATION

Some precautions must be followed while using HMOs in controlling different pests.

1. Always read and follow label instructions.
2. Cover all plant surfaces, especially the undersides of leaves and crevices of branches and stems where pests can hide.
3. To minimize the risk of plant injury, avoid application when temperatures are below 4°C or above 30°C degrees or when the relative humidity is above 90 percent.
4. Not all pests are susceptible to oils. Proper pest identification is critical before going for oil application.
5. Application of oils should never be done in combination with sulfur or sulfur-containing pesticides such as Captan or Karathane. They can react with oils to form phytotoxic compounds. Because elemental sulfur can persist for long periods, label directions on most oils prohibit their use within 30 days of a sulfur application.
6. Apply dormant oil sprays only after winter hardening.
7. Do not spray when shoots are growing. Dormant oils should not be used when the tree is in flower, since they can be deadly to pollinators.
8. Avoid mixing insecticide with the HMO.
9. Avoid spraying a huge concentration of oil, which is against horticultural scientific standards. Excessive concentration of HMO is sheer wastage and also harms the plant.
10. Oils must be mixed exactly at the right dilution rate to prevent plant damage. This is especially true for summer use of the oils.

Sprays based on the time of application

Sprays can be broadly classified into two types based on seasonal timing of the application:

• Delayed Dormant Spray: HMOs are used during delayed dormant season. The application of oils during delayed dormancy is directed primarily at killing overwintering adult pests and insect eggs before they can become active in the spring and cause plant injury. Besides being one of the most effective options, spraying oils during the dormant season is the least disruptive to beneficial insects and the environment.
• Summer Spray: Summer oils are applied when plants are in active stage (growing season) during summers at a lower concentration than dormant oils. The application is done when foliage is present on trees and insect pests are also active in the orchard.