Figuring Out Fumigants

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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.

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

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.

Crystal Snodgrass is a UF/IFAS Extension Agent based in Manatee County.

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