Physical Methods of Pest Control in Stevia Farming

Insect infestation in Stevia may cause significant damage to the crop both in greenhouse and field stage. Thus, pest control becomes is one of the most important part for overall agronomic management of the crop. The chemical control methods for most common insect pests have already been described elsewhere in this website. The chemical control methods should be adopted only as the last resort and methods which are more environment friendly with minimal adverse effect on human health should be adopted preferentially as profilactic measure. 

 

Physical insect control has been used successfully in Stevia and several other crops. Though the initial investment is higher in comparison to other pest management systems, it environment friendliness, minimal health risk and compatibility with organic farming often make them a viable option. Let us discuss some physical methods which have been used in Stevia. 

Methods:

 

The widely accepted methods for physical insect pest control are as follows –

 

  1. Barrier method

  2. Insect traps

  3. Electronic insect repellant

 

Let us now have a closer look at all the systems.

Barrier Methods

Kaolin Spray

 

In this method, a physical barrier is created between the plant surface and attacking insects. A natural mineral, kaolin is often used for this purpose. Kaolin-based sprays have been studied extensively since 1999 and research has established that these sprays deposit a "particle film" that has numerous beneficial effects on plants and in insect pest control.

 

Kaolin mixed with spreaders and stickers and applied to plants as a spray at 1–6% concentration in water form has been shown to be an effective approach to agricultural pest control and to protect plants from environmental stresses.

 

Kaolin acts as a physical barrier preventing insects from reaching vulnerable plant tissue. It acts as a repellent by creating an unsuitable surface for feeding or egg-laying. The uniform white film may also disrupt the insect’s host finding capability by masking the color of the plant tissue. Furthermore, particles of kaolin act as an irritant to the insect. After landing on a treated surface, particles of kaolin break off and attach to the insect’s body triggering an excessive grooming response that distracts the pest.

 

Kaolin-based sprays can also form a highly reflective white film over plant surfaces that is known to enhance plant photosynthesis and reduce heat stress in plants by reflecting the infra-red light spectrum. Kaolin is generally regarded as safe to humans and has long history of use in the paint, plastics, pharmaceutical and paper industry. Kaolin sprays are used for pest control and sunburn protection in both conventional and organic food production in the United States and abroad.

 

Kaolin clay is available as a wettable powder to be mixed with water. Application can be done with most commercially available spray equipment but large amounts of water are required. To prevent caking, it is suggested that the material be added while mechanical agitation is running, or to first completely mix the required amount in a small amount of water before filling up the tank to the recommended volume. It may be tank mixed with soaps, and most pesticides, but not copper, sulfur, or Bordeaux mixtures. Precipitation, curdling, uneven film formation or changes in viscosity are signs of incompatibility. Periodic shaking is recommended for a backpack sprayer or use of an automatic agitation mechanism for larger equipment in order to keep the material suspended in water. Efficacy is only successfully achieved with thorough coverage. Care should be taken to cover the entire surface of the crop.

A brand of kaolin spray

Kaolin treated plants

Kaolin treated plants

Silver Reflective Mulch

 

Plastic mulch with metalized silver sunlight reflecting upper surfaces has shown considerable insect repellant activity.

 

Silver mulches reflect sunlight up into the undersides of the canopy thereby increasing photosynthesis. The reflective surface of the silver mulch produces an increased light intensity that tends to repel whiteflies, aphids and other bottom leaf dwelling insects. Generally, these insects harbor at the dark underside of the leaves. The reflection of sunlight from the mulch illuminates the underside of the leaves and affects the phototaxis of the insects. The insects are thus repelled from the lower leaf surfaces. High light intensity at the lower surface of the leaves also interferes with the metamorphosis of some insect pests and disturbs their life cycle.

 

Other benefits of silver mulch include weed control and decreased water usage. It also provides a physical barrier against ground pests.

Reflective Much Roll

Stevia with reflective mulch

Insect Traps

Several types of insect traps can be used in Stevia plantations and greenhouses. Their mode of action and efficacy varies widely. Some of the trapping systems are described below.

Yellow Sticky Traps

 

A number of insects are found to be attracted to objects of specific colours. Aphids, whiteflies, scales, and many other insects initially locate plants on which they feed by using visual cues such as colour. However they do not see the colours reflected from the plants in the same way we do. Insects are able to distinguish among the various colours-of light that are reflected from the surface of plants. Therefore, instead of seeing leaves as a green colour, they see varying hues of yellow and blue that are reflected from the leaf surface. The colour of reflected light that we perceive as yellow (wavelength 500-600 nm) is a major component of the light reflected from plant surfaces, and a greater amount is reflected from newer growth than older growth. Most aphids, whiteflies and scales prefer the newer growth and fly towards objects reflecting large amounts of light at the proper wavelength. They will be attracted to any object strongly reflecting light in the 500-600 nm range. This strong response to yellow coloured objects can be used to help control the numbers of aphids and whiteflies in the plantation and green house.

 

Yellow sticky traps are plastic or cardboard strips with bright yellow colour and coated with strong non-drying glues. They are hung in different locations in greenhouse and nurseries. Airborne insects are attracted to the colour and whenever the land on the strips, they get trapped by the glue. Originally the sticky traps were used as a sampling tool to study insect population in certain areas, but their application area has been broadened as a pest control/management tool. In green house, one standard sticky trap per 20 square feet (approximately 2 sq. m) can reduce the white fly population be 90%.

 

The sticky traps can also be made at home with yellow paper (post it notes may work) and some common kitchen supplies. An acceptable quality non-drying glue can be made by boiling corn starch in water along with corn syrup, water and white vinegar. Otherwise, any commercial pressure sensitive adhesives may work. A lot of resources for “do it yourself” yellow sticky traps are available over the internet.

Colour of yellow sticky traps

Application in green house

Yellow pan trap

The sticky traps can also be made at home with yellow paper (post it notes may work) and some common kitchen supplies. An acceptable quality non-drying glue can be made by boiling corn starch in water along with corn syrup, water and white vinegar. Otherwise, any commercial pressure sensitive adhesives may work. A lot of resources for “do it yourself” yellow sticky traps are available over the internet.

 

The affinity of white flies and aphids towards yellow colour is also utilized in yellow pan traps. In this system, a shallow plastic pan of proper yellow colour is filled with water up to a depth of 1- 2 inch (25-50 mm). A little detergent is added in the water to break the surface tension. Insects attracted to this pan for its colour  fall in the water and gets drowned.

 

 

Sometimes, insect pheromones are used in these sticky traps to make them more effective. For some insects like grass hoppers, blue is more favoured colour, and blue sticky traps are used to control them.

Light Traps

 

Light trap is one of the very effective tools of insect pest management in organic agriculture as it mass-traps both the sexes of insect pests and also substantially reduces the carryover pest population.


A light trap consists of three major components

 

  1. Light source

  2. A funnel and container for collecting the insects

  3. A fan to create air blast for arresting the insects and guiding them to the trap container

  4. A quantity of killing/drowning liquid – often dilute detergent solution

Light source

 

Long ago, paraffin wick lamps were used as light source for these types of traps. Later, as the lighting technology progressed, incandescent electric lamps and then fluorescent lamps and mercury vapor lamps are used. At present, energy saving LED lamps are most prevalent in these systems. A lot of insects has affinity towards ultraviolet (UV) radiation, and thus, UV lamps are also used extensively in light traps.

While the light source attracts the insects, the traps need another device to kill or capture them. Often this is a container with water that is placed under the light source. The insects that circle around the light will drop in the water and get drowned. It is recommended to add a bit of soap to the water. Instead of water, it is also possible to use sticky plates.

Percentage of insects attracted at different colored light during night hours in the field

RED

2.20%

YELLOW

10.60%

GREEN

4.70%

WHITE

18.00%

UV

42.10%

BLUE

22.40%

Light traps were originally used a monitoring tools and their insect population management potential were identified later. One of the major problems with light trap is that it cannot discriminate between harmful and beneficial insects. If the traps appear to kill large numbers of beneficial insects it is better to take out the traps.

Solar powered models

 

Recently, many solar powered models of light trap have been launched. They generally employ UV and yellow LEDs as light source. The systems are provided with rechargeavle bateries, which can provede power to the light source throughout the night.

These solar powered units can run without any human intervention as they are provided with ambient light sensor and thus turn themselves on automatically in the evening and turn off at dawn. They need little maintenance too. The efficacy of light traps remains unchanged in rainy seasons when most of other pest management procedure becomes less effective.

 

For optimum effectiveness, the light traps should not be placed near competing lights (lamp posts) and should not be installed in heavy traffic areas. Traps are also to be placed in such ay way that they do not attract insects from adjoining farms across the Stevia plantation. In that case the harm will outweigh the benefit.

The traps should be placed at a height that is within the normal flight range of most flying insects. But it is just as important to position the units in a place that is easily accessible for inspection and routine service. Regular clearance of the trap pan is also necessary to ensure proper farm hygiene.

 

Local Experiences

 

My friend Mr. Ranganath Krishnan of Stevia World Agrotech Pvt Ltd, Bangalore informed me in a personal communication about successful adaptation of solar powered trap in Bangalore region. One trap in one acre of land was found to be sufficient for adequate insect control.   

Pictures of Solar Powered Insect Traps used in Karnataka

A light trap in action

A short video of a "Do It Yourself" light trap

Bug Zappers

 

A bug zapper, more formally called an electrical discharge insect control system, electric insect killer or (insect) electrocutor trap, is a device that attracts and kills flying insects that are attracted by light. A light source attracts insects to an electrical grid, where they are electrocuted by touching two wires with a high voltage between them. The name stems from the characteristic onomatopoeic zap sound produced when an insect is electrocuted. They were originally designed for indoor applications, mainly in food processing facilities, but, later they are successfully used in agricultural pest control also.

 

Bug zappers are usually housed in a protective cage of plastic or grounded metal bars to prevent people or animals from touching the high voltage grid. A light source is fitted inside, often a fluorescent lamp designed to emit violet and ultraviolet light, which is visible to insects and attracts them. The light is surrounded by a pair of interleaved bare wire grids or spirals. The distance between adjacent wires is typically about 2 mm (0.079 in). A high-voltage power supply powered by mains electricity, which may be a simple transformer-less voltage multiplier circuit made with diodes and capacitors, generates a voltage of 2,000 volts or more, high enough to conduct through the body of an insect which bridges the two grids, but not high enough to spark across the air gap. Enough electric current flows through the small body of the insect to heat it to a high temperature. The impedance of the power supply and the arrangement of the grid is such that it cannot drive a dangerous current through the body of a human.

 

Many bug zappers are fitted with trays that collect the electrocuted insects; other models are designed to allow the debris to fall to the ground below. Some use a fan to help to trap the insect.

A standard bug zapper is generally claimed to effective for controlling an area of 0.5 ha. One of the typical problems with bug japer is that it cannot differentiate between beneficial and harmful insects and indiscriminate killing of insect may have serious impact on beneficial insects.

An omnidirectional outdoor type insect zapper

Installation of solar powered insect zapper in field

Pheromone traps


Pheromones can be used as a bait to attract certain insects (especially moths) and catch them in a trap. Usually these traps use a sticky surface to catch the insects, but others use containers filled with water or other liquids to catch them. 


Most pheromones are "sex pheromones". These are chemicals released by female moths to attract a male partner. As a consequence the traps will only catch male moths and usually of only one species. To catch other species, different pheromones will have to be used.


Pheromone traps are very useful for monitoring a specific pest species and will often detect low population densities. However, the traps are not very efficient for controlling pests.

A disadvantage of the pheromone traps is that the chemicals are often expensive and not easily available. On the other hand, they have the advantage that they only attract the pest and will not kill beneficial insects. 


A type of trap, often used with pheromone baits, is the "delta-trap". It is a triangular trap, made of plastic or weatherproof card. Insects that fly into the trap are caught on a surface treated with a special type of non-drying glue.

Electronic Pest Repellants

There are two types of electronic pest control devices widely available: electromagnetic and ultrasonic.

 

Electromagnetic

 

Electromagnetic ("EM") pest repelling devices claim to affect the nervous system of many pests. The major mechanism for the lethal action of EM towards the insect was proposed to be absorption of radiofrequency energy and rise of temperature within the insect body – just like the action of a microwave oven.

 

Though there are several equipments available in the markets, which are claimed to be effective in pest control through microwave EM radiation, it is very unlikely that any affordable instrument is able to generate and radiate enough microwave energy to affect insect population even at a short distance.

 

Ultrasonic

 

Ultrasonic devices operate through emitting short wavelength, high frequency sound waves that are too high in pitch to be heard by the human ear (all frequencies greater than 20,000 Hz). Humans are unable to hear sounds higher than 20 kHz due to physiological limitations of the cochlea. Some animals, such as bats, dogs, and rodents, can hear well into the ultrasonic range. Some insects, such as grasshoppers and locusts, can detect frequencies from 50,000 Hz to 100,000 Hz, and lacewings and moths can detect ultrasound as high as 240,000 Hz produced by insect-hunting bats. The claims of effectiveness of these  equipment and application of ultrasonic sound waves for control of insects in general has been questioned by scientific fraternity.

PSEUDO SCIENCE

 
 
 
 
 
 
 

Electronic Pest Repellants