Figuring Out Fumigants

With the recent re-registration of soil fumigants, it is easy to forget the basics of soil fumigation and why it may be needed for pest control. Soil fumigation is a type of chemical control strategy used to reduce the incidence of soil-borne pests. It can be used alone or in conjunction with other methods such as cultural, mechanical, or physical as part of an Integrated Pest Management (IPM) program. There are several types of soil fumigants. Some are considered broad spectrum and kill most everything they come in contact with, while others are more pest specific. Some common pests controlled by fumigants include nematodes, fungi, bacteria, weeds, and weed seeds. Fumigants are most often used in the production of higher value crops due to cost. They have the potential to increase yield and plant quality. This ultimately translates to higher profitability for the producer.

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Soil fumigants begin as liquids or solids and when released into the soil become gases. They can be applied as liquefied gases, volatile liquids or granules. However, most growers are familiar with liquid fumigants. When fumigants are applied to the soil, they volatilize into a gaseous state and spread through the air pores in the soil. Depending on a particular fumigant’s chemical properties it may volatilize quickly or more slowly than others. Due to volatility, fumigants must be incorporated or sealed into the soil during or immediately following application. In order to kill soil-borne pests, fumigants must come in contact with the pest and must reach a lethal concentration. The lethal concentration is expressed as Fumigant Dose: Σt=1 (Concentration x Time). As with any pesticide, the lethal dose is not just the concentration of the pesticide, but the amount of time in which pests are exposed. Obtaining this lethal dose is needed to kill problem pests and is the objective of all fumigant programs.

Soil fumigation was first discovered in the 1940s when it was noticed that plant production and quality increased when the soil was treated. In the early 1950s we saw the development of the first fumigants produced for use in commercial agriculture including (DD) 1,3-dichloropropene, (EDB) ethylene dibromide and Vorlex. Since this time safer, more effective fumigants continue to be developed.

Fumigant application equipment has also improved dramatically over the years. The first fumigants were contained in 55 gallon drums and were applied via gravity flow lines to the soil through tractor pulled chisels or shanks. Now, most fumigants are contained in pressurized cylinders and can be applied through various methods on flat ground or raised beds using chisels or shanks or through drip irrigation. Fumigants can now be sealed using plastic mulch technology or rolling devices.

Types Of Pests Controlled And Fumigant Uses

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Proper use of soil fumigants can effectively control weeds, weed seeds, nematodes, insects, fungi and bacteria. Some fumigants are stronger than others at controlling certain pests. They may need to be used in conjunction with other chemical or cultural methods to be their most effective. The specific pests controlled will be listed on the fumigant’s label. As with any pest it is important to first have the pest identified before attempting any control measure. Of the pests controlled by fumigants, nematodes may be the most difficult to identify because they are microscopic and associated plant symptoms often resemble other biotic or abiotic problems. If you suspect nematodes are present, a soil test should be performed to verify presence and identify type. Soil depth at which fumigants are applied is also very important in determining a fumigant’s effectiveness. Different pests reside at different soil depths, so you want to be sure the pests you are trying to target are coming in contact with the fumigant. Now we’ll look a little closer at the different types of soil-borne pests. Nematodes are microscopic roundworms that feed on plant roots. There are several different species of nematodes. However, only a few are plant parasitic. Nematodes live in the soil either in water-filled pore spaces or within the roots themselves. They have needlelike stylets that pierce the roots and suck out the contents. Nematodes reproduce by laying eggs. Soil fungi are plant-like organisms that do not produce chlorophyll. Chlorophyll helps plants make their own food. Thus fungi cannot make their own food and must get it from another living source such as plants, animals or organic matter. Fungi produce spores that ultimately produce chemicals that cause plant diseases to develop. Bacteria are small single-celled organisms that can live in the soil and cause plant diseases. They get their nutrients from plant cells and can easily enter a plant through a wound or natural opening. Many insects spend part of their life cycle in the soil and can affect plant productivity. Insects controlled by fumigants are typically larval stages of flies or beetles. Finally, weeds are unwanted plants or “plants out of place.” They compete with wanted plants for light, nutrients and space and may be alternate hosts for insects and diseases.
 
Factors Affecting Fumigant Performance
Many factors impact the effectiveness of soil fumigation. Pest habitat can be determined after the pest is identified. This is important because it may be easier to target pests when they are at their most vulnerable. It is also important to know your soil texture before applying a fumigant. Texture can influence how much of a fumigant is needed. Typically, clay type soils have fewer pore spaces and more binding sites requiring more fumigant. In Florida, although our soils differ between regions, we typically have course, sandy soils which should require less fumigant due to fewer binding sites and few air spaces. Soil condition or permeability affects diffusion and penetration of fumigants. If soil is compacted, it decreases the number of pore spaces available and fumigant movement is restricted. It is highly recommended to cultivate soil before fumigation to allow for maximum fumigant penetration. Incorrect compaction is the number one reason for fumigation failures.
Plant debris can cause problems with fumigants because it provides organic matter which binds to the fumigant and does not allow it to move freely. Organic matter can also create avenues or chimneys through which fumigant gases may escape. It is important to incorporate all organic matter so that it has time to decompose before a fumigant is applied.
Soil moisture is also a factor that can inhibit or enhance the movement of fumigants. Too much or too little soil moisture can be detrimental. Fumigants are usually applied at 50-75% field capacity in which some soil moisture is required to hold the fumigant for the desired period of time. However, too much moisture can impede gas movement.
Soil temperature is another important factor affecting soil fumigation. Temperature is directly related to how quickly a fumigant volatilizes to a gaseous form. If the temperature is too cold, volatilization and soil penetration may be reduced. If it is too high, fumigants volatilize more quickly and thus may not be present for the length of time required to kill the target pest. Other factors such as application depth and dosage will be important in killing target pests and will vary depending on many factors. Be sure to follow label directions.
Soil sealing is very important in soil fumigation. Sealing keeps the fumigant from escaping into the atmosphere long enough to reach the lethal dosage and kill the target pest. Sealing can be achieved with plastic tarps, mechanical means or wetting. The exposure period is the amount of time the soil is undisturbed in order to kill target organisms. This will vary for different fumigants. Soil aeration may be necessary at the end of the exposure period to allow for fumigant dissipation before planting. This may prevent phytotoxicity or plant injury from the fumigant. It is important to be proactive and aware of all of these factors. Looking at fumigation in a whole systems approach will help you achieve a successful fumigation program.

Application And Calibration Basics

Application equipment and methodology vary for different types of fumigants. Depending on the type of fumigant used, it may be necessary to adjust equipment for proper application. Equipment manufacturers make fumigation equipment from chemical resistant materials because fumigants can be corrosive. Thus, it is very important to properly clean and maintain fumigation equipment.
Liquefied gases can be applied above ground for small application areas such as in greenhouses or nursery soil and trapped under sealed plastic. For soil injection applications of liquefied gases, tractor pulled shanks or chisels are typically used. Shallow applications are considered those between 6”-12” deep while deep applications are 18”-34”. These applications are commonly sealed with plastic tarps. Auger applications are used in perennial crops such as fruits, nuts, and vineyards. The fumigant is released into an auger dug hole, then filled and compacted to seal.
Volatile liquids can be applied to small areas via trench or handgun applications. Shank or chisel applications are the most commonly used to treat large areas such as agricultural row crops. As with liquefied gases, tractor pulled shanks or chisels are used. For broadcast applications, shank spacing usually equals injection depth. Volatile liquids can also be applied with sweeps or blades drawn through the soil and sealed. Drench application is typical in nurseries, ornamental plantings and orchards. Fumigating by chemigation involves injecting a liquid fumigant into the irrigation water. This can be achieved with several different types of irrigation systems – including drip irrigation – but is most commonly used with center pivots.
Volatile Solids are typically applied through broadcast applications in which granules are incorporated into the soil and sealed.
Before applying fumigants it is important to be sure your equipment is properly calibrated. There are two common calibration methods: 1. measuring the amount of material applied over a known area and 2. Collecting and measuring the amount of fumigant delivered over a period of time and calculating the amount of area covered. For method 1, this formula can be used to determine amount of fumigant used over a known area:
L x W x R = A
43,560 ft2
Where L=Length, W=Width, R=desired amount of the chemical per acre, A=Amount of chemical that should be delivered over the calibration course, and 43,560=The number of square feet in an acre.
Method 2 is the most common method for pump driven equipment in which the amount of fumigant released over a known period of time is measured. For volatile liquids, measure the flow rate and convert to an area basis. For liquefied gases do not try to collect the gas. Rather, weigh the cylinder, release the gas for a known period of time, re-weigh the cylinder, then convert to an area basis.

Safety Storage And Disposal

Fumigants are highly toxic. Restricted Use Pesticides should be applied using all available safety precautions. The first step of fumigant safety is to read the label and Material Safety Data Sheet (MSDS). Fumigants will have different safety precautions as well as different requirements for personal protective equipment (PPE). Fumigants should be applied according to the label and by or under the supervision of a certified applicator.
Fumigants should be stored in a cool, dry, locked, and well ventilated area. Always post signs indicating that it is a pesticide storage area. Never store fumigants with PPE or where humans or animals live. If possible, have fumigants delivered just before use to limit storage time. It is important to store fumigant cylinders in an upright position.
Fumigants are should be disposed of according to label recommendations. Since fumigants are toxic to fish and wildlife it is important to take precautions against contaminating the water sources. If you have questions about fumigant disposal, contact the Florida Department of Environmental Protection at (904) 448-2974.
When using fumigants it is important to follow the regulations of the Environmental Protection Agency’s Worker Protection Standards (WPS). WPS protects farm workers working on farms, forests, nurseries or greenhouses from the hazards of pesticides.

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